Programs & Examples On #Performance

For questions pertaining to the measurement or improvement of code and application efficiency.

Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs

TL;DR: Use __builtin intrinsics instead; they might happen to help.

I was able to make gcc 4.8.4 (and even 4.7.3 on generate optimal code for this by using __builtin_popcountll which uses the same assembly instruction, but gets lucky and happens to make code that doesn't have an unexpectedly long loop-carried dependency because of the false dependency bug.

I am not 100% sure of my benchmarking code, but objdump output seems to share my views. I use some other tricks (++i vs i++) to make the compiler unroll loop for me without any movl instruction (strange behaviour, I must say).


Count: 20318230000  Elapsed: 0.411156 seconds   Speed: 25.503118 GB/s

Benchmarking code:

#include <stdint.h>
#include <stddef.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

uint64_t builtin_popcnt(const uint64_t* buf, size_t len){
  uint64_t cnt = 0;
  for(size_t i = 0; i < len; ++i){
    cnt += __builtin_popcountll(buf[i]);
  return cnt;

int main(int argc, char** argv){
  if(argc != 2){
    printf("Usage: %s <buffer size in MB>\n", argv[0]);
    return -1;
  uint64_t size = atol(argv[1]) << 20;
  uint64_t* buffer = (uint64_t*)malloc((size/8)*sizeof(*buffer));

  // Spoil copy-on-write memory allocation on *nix
  for (size_t i = 0; i < (size / 8); i++) {
    buffer[i] = random();
  uint64_t count = 0;
  clock_t tic = clock();
  for(size_t i = 0; i < 10000; ++i){
    count += builtin_popcnt(buffer, size/8);
  clock_t toc = clock();
  printf("Count: %lu\tElapsed: %f seconds\tSpeed: %f GB/s\n", count, (double)(toc - tic) / CLOCKS_PER_SEC, ((10000.0*size)/(((double)(toc - tic)*1e+9) / CLOCKS_PER_SEC)));
  return 0;

Compile options:

gcc --std=gnu99 -mpopcnt -O3 -funroll-loops -march=native bench.c -o bench

GCC version:

gcc (Ubuntu 4.8.4-2ubuntu1~14.04.1) 4.8.4

Linux kernel version:


CPU information:

processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 70
model name  : Intel(R) Core(TM) i7-4870HQ CPU @ 2.50 GHz
stepping    : 1
microcode   : 0xf
cpu MHz     : 2494.226
cache size  : 6144 KB
physical id : 0
siblings    : 1
core id     : 0
cpu cores   : 1
apicid      : 0
initial apicid  : 0
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss ht syscall nx rdtscp lm constant_tsc nopl xtopology nonstop_tsc eagerfpu pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm arat pln pts dtherm fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 invpcid xsaveopt
bugs        :
bogomips    : 4988.45
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

Find the least number of coins required that can make any change from 1 to 99 cents

Came across this one today, while studying

DPCHANGE(money, coins)
 MinNumCoins(0) ? 0
 for m ? 1 to money
        MinNumCoins(m) ? 8
        for i ? 1 to |coins|
            if m = coini
                if MinNumCoins(m - coini) + 1 < MinNumCoins(m)
                    MinNumCoins(m) ? MinNumCoins(m - coini) + 1
    output MinNumCoins(money)

Takes a comma-separated string of denominations available, and the target amount.

C# implementation:

    public static void DPCHANGE(int val, string denoms)
        int[] idenoms = Array.ConvertAll(denoms.Split(','), int.Parse);
        int[] minNumCoins = new int[val + 1];

        minNumCoins[0] = 0;
        for (int m = 1; m <= val; m++)
            minNumCoins[m] = Int32.MaxValue - 1;
            for (int i = 1; i <= idenoms.Count() - 1; i++)
                if (m >= idenoms[i])
                    if (minNumCoins[m - idenoms[i]] + 1 < minNumCoins[m])
                        minNumCoins[m] = minNumCoins[m - idenoms[i]] + 1;

Is String.Contains() faster than String.IndexOf()?

Use a benchmark library, like this recent foray from Jon Skeet to measure it.

Caveat Emptor

As all (micro-)performance questions, this depends on the versions of software you are using, the details of the data inspected and the code surrounding the call.

As all (micro-)performance questions, the first step has to be to get a running version which is easily maintainable. Then benchmarking, profiling and tuning can be applied to the measured bottlenecks instead of guessing.

Which is faster: Stack allocation or Heap allocation

class Foo {
    Foo(int a) {

int func() {
    int a1, a2;
    std::cin >> a1;
    std::cin >> a2;

    Foo f1(a1);
    __asm push a1;
    __asm lea ecx, [this];
    __asm call Foo::Foo(int);

    Foo* f2 = new Foo(a2);
    __asm push sizeof(Foo);
    __asm call operator new;//there's a lot instruction here(depends on system)
    __asm push a2;
    __asm call Foo::Foo(int);

    delete f2;

It would be like this in asm. When you're in func, the f1 and pointer f2 has been allocated on stack (automated storage). And by the way, Foo f1(a1) has no instruction effects on stack pointer (esp),It has been allocated, if func wants get the member f1, it's instruction is something like this: lea ecx [ebp+f1], call Foo::SomeFunc(). Another thing the stack allocate may make someone think the memory is something like FIFO, the FIFO just happened when you go into some function, if you are in the function and allocate something like int i = 0, there no push happened.

Is it safe to shallow clone with --depth 1, create commits, and pull updates again?

See some of the answers to my similar question why-cant-i-push-from-a-shallow-clone and the link to the recent thread on the git list.

Ultimately, the 'depth' measurement isn't consistent between repos, because they measure from their individual HEADs, rather than (a) your Head, or (b) the commit(s) you cloned/fetched, or (c) something else you had in mind.

The hard bit is getting one's Use Case right (i.e. self-consistent), so that distributed, and therefore probably divergent repos will still work happily together.

It does look like the checkout --orphan is the right 'set-up' stage, but still lacks clean (i.e. a simple understandable one line command) guidance on the "clone" step. Rather it looks like you have to init a repo, set up a remote tracking branch (you do want the one branch only?), and then fetch that single branch, which feels long winded with more opportunity for mistakes.

Edit: For the 'clone' step see this answer

MyISAM versus InnoDB

I've worked on a high-volume system using MySQL and I've tried both MyISAM and InnoDB.

I found that the table-level locking in MyISAM caused serious performance problems for our workload which sounds similar to yours. Unfortunately I also found that performance under InnoDB was also worse than I'd hoped.

In the end I resolved the contention issue by fragmenting the data such that inserts went into a "hot" table and selects never queried the hot table.

This also allowed deletes (the data was time-sensitive and we only retained X days worth) to occur on "stale" tables that again weren't touched by select queries. InnoDB seems to have poor performance on bulk deletes so if you're planning on purging data you might want to structure it in such a way that the old data is in a stale table which can simply be dropped instead of running deletes on it.

Of course I have no idea what your application is but hopefully this gives you some insight into some of the issues with MyISAM and InnoDB.

Getting HTTP code in PHP using curl

curl_exec is necessary. Try CURLOPT_NOBODY to not download the body. That might be faster.

Case vs If Else If: Which is more efficient?

It seems that the compiler is better in optimizing a switch-statement than an if-statement.

The compiler doesn't know if the order of evaluating the if-statements is important to you, and can't perform any optimizations there. You could be calling methods in the if-statements, influencing variables. With the switch-statement it knows that all clauses can be evaluated at the same time and can put them in whatever order is most efficient.

Here's a small comparison:

Most efficient way to concatenate strings in JavaScript?

I have no comment on the concatenation itself, but I'd like to point out that @Jakub Hampl's suggestion:

For building strings in the DOM, in some cases it might be better to iteratively add to the DOM, rather then add a huge string at once.

is wrong, because it's based on a flawed test. That test never actually appends into the DOM.

This fixed test shows that creating the string all at once before rendering it is much, MUCH faster. It's not even a contest.

(Sorry this is a separate answer, but I don't have enough rep to comment on answers yet.)

Efficiency of Java "Double Brace Initialization"?

Mario Gleichman describes how to use Java 1.5 generic functions to simulate Scala List literals, though sadly you wind up with immutable Lists.

He defines this class:

package literal;

public class collection {
    public static <T> List<T> List(T...elems){
        return Arrays.asList( elems );

and uses it thusly:

import static literal.collection.List;
import static*;

public class CollectionDemo {
    public void demoList(){
        List<String> slist = List( "a", "b", "c" );
        List<Integer> iList = List( 1, 2, 3 );
        for( String elem : List( "a", "java", "list" ) )
            System.out.println( elem );

Google Collections, now part of Guava supports a similar idea for list construction. In this interview, Jared Levy says:

[...] the most heavily-used features, which appear in almost every Java class I write, are static methods that reduce the number of repetitive keystrokes in your Java code. It's so convenient being able to enter commands like the following:

Map<OneClassWithALongName, AnotherClassWithALongName> = Maps.newHashMap();

List<String> animals = Lists.immutableList("cat", "dog", "horse");

7/10/2014: If only it could be as simple as Python's:

animals = ['cat', 'dog', 'horse']

2/21/2020: In Java 11 you can now say:

animals = List.of(“cat”, “dog”, “horse”)

What Process is using all of my disk IO

To find out which processes in state 'D' (waiting for disk response) are currently running:

while true; do date; ps aux | awk '{if($8=="D") print $0;}'; sleep 1; done


watch -n1 -d "ps axu | awk '{if (\$8==\"D\") {print \$0}}'"

Wed Aug 29 13:00:46 CLT 2012
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:00:47 CLT 2012
Wed Aug 29 13:00:48 CLT 2012
Wed Aug 29 13:00:49 CLT 2012
Wed Aug 29 13:00:50 CLT 2012
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:00:51 CLT 2012
Wed Aug 29 13:00:52 CLT 2012
Wed Aug 29 13:00:53 CLT 2012
Wed Aug 29 13:00:55 CLT 2012
Wed Aug 29 13:00:56 CLT 2012
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:00:57 CLT 2012
root       302  0.0  0.0      0     0 ?        D    May28   3:07  \_ [kdmflush]
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:00:58 CLT 2012
root       302  0.0  0.0      0     0 ?        D    May28   3:07  \_ [kdmflush]
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:00:59 CLT 2012
root       302  0.0  0.0      0     0 ?        D    May28   3:07  \_ [kdmflush]
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:01:00 CLT 2012
root       302  0.0  0.0      0     0 ?        D    May28   3:07  \_ [kdmflush]
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:01:01 CLT 2012
root       302  0.0  0.0      0     0 ?        D    May28   3:07  \_ [kdmflush]
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]
Wed Aug 29 13:01:02 CLT 2012
Wed Aug 29 13:01:03 CLT 2012
root       321  0.0  0.0      0     0 ?        D    May28   4:25  \_ [jbd2/dm-0-8]

As you can see from the result, the jdb2/dm-0-8 (ext4 journal process), and kdmflush are constantly block your Linux.

For more details this URL could be helpful: Linux Wait-IO Problem

Most efficient way to find smallest of 3 numbers Java?

For pure characters-of-code efficiency, I can't find anything better than

smallest = a<b&&a<c?a:b<c?b:c;

How to change the playing speed of videos in HTML5?

Just type

document.querySelector('video').playbackRate = 1.25;

in JS console of your modern browser.

High CPU Utilization in java application - why?

During these peak CPU times, what is the user load like? You say this is a web based application, so the culprits that come to mind is memory utilization issues. If you store a lot of stuff in the session, for instance, and the session count gets high enough, the app server will start thrashing about. This is also a case where the GC might make matters worse depending on the scheme you are using. More information about the app and the server configuration would be helpful in pointing towards more debugging ideas.

When to use StringBuilder in Java

For two strings concat is faster, in other cases StringBuilder is a better choice, see my explanation in concatenation operator (+) vs concat()

Fastest way to remove non-numeric characters from a VARCHAR in SQL Server

Working with varchars is fundamentally slow and inefficient compared to working with numerics, for obvious reasons. The functions you link to in the original post will indeed be quite slow, as they loop through each character in the string to determine whether or not it's a number. Do that for thousands of records and the process is bound to be slow. This is the perfect job for Regular Expressions, but they're not natively supported in SQL Server. You can add support using a CLR function, but it's hard to say how slow this will be without trying it I would definitely expect it to be significantly faster than looping through each character of each phone number, however!

Once you get the phone numbers formatted in your database so that they're only numbers, you could switch to a numeric type in SQL which would yield lightning-fast comparisons against other numeric types. You might find that, depending on how fast your new data is coming in, doing the trimming and conversion to numeric on the database side is plenty fast enough once what you're comparing to is properly formatted, but if possible, you would be better off writing an import utility in a .NET language that would take care of these formatting issues before hitting the database.

Either way though, you're going to have a big problem regarding optional formatting. Even if your numbers are guaranteed to be only North American in origin, some people will put the 1 in front of a fully area-code qualified phone number and others will not, which will cause the potential for multiple entries of the same phone number. Furthermore, depending on what your data represents, some people will be using their home phone number which might have several people living there, so a unique constraint on it would only allow one database member per household. Some would use their work number and have the same problem, and some would or wouldn't include the extension which would cause artificial uniqueness potential again.

All of that may or may not impact you, depending on your particular data and usages, but it's important to keep in mind!

What is the relative performance difference of if/else versus switch statement in Java?

That's micro optimization and premature optimization, which are evil. Rather worry about readabililty and maintainability of the code in question. If there are more than two if/else blocks glued together or its size is unpredictable, then you may highly consider a switch statement.

Alternatively, you can also grab Polymorphism. First create some interface:

public interface Action { 
    void execute(String input);

And get hold of all implementations in some Map. You can do this either statically or dynamically:

Map<String, Action> actions = new HashMap<String, Action>();

Finally replace the if/else or switch by something like this (leaving trivial checks like nullpointers aside):


It might be microslower than if/else or switch, but the code is at least far better maintainable.

As you're talking about webapplications, you can make use of HttpServletRequest#getPathInfo() as action key (eventually write some more code to split the last part of pathinfo away in a loop until an action is found). You can find here similar answers:

If you're worrying about Java EE webapplication performance in general, then you may find this article useful as well. There are other areas which gives a much more performance gain than only (micro)optimizing the raw Java code.

How to get object size in memory?

The following code fragment should return the size in bytes of any object passed to it, so long as it can be serialized. I got this from a colleague at Quixant to resolve a problem of writing to SRAM on a gaming platform. Hope it helps out. Credit and thanks to Carlo Vittuci.

/// <summary>
/// Calculates the lenght in bytes of an object 
/// and returns the size 
/// </summary>
/// <param name="TestObject"></param>
/// <returns></returns>
private int GetObjectSize(object TestObject)
    BinaryFormatter bf = new BinaryFormatter();
    MemoryStream ms = new MemoryStream();
    byte[] Array;
    bf.Serialize(ms, TestObject);
    Array = ms.ToArray();
    return Array.Length;

Handling warning for possible multiple enumeration of IEnumerable

If you only need to check the first element you can peek on it without iterating the whole collection:

public List<object> Foo(IEnumerable<object> objects)
    object firstObject;
    if (objects == null || !TryPeek(ref objects, out firstObject))
        throw new ArgumentException();

    var list = DoSomeThing(firstObject);
    var secondList = DoSomeThingElse(objects);

    return list;

public static bool TryPeek<T>(ref IEnumerable<T> source, out T first)
    if (source == null)
        throw new ArgumentNullException(nameof(source));

    IEnumerator<T> enumerator = source.GetEnumerator();
    if (!enumerator.MoveNext())
        first = default(T);
        source = Enumerable.Empty<T>();
        return false;

    first = enumerator.Current;
    T firstElement = first;
    source = Iterate();
    return true;

    IEnumerable<T> Iterate()
        yield return firstElement;
        using (enumerator)
            while (enumerator.MoveNext())
                yield return enumerator.Current;

What are the performance characteristics of sqlite with very large database files?

Besides the usual recommendation:

  1. Drop index for bulk insert.
  2. Batch inserts/updates in large transactions.
  3. Tune your buffer cache/disable journal /w PRAGMAs.
  4. Use a 64bit machine (to be able to use lots of cache™).
  5. [added July 2014] Use common table expression (CTE) instead of running multiple SQL queries! Requires SQLite release 3.8.3.

I have learnt the following from my experience with SQLite3:

  1. For maximum insert speed, don't use schema with any column constraint. (Alter table later as needed You can't add constraints with ALTER TABLE).
  2. Optimize your schema to store what you need. Sometimes this means breaking down tables and/or even compressing/transforming your data before inserting to the database. A great example is to storing IP addresses as (long) integers.
  3. One table per db file - to minimize lock contention. (Use ATTACH DATABASE if you want to have a single connection object.
  4. SQLite can store different types of data in the same column (dynamic typing), use that to your advantage.

Question/comment welcome. ;-)

Exact time measurement for performance testing

A better way is to use the Stopwatch class:

using System.Diagnostics;
// ...

Stopwatch sw = new Stopwatch();


// ...



When is assembly faster than C?

Here is a real world example: Fixed point multiplies on old compilers.

These don't only come handy on devices without floating point, they shine when it comes to precision as they give you 32 bits of precision with a predictable error (float only has 23 bit and it's harder to predict precision loss). i.e. uniform absolute precision over the entire range, instead of close-to-uniform relative precision (float).

Modern compilers optimize this fixed-point example nicely, so for more modern examples that still need compiler-specific code, see

C doesn't have a full-multiplication operator (2N-bit result from N-bit inputs). The usual way to express it in C is to cast the inputs to the wider type and hope the compiler recognizes that the upper bits of the inputs aren't interesting:

// on a 32-bit machine, int can hold 32-bit fixed-point integers.
int inline FixedPointMul (int a, int b)
  long long a_long = a; // cast to 64 bit.

  long long product = a_long * b; // perform multiplication

  return (int) (product >> 16);  // shift by the fixed point bias

The problem with this code is that we do something that can't be directly expressed in the C-language. We want to multiply two 32 bit numbers and get a 64 bit result of which we return the middle 32 bit. However, in C this multiply does not exist. All you can do is to promote the integers to 64 bit and do a 64*64 = 64 multiply.

x86 (and ARM, MIPS and others) can however do the multiply in a single instruction. Some compilers used to ignore this fact and generate code that calls a runtime library function to do the multiply. The shift by 16 is also often done by a library routine (also the x86 can do such shifts).

So we're left with one or two library calls just for a multiply. This has serious consequences. Not only is the shift slower, registers must be preserved across the function calls and it does not help inlining and code-unrolling either.

If you rewrite the same code in (inline) assembler you can gain a significant speed boost.

In addition to this: using ASM is not the best way to solve the problem. Most compilers allow you to use some assembler instructions in intrinsic form if you can't express them in C. The VS.NET2008 compiler for example exposes the 32*32=64 bit mul as __emul and the 64 bit shift as __ll_rshift.

Using intrinsics you can rewrite the function in a way that the C-compiler has a chance to understand what's going on. This allows the code to be inlined, register allocated, common subexpression elimination and constant propagation can be done as well. You'll get a huge performance improvement over the hand-written assembler code that way.

For reference: The end-result for the fixed-point mul for the VS.NET compiler is:

int inline FixedPointMul (int a, int b)
    return (int) __ll_rshift(__emul(a,b),16);

The performance difference of fixed point divides is even bigger. I had improvements up to factor 10 for division heavy fixed point code by writing a couple of asm-lines.

Using Visual C++ 2013 gives the same assembly code for both ways.

gcc4.1 from 2007 also optimizes the pure C version nicely. (The Godbolt compiler explorer doesn't have any earlier versions of gcc installed, but presumably even older GCC versions could do this without intrinsics.)

See source + asm for x86 (32-bit) and ARM on the Godbolt compiler explorer. (Unfortunately it doesn't have any compilers old enough to produce bad code from the simple pure C version.)

Modern CPUs can do things C doesn't have operators for at all, like popcnt or bit-scan to find the first or last set bit. (POSIX has a ffs() function, but its semantics don't match x86 bsf / bsr. See

Some compilers can sometimes recognize a loop that counts the number of set bits in an integer and compile it to a popcnt instruction (if enabled at compile time), but it's much more reliable to use __builtin_popcnt in GNU C, or on x86 if you're only targeting hardware with SSE4.2: _mm_popcnt_u32 from <immintrin.h>.

Or in C++, assign to a std::bitset<32> and use .count(). (This is a case where the language has found a way to portably expose an optimized implementation of popcount through the standard library, in a way that will always compile to something correct, and can take advantage of whatever the target supports.) See also

Similarly, ntohl can compile to bswap (x86 32-bit byte swap for endian conversion) on some C implementations that have it.

Another major area for intrinsics or hand-written asm is manual vectorization with SIMD instructions. Compilers are not bad with simple loops like dst[i] += src[i] * 10.0;, but often do badly or don't auto-vectorize at all when things get more complicated. For example, you're unlikely to get anything like How to implement atoi using SIMD? generated automatically by the compiler from scalar code.

List of strings to one string

I would go with option A:

String.Join(String.Empty, los.ToArray());

My reasoning is because the Join method was written for that purpose. In fact if you look at Reflector, you'll see that unsafe code was used to really optimize it. The other two also WORK, but I think the Join function was written for this purpose, and I would guess, the most efficient. I could be wrong though...

As per @Nuri YILMAZ without .ToArray(), but this is .NET 4+:

String.Join(String.Empty, los);

How much overhead does SSL impose?

Assuming you don't count connection set-up (as you indicated in your update), it strongly depends on the cipher chosen. Network overhead (in terms of bandwidth) will be negligible. CPU overhead will be dominated by cryptography. On my mobile Core i5, I can encrypt around 250 MB per second with RC4 on a single core. (RC4 is what you should choose for maximum performance.) AES is slower, providing "only" around 50 MB/s. So, if you choose correct ciphers, you won't manage to keep a single current core busy with the crypto overhead even if you have a fully utilized 1 Gbit line. [Edit: RC4 should not be used because it is no longer secure. However, AES hardware support is now present in many CPUs, which makes AES encryption really fast on such platforms.]

Connection establishment, however, is different. Depending on the implementation (e.g. support for TLS false start), it will add round-trips, which can cause noticable delays. Additionally, expensive crypto takes place on the first connection establishment (above-mentioned CPU could only accept 14 connections per core per second if you foolishly used 4096-bit keys and 100 if you use 2048-bit keys). On subsequent connections, previous sessions are often reused, avoiding the expensive crypto.

So, to summarize:

Transfer on established connection:

  • Delay: nearly none
  • CPU: negligible
  • Bandwidth: negligible

First connection establishment:

  • Delay: additional round-trips
  • Bandwidth: several kilobytes (certificates)
  • CPU on client: medium
  • CPU on server: high

Subsequent connection establishments:

  • Delay: additional round-trip (not sure if one or multiple, may be implementation-dependant)
  • Bandwidth: negligible
  • CPU: nearly none

What is the difference between parseInt() and Number()?

If you are looking for performance then probably best results you'll get with bitwise right shift "10">>0. Also multiply ("10" * 1) or not not (~~"10"). All of them are much faster of Number and parseInt. They even have "feature" returning 0 for not number argument. Here are Performance tests.

delete all from table

You can use the below query to remove all the rows from the table, also you should keep it in mind that it will reset the Identity too.


How to reduce the image size without losing quality in PHP

I'd go for jpeg. Read this post regarding image size reduction and after deciding on the technique, use ImageMagick

Hope this helps

How to deal with a slow SecureRandom generator?

Many Linux distros (mostly Debian-based) configure OpenJDK to use /dev/random for entropy.

/dev/random is by definition slow (and can even block).

From here you have two options on how to unblock it:

  1. Improve entropy, or
  2. Reduce randomness requirements.

Option 1, Improve entropy

To get more entropy into /dev/random, try the haveged daemon. It's a daemon that continuously collects HAVEGE entropy, and works also in a virtualized environment because it doesn't require any special hardware, only the CPU itself and a clock.

On Ubuntu/Debian:

apt-get install haveged
update-rc.d haveged defaults
service haveged start


yum install haveged
systemctl enable haveged
systemctl start haveged

Option 2. Reduce randomness requirements

If for some reason the solution above doesn't help or you don't care about cryptographically strong randomness, you can switch to /dev/urandom instead, which is guaranteed not to block.

To do it globally, edit the file jre/lib/security/ in your default Java installation to use /dev/urandom (due to another bug it needs to be specified as /dev/./urandom).

Like this:


Then you won't ever have to specify it on the command line.

Note: If you do cryptography, you need good entropy. Case in point - android PRNG issue reduced the security of Bitcoin wallets.

Why is it faster to check if dictionary contains the key, rather than catch the exception in case it doesn't?

On the one hand, throwing exceptions is inherently expensive, because the stack has to be unwound etc.
On the other hand, accessing a value in a dictionary by its key is cheap, because it's a fast, O(1) operation.

BTW: The correct way to do this is to use TryGetValue

obj item;
if(!dict.TryGetValue(name, out item))
    return null;
return item;

This accesses the dictionary only once instead of twice.
If you really want to just return null if the key doesn't exist, the above code can be simplified further:

obj item;
dict.TryGetValue(name, out item);
return item;

This works, because TryGetValue sets item to null if no key with name exists.

Speed comparison with Project Euler: C vs Python vs Erlang vs Haskell

C++11, < 20ms for me - Run it here

I understand that you want tips to help improve your language specific knowledge, but since that has been well covered here, I thought I would add some context for people who may have looked at the mathematica comment on your question, etc, and wondered why this code was so much slower.

This answer is mainly to provide context to hopefully help people evaluate the code in your question / other answers more easily.

This code uses only a couple of (uglyish) optimisations, unrelated to the language used, based on:

  1. every traingle number is of the form n(n+1)/2
  2. n and n+1 are coprime
  3. the number of divisors is a multiplicative function

#include <iostream>
#include <cmath>
#include <tuple>
#include <chrono>

using namespace std;

// Calculates the divisors of an integer by determining its prime factorisation.

int get_divisors(long long n)
    int divisors_count = 1;

    for(long long i = 2;
        i <= sqrt(n);
        /* empty */)
        int divisions = 0;
        while(n % i == 0)
            n /= i;

        divisors_count *= (divisions + 1);

        //here, we try to iterate more efficiently by skipping
        //obvious non-primes like 4, 6, etc
        if(i == 2)
            i += 2;

    if(n != 1) //n is a prime
        return divisors_count * 2;
        return divisors_count;

long long euler12()
    //n and n + 1
    long long n, n_p_1;

    n = 1; n_p_1 = 2;

    // divisors_x will store either the divisors of x or x/2
    // (the later iff x is divisible by two)
    long long divisors_n = 1;
    long long divisors_n_p_1 = 2;

        /* This loop has been unwound, so two iterations are completed at a time
         * n and n + 1 have no prime factors in common and therefore we can
         * calculate their divisors separately

        long long total_divisors;                 //the divisors of the triangle number
                                                  // n(n+1)/2

        //the first (unwound) iteration

        divisors_n_p_1 = get_divisors(n_p_1 / 2); //here n+1 is even and we

        total_divisors =
                * divisors_n_p_1;

        if(total_divisors > 1000)

        //move n and n+1 forward
        n = n_p_1;
        n_p_1 = n + 1;

        //fix the divisors
        divisors_n = divisors_n_p_1;
        divisors_n_p_1 = get_divisors(n_p_1);   //n_p_1 is now odd!

        //now the second (unwound) iteration

        total_divisors =
                * divisors_n_p_1;

        if(total_divisors > 1000)

        //move n and n+1 forward
        n = n_p_1;
        n_p_1 = n + 1;

        //fix the divisors
        divisors_n = divisors_n_p_1;
        divisors_n_p_1 = get_divisors(n_p_1 / 2);   //n_p_1 is now even!

    return (n * n_p_1) / 2;

int main()
    for(int i = 0; i < 1000; i++)
        using namespace std::chrono;
        auto start = high_resolution_clock::now();
        auto result = euler12();
        auto end = high_resolution_clock::now();

        double time_elapsed = duration_cast<milliseconds>(end - start).count();

        cout << result << " " << time_elapsed << '\n';
    return 0;

That takes around 19ms on average for my desktop and 80ms for my laptop, a far cry from most of the other code I've seen here. And there are, no doubt, many optimisations still available.

Speed tradeoff of Java's -Xms and -Xmx options

> C:\java -X

-Xmixed           mixed mode execution (default)
-Xint             interpreted mode execution only
-Xbootclasspath:<directories and zip/jar files separated by ;>
                  set search path for bootstrap classes and resources
-Xbootclasspath/a:<directories and zip/jar files separated by ;>
                  append to end of bootstrap class path
-Xbootclasspath/p:<directories and zip/jar files separated by ;>
                  prepend in front of bootstrap class path
-Xnoclassgc       disable class garbage collection
-Xincgc           enable incremental garbage collection
-Xloggc:<file>    log GC status to a file with time stamps
-Xbatch           disable background compilation
-Xms<size>        set initial Java heap size
-Xmx<size>        set maximum Java heap size
-Xss<size>        set java thread stack size
-Xprof            output cpu profiling data
-Xfuture          enable strictest checks, anticipating future default
-Xrs              reduce use of OS signals by Java/VM (see documentation)
-Xcheck:jni       perform additional checks for JNI functions
-Xshare:off       do not attempt to use shared class data
-Xshare:auto      use shared class data if possible (default)
-Xshare:on        require using shared class data, otherwise fail.

The -X options are non-standard and subject to change without notice.



I tested JonVD's code, but found it was very slow, 6s.

This code took 0s.

FROM Orders where EmployeeID=5    
Order By OrderDate DESC

MySQL vs MongoDB 1000 reads

MongoDB is not magically faster. If you store the same data, organised in basically the same fashion, and access it exactly the same way, then you really shouldn't expect your results to be wildly different. After all, MySQL and MongoDB are both GPL, so if Mongo had some magically better IO code in it, then the MySQL team could just incorporate it into their codebase.

People are seeing real world MongoDB performance largely because MongoDB allows you to query in a different manner that is more sensible to your workload.

For example, consider a design that persisted a lot of information about a complicated entity in a normalised fashion. This could easily use dozens of tables in MySQL (or any relational db) to store the data in normal form, with many indexes needed to ensure relational integrity between tables.

Now consider the same design with a document store. If all of those related tables are subordinate to the main table (and they often are), then you might be able to model the data such that the entire entity is stored in a single document. In MongoDB you can store this as a single document, in a single collection. This is where MongoDB starts enabling superior performance.

In MongoDB, to retrieve the whole entity, you have to perform:

  • One index lookup on the collection (assuming the entity is fetched by id)
  • Retrieve the contents of one database page (the actual binary json document)

So a b-tree lookup, and a binary page read. Log(n) + 1 IOs. If the indexes can reside entirely in memory, then 1 IO.

In MySQL with 20 tables, you have to perform:

  • One index lookup on the root table (again, assuming the entity is fetched by id)
  • With a clustered index, we can assume that the values for the root row are in the index
  • 20+ range lookups (hopefully on an index) for the entity's pk value
  • These probably aren't clustered indexes, so the same 20+ data lookups once we figure out what the appropriate child rows are.

So the total for mysql, even assuming that all indexes are in memory (which is harder since there are 20 times more of them) is about 20 range lookups.

These range lookups are likely comprised of random IO — different tables will definitely reside in different spots on disk, and it's possible that different rows in the same range in the same table for an entity might not be contiguous (depending on how the entity has been updated, etc).

So for this example, the final tally is about 20 times more IO with MySQL per logical access, compared to MongoDB.

This is how MongoDB can boost performance in some use cases.

Huge performance difference when using group by vs distinct

The two queries express the same question. Apparently the query optimizer chooses two different execution plans. My guess would be that the distinct approach is executed like:

  • Copy all business_key values to a temporary table
  • Sort the temporary table
  • Scan the temporary table, returning each item that is different from the one before it

The group by could be executed like:

  • Scan the full table, storing each value of business key in a hashtable
  • Return the keys of the hashtable

The first method optimizes for memory usage: it would still perform reasonably well when part of the temporary table has to be swapped out. The second method optimizes for speed, but potentially requires a large amount of memory if there are a lot of different keys.

Since you either have enough memory or few different keys, the second method outperforms the first. It's not unusual to see performance differences of 10x or even 100x between two execution plans.

Fastest way to check if a value exists in a list

This is not the code, but the algorithm for very fast searching.

If your list and the value you are looking for are all numbers, this is pretty straightforward. If strings: look at the bottom:

  • -Let "n" be the length of your list
  • -Optional step: if you need the index of the element: add a second column to the list with current index of elements (0 to n-1) - see later
  • Order your list or a copy of it (.sort())
  • Loop through:
    • Compare your number to the n/2th element of the list
      • If larger, loop again between indexes n/2-n
      • If smaller, loop again between indexes 0-n/2
      • If the same: you found it
  • Keep narrowing the list until you have found it or only have 2 numbers (below and above the one you are looking for)
  • This will find any element in at most 19 steps for a list of 1.000.000 (log(2)n to be precise)

If you also need the original position of your number, look for it in the second, index column.

If your list is not made of numbers, the method still works and will be fastest, but you may need to define a function which can compare/order strings.

Of course, this needs the investment of the sorted() method, but if you keep reusing the same list for checking, it may be worth it.

How to identify unused CSS definitions from multiple CSS files in a project

Chrome Developer Tools has an Audits tab which can show unused CSS selectors.

Run an audit, then, under Web Page Performance see Remove unused CSS rules

enter image description here

MySQL my.cnf performance tuning recommendations

Try starting with the Percona wizard and comparing their recommendations against your current settings one by one. Don't worry there aren't as many applicable settings as you might think.

Update circa 2020: Sorry, this tool reached it's end of life:

Everyone points to key_buffer_size first which you have addressed. With 96GB memory I'd be wary of any tiny default value (likely to be only 96M!).

Make first letter of a string upper case (with maximum performance)

If performance/memory usage is an issue then, this one only creates one (1) StringBuilder and one (1) new String of the same size as the Original string.

public static string ToUpperFirst(this string str) {
  if( !string.IsNullOrEmpty( str ) ) {
    StringBuilder sb = new StringBuilder(str);
    sb[0] = char.ToUpper(sb[0]);

    return sb.ToString();

  } else return str;

Python vs Bash - In which kind of tasks each one outruns the other performance-wise?

Typical mainframe flow...

Input Disk/Tape/User (runtime) --> Job Control Language (JCL) --> Output Disk/Tape/Screen/Printer
                                   |                          ^
                                   v                          |
                                   `--> COBOL Program --------' 

Typical Linux flow...

Input Disk/SSD/User (runtime) --> sh/bash/ksh/zsh/... ----------> Output Disk/SSD/Screen/Printer
                                   |                          ^
                                   v                          |
                                   `--> Python script --------'
                                   |                          ^
                                   v                          |
                                   `--> awk script -----------'
                                   |                          ^
                                   v                          |
                                   `--> sed script -----------'
                                   |                          ^
                                   v                          |
                                   `--> C/C++ program --------'
                                   |                          ^
                                   v                          |
                                   `--- Java program ---------'
                                   |                          ^
                                   v                          |
                                   :                          :

Shells are the glue of Linux

Linux shells like sh/ksh/bash/... provide input/output/flow-control designation facilities much like the old mainframe Job Control Language... but on steroids! They are Turing complete languages in their own right while being optimized to efficiently pass data and control to and from other executing processes written in any language the O/S supports.

Most Linux applications, regardless what language the bulk of the program is written in, depend on shell scripts and Bash has become the most common. Clicking an icon on the desktop usually runs a short Bash script. That script, either directly or indirectly, knows where all the files needed are and sets variables and command line parameters, finally calling the program. That's a shell's simplest use.

Linux as we know it however would hardly be Linux without the thousands of shell scripts that startup the system, respond to events, control execution priorities and compile, configure and run programs. Many of these are quite large and complex.

Shells provide an infrastructure that lets us use pre-built components that are linked together at run time rather than compile time. Those components are free-standing programs in their own right that can be used alone or in other combinations without recompiling. The syntax for calling them is indistinguishable from that of a Bash builtin command, and there are in fact numerous builtin commands for which there is also a stand-alone executable on the system, often having additional options.

There is no language-wide difference between Python and Bash in performance. It entirely depends on how each is coded and which external tools are called.

Any of the well known tools like awk, sed, grep, bc, dc, tr, etc. will leave doing those operations in either language in the dust. Bash then is preferred for anything without a graphical user interface since it is easier and more efficient to call and pass data back from a tool like those with Bash than Python.


It depends on which programs the Bash shell script calls and their suitability for the subtask they are given whether the overall throughput and/or responsiveness will be better or worse than the equivalent Python. To complicate matters Python, like most languages, can also call other executables, though it is more cumbersome and thus not as often used.

User Interface

One area where Python is the clear winner is user interface. That makes it an excellent language for building local or client-server applications as it natively supports GTK graphics and is far more intuitive than Bash.

Bash only understands text. Other tools must be called for a GUI and data passed back from them. A Python script is one option. Faster but less flexible options are the binaries like YAD, Zenity, and GTKDialog.

While shells like Bash work well with GUIs like Yad, GtkDialog (embedded XML-like interface to GTK+ functions), dialog, and xmessage, Python is much more capable and so better for complex GUI windows.


Building with shell scripts is like assembling a computer with off-the-shelf components the way desktop PCs are.

Building with Python, C++ or most any other language is more like building a computer by soldering the chips (libraries) and other electronic parts together the way smartphones are.

The best results are usually obtained by using a combination of languages where each can do what they do best. One developer calls this "polyglot programming".

What is the difference between call and apply?

The difference is that call() takes the function arguments separately, and apply() takes the function arguments in an array.

Java HashMap performance optimization / alternative

As many people pointed out the hashCode() method was to blame. It was only generating around 20,000 codes for 26 million distinct objects. That is an average of 1,300 objects per hash bucket = very very bad. However if I turn the two arrays into a number in base 52 I am guaranteed to get a unique hash code for every object:

public int hashCode() {       
    // assume that both a and b are sorted       
    return a[0] + powerOf52(a[1], 1) + powerOf52(b[0], 2) + powerOf52(b[1], 3) + powerOf52(b[2], 4);

public static int powerOf52(byte b, int power) {
    int result = b;
    for (int i = 0; i < power; i++) {
        result *= 52;
    return result;

The arrays are sorted to ensure this methods fulfills the hashCode() contract that equal objects have the same hash code. Using the old method the average number of puts per second over blocks of 100,000 puts, 100,000 to 2,000,000 was:


Using the new method gives:


Much much better. The old method tailed off very quickly while the new one keeps up a good throughput.

jQuery hasClass() - check for more than one class

This worked for me:


Algorithm to calculate the number of divisors of a given number

Try something along these lines:

int divisors(int myNum) {
    int limit = myNum;
    int divisorCount = 0;
    if (x == 1) 
        return 1;
    for (int i = 1; i < limit; ++i) {
        if (myNum % i == 0) {
            limit = myNum / i;
            if (limit != i)
    return divisorCount;

compareTo with primitives -> Integer / int

If you are using java 8, you can create Comparator by this method:

Comparator.comparingInt(i -> i);

if you would like to compare with reversed order:

Comparator.comparingInt(i -> -i);

How many threads is too many?

One thing you should keep in mind is that python (at least the C based version) uses what's called a global interpreter lock that can have a huge impact on performance on mult-core machines.

If you really need the most out of multithreaded python, you might want to consider using Jython or something.

Difference between declaring variables before or in loop?

It is language dependent - IIRC C# optimises this, so there isn't any difference, but JavaScript (for example) will do the whole memory allocation shebang each time.

Check if property has attribute

This can now be done without expression trees and extension methods in a type safe manner with the new C# feature nameof() like this:

Attribute.IsDefined(typeof(YourClass).GetProperty(nameof(YourClass.Id)), typeof(IsIdentity));

nameof() was introduced in C# 6

Java Reflection Performance

Yes, it is significantly slower. We were running some code that did that, and while I don't have the metrics available at the moment, the end result was that we had to refactor that code to not use reflection. If you know what the class is, just call the constructor directly.

Efficient evaluation of a function at every cell of a NumPy array

If you are working with numbers and f(A(i,j)) = f(A(j,i)), you could use scipy.spatial.distance.cdist defining f as a distance between A(i) and A(j).

I just discovered why all ASP.Net websites are slow, and I am trying to work out what to do about it

If you are using the updated Microsoft.Web.RedisSessionStateProvider(starting from 3.0.2) you can add this to your web.config to allow concurrent sessions.

    <add key="aspnet:AllowConcurrentRequestsPerSession" value="true"/>


The most efficient way to remove first N elements in a list?

You can use list slicing to archive your goal:

n = 5
mylist = [1,2,3,4,5,6,7,8,9]
newlist = mylist[n:]
print newlist


[6, 7, 8, 9]

Or del if you only want to use one list:

n = 5
mylist = [1,2,3,4,5,6,7,8,9]
del mylist[:n]
print mylist


[6, 7, 8, 9]

fastest MD5 Implementation in JavaScript

I've heard Joseph's Myers implementation is quite fast. Additionally, he has a lengthy article on Javascript optimization describing what he learned while writing his implementation. It's a good read for anyone interested in performant javascript.

His MD5 implementation can be found here

JavaScript style.display="none" or jQuery .hide() is more efficient?

Talking about efficiency:

document.getElementById( 'elemtId' ).style.display = 'none';

What jQuery does with its .show() and .hide() methods is, that it remembers the last state of an element. That can come in handy sometimes, but since you asked about efficiency that doesn't matter here.

C# Sort and OrderBy comparison

Why not measure it:

class Program
    class NameComparer : IComparer<string>
        public int Compare(string x, string y)
            return string.Compare(x, y, true);

    class Person
        public Person(string id, string name)
            Id = id;
            Name = name;
        public string Id { get; set; }
        public string Name { get; set; }

    static void Main()
        List<Person> persons = new List<Person>();
        persons.Add(new Person("P005", "Janson"));
        persons.Add(new Person("P002", "Aravind"));
        persons.Add(new Person("P007", "Kazhal"));


        const int COUNT = 1000000;
        Stopwatch watch = Stopwatch.StartNew();
        for (int i = 0; i < COUNT; i++)
        Console.WriteLine("Sort: {0}ms", watch.ElapsedMilliseconds);

        watch = Stopwatch.StartNew();
        for (int i = 0; i < COUNT; i++)
        Console.WriteLine("OrderBy: {0}ms", watch.ElapsedMilliseconds);

    static void Sort(List<Person> list)
        list.Sort((p1, p2) => string.Compare(p1.Name, p2.Name, true));

    static void OrderBy(List<Person> list)
        var result = list.OrderBy(n => n.Name, new NameComparer()).ToArray();

On my computer when compiled in Release mode this program prints:

Sort: 1162ms
OrderBy: 1269ms


As suggested by @Stefan here are the results of sorting a big list fewer times:

List<Person> persons = new List<Person>();
for (int i = 0; i < 100000; i++)
    persons.Add(new Person("P" + i.ToString(), "Janson" + i.ToString()));


const int COUNT = 30;
Stopwatch watch = Stopwatch.StartNew();
for (int i = 0; i < COUNT; i++)
Console.WriteLine("Sort: {0}ms", watch.ElapsedMilliseconds);

watch = Stopwatch.StartNew();
for (int i = 0; i < COUNT; i++)
Console.WriteLine("OrderBy: {0}ms", watch.ElapsedMilliseconds);


Sort: 8965ms
OrderBy: 8460ms

In this scenario it looks like OrderBy performs better.


And using random names:

List<Person> persons = new List<Person>();
for (int i = 0; i < 100000; i++)
    persons.Add(new Person("P" + i.ToString(), RandomString(5, true)));


private static Random randomSeed = new Random();
public static string RandomString(int size, bool lowerCase)
    var sb = new StringBuilder(size);
    int start = (lowerCase) ? 97 : 65;
    for (int i = 0; i < size; i++)
        sb.Append((char)(26 * randomSeed.NextDouble() + start));
    return sb.ToString();


Sort: 8968ms
OrderBy: 8728ms

Still OrderBy is faster

How to remove all null elements from a ArrayList or String Array?

 for (Iterator<Tourist> itr = tourists.iterator(); itr.hasNext();) {
      if ( == null) { itr.remove(); }

Which are more performant, CTE or temporary tables?

CTE won't take any physical space. It is just a result set we can use join.

Temp tables are temporary. We can create indexes, constrains as like normal tables for that we need to define all variables.

Temp table's scope only within the session. EX: Open two SQL query window

create table #temp(empid int,empname varchar)
insert into #temp 
select 101,'xxx'

select * from #temp

Run this query in first window then run the below query in second window you can find the difference.

select * from #temp

What's the most efficient way to test two integer ranges for overlap?

This can easily warp a normal human brain, so I've found a visual approach to be easier to understand:

overlap madness

le Explanation

If two ranges are "too fat" to fit in a slot that is exactly the sum of the width of both, then they overlap.

For ranges [a1, a2] and [b1, b2] this would be:

 * we are testing for:
 *     max point - min point < w1 + w2    
if max(a2, b2) - min(a1, b1) < (a2 - a1) + (b2 - b1) {
  // too fat -- they overlap!

Export data from Chrome developer tool

I came across the same problem, and found that easier way is to undock the developer tool's video to a separate window! (Using the right hand top corner toolbar button of developer tools window) and in the new window , simply say select all and copy and paste to excel!!

It is more efficient to use if-return-return or if-else-return?

This is a question of style (or preference) since the interpreter does not care. Personally I would try not to make the final statement of a function which returns a value at an indent level other than the function base. The else in example 1 obscures, if only slightly, where the end of the function is.

By preference I use:

return A+1 if (A > B) else A-1

As it obeys both the good convention of having a single return statement as the last statement in the function (as already mentioned) and the good functional programming paradigm of avoiding imperative style intermediate results.

For more complex functions I prefer to break the function into multiple sub-functions to avoid premature returns if possible. Otherwise I revert to using an imperative style variable called rval. I try not to use multiple return statements unless the function is trivial or the return statement before the end is as a result of an error. Returning prematurely highlights the fact that you cannot go on. For complex functions that are designed to branch off into multiple subfunctions I try to code them as case statements (driven by a dict for instance).

Some posters have mentioned speed of operation. Speed of Run-time is secondary for me since if you need speed of execution Python is not the best language to use. I use Python as its the efficiency of coding (i.e. writing error free code) that matters to me.

How to check which locks are held on a table

I use a Dynamic Management View (DMV) to capture locks as well as the object_id or partition_id of the item that is locked.

(MUST switch to the Database you want to observe to get object_id)

     WT.blocking_session_id, AS [object name],
     O.type_desc AS [object descr],
     P.partition_id AS [partition id],
     P.rows AS [partition/page rows],
     AU.type_desc AS [index descr],
     AU.container_id AS [index/page container_id]
FROM sys.dm_tran_locks AS TL
INNER JOIN sys.dm_os_waiting_tasks AS WT 
 ON TL.lock_owner_address = WT.resource_address
LEFT OUTER JOIN sys.objects AS O 
 ON O.object_id = TL.resource_associated_entity_id
LEFT OUTER JOIN sys.partitions AS P 
 ON P.hobt_id = TL.resource_associated_entity_id
LEFT OUTER JOIN sys.allocation_units AS AU 
 ON AU.allocation_unit_id = TL.resource_associated_entity_id;

Is there a performance difference between a for loop and a for-each loop?

It's always better to use the iterator instead of indexing. This is because iterator is most likely optimzied for the List implementation while indexed (calling get) might not be. For example LinkedList is a List but indexing through its elements will be slower than iterating using the iterator.

Equals(=) vs. LIKE

This is a copy/paste of another answer of mine for question SQL 'like' vs '=' performance:

A personal example using mysql 5.5: I had an inner join between 2 tables, one of 3 million rows and one of 10 thousand rows.

When using a like on an index as below(no wildcards), it took about 30 seconds:

where login like '12345678'

using 'explain' I get:

enter image description here

When using an '=' on the same query, it took about 0.1 seconds:

where login ='12345678'

Using 'explain' I get:

enter image description here

As you can see, the like completely cancelled the index seek, so query took 300 times more time.

Should I use Java's String.format() if performance is important?

I took hhafez code and added a memory test:

private static void test() {
    Runtime runtime = Runtime.getRuntime();
    long memory;
    memory = runtime.freeMemory();
    // for loop code
    memory = memory-runtime.freeMemory();

I run this separately for each approach, the '+' operator, String.format and StringBuilder (calling toString()), so the memory used will not be affected by other approaches. I added more concatenations, making the string as "Blah" + i + "Blah"+ i +"Blah" + i + "Blah".

The result are as follow (average of 5 runs each):
Approach       Time(ms)  Memory allocated (long)
'+' operator     747           320,504
String.format  16484       373,312
StringBuilder  769           57,344

We can see that String '+' and StringBuilder are practically identical time-wise, but StringBuilder is much more efficient in memory use. This is very important when we have many log calls (or any other statements involving strings) in a time interval short enough so the Garbage Collector won't get to clean the many string instances resulting of the '+' operator.

And a note, BTW, don't forget to check the logging level before constructing the message.


  1. I'll keep on using StringBuilder.
  2. I have too much time or too little life.

Fastest way to check a string is alphanumeric in Java

I've written the tests that compare using regular expressions (as per other answers) against not using regular expressions. Tests done on a quad core OSX10.8 machine running Java 1.6

Interestingly using regular expressions turns out to be about 5-10 times slower than manually iterating over a string. Furthermore the isAlphanumeric2() function is marginally faster than isAlphanumeric(). One supports the case where extended Unicode numbers are allowed, and the other is for when only standard ASCII numbers are allowed.

public class QuickTest extends TestCase {

    private final int reps = 1000000;

    public void testRegexp() {
        for(int i = 0; i < reps; i++)

public void testIsAlphanumeric() {
    for(int i = 0; i < reps; i++)

public void testIsAlphanumeric2() {
    for(int i = 0; i < reps; i++)

    public boolean isAlphanumeric(String str) {
        for (int i=0; i<str.length(); i++) {
            char c = str.charAt(i);
            if (!Character.isLetterOrDigit(c))
                return false;

        return true;

    public boolean isAlphanumeric2(String str) {
        for (int i=0; i<str.length(); i++) {
            char c = str.charAt(i);
            if (c < 0x30 || (c >= 0x3a && c <= 0x40) || (c > 0x5a && c <= 0x60) || c > 0x7a)
                return false;
        return true;


Best way to reverse a string

     using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
    class Program
        public static string ReverseString(string str)
            int totalLength = str.Length;
            int iCount = 0;
            string strRev = string.Empty;
            iCount = totalLength;
            while (iCount != 0)
                strRev += str[iCount]; 
            return strRev;
        static void Main(string[] args)
            string str = "Punit Pandya";
            string strResult = ReverseString(str);


Comparing two byte arrays in .NET

I settled on a solution inspired by the EqualBytesLongUnrolled method posted by ArekBulski with an additional optimization. In my instance, array differences in arrays tend to be near the tail of the arrays. In testing, I found that when this is the case for large arrays, being able to compare array elements in reverse order gives this solution a huge performance gain over the memcmp based solution. Here is that solution:

public enum CompareDirection { Forward, Backward }

private static unsafe bool UnsafeEquals(byte[] a, byte[] b, CompareDirection direction = CompareDirection.Forward)
    // returns when a and b are same array or both null
    if (a == b) return true;

    // if either is null or different lengths, can't be equal
    if (a == null || b == null || a.Length != b.Length)
        return false;

    const int UNROLLED = 16;                // count of longs 'unrolled' in optimization
    int size = sizeof(long) * UNROLLED;     // 128 bytes (min size for 'unrolled' optimization)
    int len = a.Length;
    int n = len / size;         // count of full 128 byte segments
    int r = len % size;         // count of remaining 'unoptimized' bytes

    // pin the arrays and access them via pointers
    fixed (byte* pb_a = a, pb_b = b)
        if (r > 0 && direction == CompareDirection.Backward)
            byte* pa = pb_a + len - 1;
            byte* pb = pb_b + len - 1;
            byte* phead = pb_a + len - r;
            while(pa >= phead)
                if (*pa != *pb) return false;

        if (n > 0)
            int nOffset = n * size;
            if (direction == CompareDirection.Forward)
                long* pa = (long*)pb_a;
                long* pb = (long*)pb_b;
                long* ptail = (long*)(pb_a + nOffset);
                while (pa < ptail)
                    if (*(pa + 0) != *(pb + 0) || *(pa + 1) != *(pb + 1) ||
                        *(pa + 2) != *(pb + 2) || *(pa + 3) != *(pb + 3) ||
                        *(pa + 4) != *(pb + 4) || *(pa + 5) != *(pb + 5) ||
                        *(pa + 6) != *(pb + 6) || *(pa + 7) != *(pb + 7) ||
                        *(pa + 8) != *(pb + 8) || *(pa + 9) != *(pb + 9) ||
                        *(pa + 10) != *(pb + 10) || *(pa + 11) != *(pb + 11) ||
                        *(pa + 12) != *(pb + 12) || *(pa + 13) != *(pb + 13) ||
                        *(pa + 14) != *(pb + 14) || *(pa + 15) != *(pb + 15)
                        return false;
                    pa += UNROLLED;
                    pb += UNROLLED;
                long* pa = (long*)(pb_a + nOffset);
                long* pb = (long*)(pb_b + nOffset);
                long* phead = (long*)pb_a;
                while (phead < pa)
                    if (*(pa - 1) != *(pb - 1) || *(pa - 2) != *(pb - 2) ||
                        *(pa - 3) != *(pb - 3) || *(pa - 4) != *(pb - 4) ||
                        *(pa - 5) != *(pb - 5) || *(pa - 6) != *(pb - 6) ||
                        *(pa - 7) != *(pb - 7) || *(pa - 8) != *(pb - 8) ||
                        *(pa - 9) != *(pb - 9) || *(pa - 10) != *(pb - 10) ||
                        *(pa - 11) != *(pb - 11) || *(pa - 12) != *(pb - 12) ||
                        *(pa - 13) != *(pb - 13) || *(pa - 14) != *(pb - 14) ||
                        *(pa - 15) != *(pb - 15) || *(pa - 16) != *(pb - 16)
                        return false;
                    pa -= UNROLLED;
                    pb -= UNROLLED;

        if (r > 0 && direction == CompareDirection.Forward)
            byte* pa = pb_a + len - r;
            byte* pb = pb_b + len - r;
            byte* ptail = pb_a + len;
            while(pa < ptail)
                if (*pa != *pb) return false;

    return true;

Why do people say that Ruby is slow?

The way to deal with Ruby's performance in Web application is the same as with any other programming language:


This is easier to do in Rails than in most other Web Frameworks.

At the application level, by caching whatever is supposed to be cached and by managing the access to the DB in an intelligent way (since the bottleneck is usually on the "DB" access for most WEB apps).

Rails makes it very easy and natural to solve these problems. There are several abstractions for caching data, pages and fragments, and there are also very nice abstractions to deal with the SQL part in an optimised and reusable fashion (Active Record and AREL).

This is the reason why so many applications written in faster and not-so-expressive languages (like php) end up being slower than the Ruby counterparts. It's not so easy and elegant to tackle caching and querying with these languages than it is with Ruby.

At the infrastructure level it is reasonable to think of load balancing and all that stuff that I do not happen to know a lot about. I'd outsource that problem by hiring some platform as service provider, like Heroku or Engine Yard. Anyway. Deploying rails with load balancing is probably not very hard to do.

How to write super-fast file-streaming code in C#?

I don't believe there's anything within .NET to allow copying a section of a file without buffering it in memory. However, it strikes me that this is inefficient anyway, as it needs to open the input file and seek many times. If you're just splitting up the file, why not open the input file once, and then just write something like:

public static void CopySection(Stream input, string targetFile, int length)
    byte[] buffer = new byte[8192];

    using (Stream output = File.OpenWrite(targetFile))
        int bytesRead = 1;
        // This will finish silently if we couldn't read "length" bytes.
        // An alternative would be to throw an exception
        while (length > 0 && bytesRead > 0)
            bytesRead = input.Read(buffer, 0, Math.Min(length, buffer.Length));
            output.Write(buffer, 0, bytesRead);
            length -= bytesRead;

This has a minor inefficiency in creating a buffer on each invocation - you might want to create the buffer once and pass that into the method as well:

public static void CopySection(Stream input, string targetFile,
                               int length, byte[] buffer)
    using (Stream output = File.OpenWrite(targetFile))
        int bytesRead = 1;
        // This will finish silently if we couldn't read "length" bytes.
        // An alternative would be to throw an exception
        while (length > 0 && bytesRead > 0)
            bytesRead = input.Read(buffer, 0, Math.Min(length, buffer.Length));
            output.Write(buffer, 0, bytesRead);
            length -= bytesRead;

Note that this also closes the output stream (due to the using statement) which your original code didn't.

The important point is that this will use the operating system file buffering more efficiently, because you reuse the same input stream, instead of reopening the file at the beginning and then seeking.

I think it'll be significantly faster, but obviously you'll need to try it to see...

This assumes contiguous chunks, of course. If you need to skip bits of the file, you can do that from outside the method. Also, if you're writing very small files, you may want to optimise for that situation too - the easiest way to do that would probably be to introduce a BufferedStream wrapping the input stream.

How do you test running time of VBA code?

Unless your functions are very slow, you're going to need a very high-resolution timer. The most accurate one I know is QueryPerformanceCounter. Google it for more info. Try pushing the following into a class, call it CTimer say, then you can make an instance somewhere global and just call .StartCounter and .TimeElapsed

Option Explicit

    lowpart As Long
    highpart As Long
End Type

Private Declare Function QueryPerformanceCounter Lib "kernel32" (lpPerformanceCount As LARGE_INTEGER) As Long
Private Declare Function QueryPerformanceFrequency Lib "kernel32" (lpFrequency As LARGE_INTEGER) As Long

Private m_CounterStart As LARGE_INTEGER
Private m_CounterEnd As LARGE_INTEGER
Private m_crFrequency As Double

Private Const TWO_32 = 4294967296# ' = 256# * 256# * 256# * 256#

Private Function LI2Double(LI As LARGE_INTEGER) As Double
Dim Low As Double
    Low = LI.lowpart
    If Low < 0 Then
        Low = Low + TWO_32
    End If
    LI2Double = LI.highpart * TWO_32 + Low
End Function

Private Sub Class_Initialize()
Dim PerfFrequency As LARGE_INTEGER
    QueryPerformanceFrequency PerfFrequency
    m_crFrequency = LI2Double(PerfFrequency)
End Sub

Public Sub StartCounter()
    QueryPerformanceCounter m_CounterStart
End Sub

Property Get TimeElapsed() As Double
Dim crStart As Double
Dim crStop As Double
    QueryPerformanceCounter m_CounterEnd
    crStart = LI2Double(m_CounterStart)
    crStop = LI2Double(m_CounterEnd)
    TimeElapsed = 1000# * (crStop - crStart) / m_crFrequency
End Property

Android - how do I investigate an ANR?

You need to look for "waiting to lock" in /data/anr/traces.txt file

enter image description here

for more details: Engineer for High Performance with Tools from Android & Play (Google I/O '17)

Is "else if" faster than "switch() case"?

Believing this performance evaluation, the switch case is faster.

This is the conclusion:

The results show that the switch statement is faster to execute than the if-else-if ladder. This is due to the compiler's ability to optimise the switch statement. In the case of the if-else-if ladder, the code must process each if statement in the order determined by the programmer. However, because each case within a switch statement does not rely on earlier cases, the compiler is able to re-order the testing in such a way as to provide the fastest execution.

Get the second largest number in a list in linear time

use defalut sort() method to get second largest number in the list. sort is in built method you do not need to import module for this.

lis = [11,52,63,85,14]

Fastest way to iterate over all the chars in a String

This is just micro-optimisation that you shouldn't worry about.

char[] chars = str.toCharArray();

returns you a copy of str character arrays (in JDK, it returns a copy of characters by calling System.arrayCopy).

Other than that, str.charAt() only checks if the index is indeed in bounds and returns a character within the array index.

The first one doesn't create additional memory in JVM.

Performing a Stress Test on Web Application?

A little late to this party. I agree that Pylot is the best up-and-coming open source tool out there. It's simple to use and is actively worked on by a great guy (Corey Goldberg). As the founder of OpenQA, I'm also happy that Pylot now is listed on our home page and uses some of our infrastructure (namely the forums).

However, I also recently decided that the entire concept of load testing was flawed: emulating HTTP traffic, with applications as complex as they have become, is a pain in the butt. That's why I created the commercial tool BrowserMob. It's an external load testing service that uses Selenium to control real web browsers when playing back load.

The approach obviously requires a ton more hardware than normal load testing techniques, but hardware is actually pretty cheap when you are using cloud computing. And a nice side effect of this is that the scripting is much easier than normal load testing. You don't have to do any advanced regex matching (like JMeter requires) to extract out cookies, .NET session state, Ajax request parameters, etc. Since you're using real browsers, they just do what they are supposed to do.

Sorry to blatantly pitch a commercial product, but hopefully the concept is interesting to some folks and at least gets them thinking about some new ways to deal with load testing when you have access to a bunch of extra hardware!

SQL, Postgres OIDs, What are they and why are they useful?

To remove all OIDs from your database tables, you can use this Linux script:

First, login as PostgreSQL superuser:

sudo su postgres

Now run this script, changing YOUR_DATABASE_NAME with you database name:

for tbl in `psql -qAt -c "select schemaname || '.' || tablename from pg_tables WHERE schemaname <> 'pg_catalog' AND schemaname <> 'information_schema';" YOUR_DATABASE_NAME` ; do  psql -c "alter table $tbl SET WITHOUT OIDS" YOUR_DATABASE_NAME ; done

I used this script to remove all my OIDs, since Npgsql 3.0 doesn't work with this, and it isn't important to PostgreSQL anymore.

Ruby Array find_first object?

Do you need the object itself or do you just need to know if there is an object that satisfies. If the former then yes: use find:

found_object = my_array.find { |e| e.satisfies_condition? }

otherwise you can use any?

found_it = my_array.any?  { |e| e.satisfies_condition? }

The latter will bail with "true" when it finds one that satisfies the condition. The former will do the same, but return the object.

How do I improve ASP.NET MVC application performance?

Following are things to do

  1. Kernel mode Cache
  2. Pipeline mode
  3. Remove unused modules
  4. runAllManagedModulesForAllRequests
  5. Don't write in wwwroot
  6. Remove unused view engines and language

How can I increase the cursor speed in terminal?

If by "cursor speed", you mean the repeat rate when holding down a key - then have a look here:

To summarize, open up a Terminal window and type the following command:

defaults write NSGlobalDomain KeyRepeat -int 0

More detail from the article:

Everybody knows that you can get a pretty fast keyboard repeat rate by changing a slider on the Keyboard tab of the Keyboard & Mouse System Preferences panel. But you can make it even faster! In Terminal, run this command:

defaults write NSGlobalDomain KeyRepeat -int 0

Then log out and log in again. The fastest setting obtainable via System Preferences is 2 (lower numbers are faster), so you may also want to try a value of 1 if 0 seems too fast. You can always visit the Keyboard & Mouse System Preferences panel to undo your changes.

You may find that a few applications don't handle extremely fast keyboard input very well, but most will do just fine with it.

Why are elementwise additions much faster in separate loops than in a combined loop?

The Original Question

Why is one loop so much slower than two loops?


Case 1 is a classic interpolation problem that happens to be an inefficient one. I also think that this was one of the leading reasons why many machine architectures and developers ended up building and designing multi-core systems with the ability to do multi-threaded applications as well as parallel programming.

Looking at it from this kind of an approach without involving how the hardware, OS, and compiler(s) work together to do heap allocations that involve working with RAM, cache, page files, etc.; the mathematics that is at the foundation of these algorithms shows us which of these two is the better solution.

We can use an analogy of a Boss being a Summation that will represent a For Loop that has to travel between workers A & B.

We can easily see that Case 2 is at least half as fast if not a little more than Case 1 due to the difference in the distance that is needed to travel and the time taken between the workers. This math lines up almost virtually and perfectly with both the benchmark times as well as the number of differences in assembly instructions.

I will now begin to explain how all of this works below.

Assessing The Problem

The OP's code:

const int n=100000;

for(int j=0;j<n;j++){
    a1[j] += b1[j];
    c1[j] += d1[j];


for(int j=0;j<n;j++){
    a1[j] += b1[j];
for(int j=0;j<n;j++){
    c1[j] += d1[j];

The Consideration

Considering the OP's original question about the two variants of the for loops and his amended question towards the behavior of caches along with many of the other excellent answers and useful comments; I'd like to try and do something different here by taking a different approach about this situation and problem.

The Approach

Considering the two loops and all of the discussion about cache and page filing I'd like to take another approach as to looking at this from a different perspective. One that doesn't involve the cache and page files nor the executions to allocate memory, in fact, this approach doesn't even concern the actual hardware or the software at all.

The Perspective

After looking at the code for a while it became quite apparent what the problem is and what is generating it. Let's break this down into an algorithmic problem and look at it from the perspective of using mathematical notations then apply an analogy to the math problems as well as to the algorithms.

What We Do Know

We know is that this loop will run 100,000 times. We also know that a1, b1, c1 & d1 are pointers on a 64-bit architecture. Within C++ on a 32-bit machine, all pointers are 4 bytes and on a 64-bit machine, they are 8 bytes in size since pointers are of a fixed length.

We know that we have 32 bytes in which to allocate for in both cases. The only difference is we are allocating 32 bytes or two sets of 2-8 bytes on each iteration wherein the second case we are allocating 16 bytes for each iteration for both of the independent loops.

Both loops still equal 32 bytes in total allocations. With this information let's now go ahead and show the general math, algorithms, and analogy of these concepts.

We do know the number of times that the same set or group of operations that will have to be performed in both cases. We do know the amount of memory that needs to be allocated in both cases. We can assess that the overall workload of the allocations between both cases will be approximately the same.

What We Don't Know

We do not know how long it will take for each case unless if we set a counter and run a benchmark test. However, the benchmarks were already included from the original question and from some of the answers and comments as well; and we can see a significant difference between the two and this is the whole reasoning for this proposal to this problem.

Let's Investigate

It is already apparent that many have already done this by looking at the heap allocations, benchmark tests, looking at RAM, cache, and page files. Looking at specific data points and specific iteration indices were also included and the various conversations about this specific problem have many people starting to question other related things about it. How do we begin to look at this problem by using mathematical algorithms and applying an analogy to it? We start off by making a couple of assertions! Then we build out our algorithm from there.

Our Assertions:

  • We will let our loop and its iterations be a Summation that starts at 1 and ends at 100000 instead of starting with 0 as in the loops for we don't need to worry about the 0 indexing scheme of memory addressing since we are just interested in the algorithm itself.
  • In both cases we have four functions to work with and two function calls with two operations being done on each function call. We will set these up as functions and calls to functions as the following: F1(), F2(), f(a), f(b), f(c) and f(d).

The Algorithms:

1st Case: - Only one summation but two independent function calls.

Sum n=1 : [1,100000] = F1(), F2();
                       F1() = { f(a) = f(a) + f(b); }
                       F2() = { f(c) = f(c) + f(d); }

2nd Case: - Two summations but each has its own function call.

Sum1 n=1 : [1,100000] = F1();
                        F1() = { f(a) = f(a) + f(b); }

Sum2 n=1 : [1,100000] = F1();
                        F1() = { f(c) = f(c) + f(d); }

If you noticed F2() only exists in Sum from Case1 where F1() is contained in Sum from Case1 and in both Sum1 and Sum2 from Case2. This will be evident later on when we begin to conclude that there is an optimization that is happening within the second algorithm.

The iterations through the first case Sum calls f(a) that will add to its self f(b) then it calls f(c) that will do the same but add f(d) to itself for each 100000 iterations. In the second case, we have Sum1 and Sum2 that both act the same as if they were the same function being called twice in a row.

In this case we can treat Sum1 and Sum2 as just plain old Sum where Sum in this case looks like this: Sum n=1 : [1,100000] { f(a) = f(a) + f(b); } and now this looks like an optimization where we can just consider it to be the same function.

Summary with Analogy

With what we have seen in the second case it almost appears as if there is optimization since both for loops have the same exact signature, but this isn't the real issue. The issue isn't the work that is being done by f(a), f(b), f(c), and f(d). In both cases and the comparison between the two, it is the difference in the distance that the Summation has to travel in each case that gives you the difference in execution time.

Think of the for loops as being the summations that does the iterations as being a Boss that is giving orders to two people A & B and that their jobs are to meat C & D respectively and to pick up some package from them and return it. In this analogy, the for loops or summation iterations and condition checks themselves don't actually represent the Boss. What actually represents the Boss is not from the actual mathematical algorithms directly but from the actual concept of Scope and Code Block within a routine or subroutine, method, function, translation unit, etc. The first algorithm has one scope where the second algorithm has two consecutive scopes.

Within the first case on each call slip, the Boss goes to A and gives the order and A goes off to fetch B's package then the Boss goes to C and gives the orders to do the same and receive the package from D on each iteration.

Within the second case, the Boss works directly with A to go and fetch B's package until all packages are received. Then the Boss works with C to do the same for getting all of D's packages.

Since we are working with an 8-byte pointer and dealing with heap allocation let's consider the following problem. Let's say that the Boss is 100 feet from A and that A is 500 feet from C. We don't need to worry about how far the Boss is initially from C because of the order of executions. In both cases, the Boss initially travels from A first then to B. This analogy isn't to say that this distance is exact; it is just a useful test case scenario to show the workings of the algorithms.

In many cases when doing heap allocations and working with the cache and page files, these distances between address locations may not vary that much or they can vary significantly depending on the nature of the data types and the array sizes.

The Test Cases:

First Case: On first iteration the Boss has to initially go 100 feet to give the order slip to A and A goes off and does his thing, but then the Boss has to travel 500 feet to C to give him his order slip. Then on the next iteration and every other iteration after the Boss has to go back and forth 500 feet between the two.

Second Case: The Boss has to travel 100 feet on the first iteration to A, but after that, he is already there and just waits for A to get back until all slips are filled. Then the Boss has to travel 500 feet on the first iteration to C because C is 500 feet from A. Since this Boss( Summation, For Loop ) is being called right after working with A he then just waits there as he did with A until all of C's order slips are done.

The Difference In Distances Traveled

const n = 100000
distTraveledOfFirst = (100 + 500) + ((n-1)*(500 + 500);
// Simplify
distTraveledOfFirst = 600 + (99999*100);
distTraveledOfFirst = 600 + 9999900;
distTraveledOfFirst =  10000500;
// Distance Traveled On First Algorithm = 10,000,500ft

distTraveledOfSecond = 100 + 500 = 600;
// Distance Traveled On Second Algorithm = 600ft;

The Comparison of Arbitrary Values

We can easily see that 600 is far less than 10 million. Now, this isn't exact, because we don't know the actual difference in distance between which address of RAM or from which cache or page file each call on each iteration is going to be due to many other unseen variables. This is just an assessment of the situation to be aware of and looking at it from the worst-case scenario.

From these numbers it would almost appear as if algorithm one should be 99% slower than algorithm two; however, this is only the Boss's part or responsibility of the algorithms and it doesn't account for the actual workers A, B, C, & D and what they have to do on each and every iteration of the Loop. So the boss's job only accounts for about 15 - 40% of the total work being done. The bulk of the work that is done through the workers has a slightly bigger impact towards keeping the ratio of the speed rate differences to about 50-70%

The Observation: - The differences between the two algorithms

In this situation, it is the structure of the process of the work being done. It goes to show that Case 2 is more efficient from both the partial optimization of having a similar function declaration and definition where it is only the variables that differ by name and the distance traveled.

We also see that the total distance traveled in Case 1 is much farther than it is in Case 2 and we can consider this distance traveled our Time Factor between the two algorithms. Case 1 has considerable more work to do than Case 2 does.

This is observable from the evidence of the assembly instructions that were shown in both cases. Along with what was already stated about these cases, this doesn't account for the fact that in Case 1 the boss will have to wait for both A & C to get back before he can go back to A again for each iteration. It also doesn't account for the fact that if A or B is taking an extremely long time then both the Boss and the other worker(s) are idle waiting to be executed.

In Case 2 the only one being idle is the Boss until the worker gets back. So even this has an impact on the algorithm.

The OP's Amended Question(s)

EDIT: The question turned out to be of no relevance, as the behavior severely depends on the sizes of the arrays (n) and the CPU cache. So if there is further interest, I rephrase the question:

Could you provide some solid insight into the details that lead to the different cache behaviors as illustrated by the five regions on the following graph?

It might also be interesting to point out the differences between CPU/cache architectures, by providing a similar graph for these CPUs.

Regarding These Questions

As I have demonstrated without a doubt, there is an underlying issue even before the Hardware and Software becomes involved.

Now as for the management of memory and caching along with page files, etc. which all work together in an integrated set of systems between the following:

  • The architecture (hardware, firmware, some embedded drivers, kernels and assembly instruction sets).
  • The OS (file and memory management systems, drivers and the registry).
  • The compiler (translation units and optimizations of the source code).
  • And even the source code itself with its set(s) of distinctive algorithms.

We can already see that there is a bottleneck that is happening within the first algorithm before we even apply it to any machine with any arbitrary architecture, OS, and programmable language compared to the second algorithm. There already existed a problem before involving the intrinsics of a modern computer.

The Ending Results

However; it is not to say that these new questions are not of importance because they themselves are and they do play a role after all. They do impact the procedures and the overall performance and that is evident with the various graphs and assessments from many who have given their answer(s) and or comment(s).

If you paid attention to the analogy of the Boss and the two workers A & B who had to go and retrieve packages from C & D respectively and considering the mathematical notations of the two algorithms in question; you can see without the involvement of the computer hardware and software Case 2 is approximately 60% faster than Case 1.

When you look at the graphs and charts after these algorithms have been applied to some source code, compiled, optimized, and executed through the OS to perform their operations on a given piece of hardware, you can even see a little more degradation between the differences in these algorithms.

If the Data set is fairly small it may not seem all that bad of a difference at first. However, since Case 1 is about 60 - 70% slower than Case 2 we can look at the growth of this function in terms of the differences in time executions:

DeltaTimeDifference approximately = Loop1(time) - Loop2(time)
Loop1(time) = Loop2(time) + (Loop2(time)*[0.6,0.7]) // approximately
// So when we substitute this back into the difference equation we end up with
DeltaTimeDifference approximately = (Loop2(time) + (Loop2(time)*[0.6,0.7])) - Loop2(time)
// And finally we can simplify this to
DeltaTimeDifference approximately = [0.6,0.7]*Loop2(time)

This approximation is the average difference between these two loops both algorithmically and machine operations involving software optimizations and machine instructions.

When the data set grows linearly, so does the difference in time between the two. Algorithm 1 has more fetches than algorithm 2 which is evident when the Boss has to travel back and forth the maximum distance between A & C for every iteration after the first iteration while algorithm 2 the Boss has to travel to A once and then after being done with A he has to travel a maximum distance only one time when going from A to C.

Trying to have the Boss focusing on doing two similar things at once and juggling them back and forth instead of focusing on similar consecutive tasks is going to make him quite angry by the end of the day since he had to travel and work twice as much. Therefore do not lose the scope of the situation by letting your boss getting into an interpolated bottleneck because the boss's spouse and children wouldn't appreciate it.

Amendment: Software Engineering Design Principles

-- The difference between local Stack and heap allocated computations within iterative for loops and the difference between their usages, their efficiencies, and effectiveness --

The mathematical algorithm that I proposed above mainly applies to loops that perform operations on data that is allocated on the heap.

  • Consecutive Stack Operations:
    • If the loops are performing operations on data locally within a single code block or scope that is within the stack frame it will still sort of apply, but the memory locations are much closer where they are typically sequential and the difference in distance traveled or execution time is almost negligible. Since there are no allocations being done within the heap, the memory isn't scattered, and the memory isn't being fetched through ram. The memory is typically sequential and relative to the stack frame and stack pointer.
  • When consecutive operations are being done on the stack, a modern processor will cache repetitive values and addresses keeping these values within local cache registers. The time of operations or instructions here is on the order of nano-seconds.
  • Consecutive Heap Allocated Operations:
    • When you begin to apply heap allocations and the processor has to fetch the memory addresses on consecutive calls, depending on the architecture of the CPU, the bus controller, and the RAM modules the time of operations or execution can be on the order of micro to milliseconds. In comparison to cached stack operations, these are quite slow.
    • The CPU will have to fetch the memory address from RAM and typically anything across the system bus is slow compared to the internal data paths or data buses within the CPU itself.

So when you are working with data that needs to be on the heap and you are traversing through them in loops, it is more efficient to keep each data set and its corresponding algorithms within its own single loop. You will get better optimizations compared to trying to factor out consecutive loops by putting multiple operations of different data sets that are on the heap into a single loop.

It is okay to do this with data that is on the stack since they are frequently cached, but not for data that has to have its memory address queried every iteration.

This is where software engineering and software architecture design comes into play. It is the ability to know how to organize your data, knowing when to cache your data, knowing when to allocate your data on the heap, knowing how to design and implement your algorithms, and knowing when and where to call them.

You might have the same algorithm that pertains to the same data set, but you might want one implementation design for its stack variant and another for its heap-allocated variant just because of the above issue that is seen from its O(n) complexity of the algorithm when working with the heap.

From what I've noticed over the years, many people do not take this fact into consideration. They will tend to design one algorithm that works on a particular data set and they will use it regardless of the data set being locally cached on the stack or if it was allocated on the heap.

If you want true optimization, yes it might seem like code duplication, but to generalize it would be more efficient to have two variants of the same algorithm. One for stack operations, and the other for heap operations that are performed in iterative loops!

Here's a pseudo example: Two simple structs, one algorithm.

struct A {
    int data;
    A() : data{0}{}
    A(int a) : data{a}{}
struct B {
    int data;
    B() : data{0}{}
    A(int b) : data{b}{}

template<typename T>
void Foo( T& t ) {
    // Do something with t

// Some looping operation: first stack then heap.

// Stack data:
A dataSetA[10] = {};
B dataSetB[10] = {};

// For stack operations this is okay and efficient
for (int i = 0; i < 10; i++ ) {

// If the above two were on the heap then performing
// the same algorithm to both within the same loop
// will create that bottleneck
A* dataSetA = new [] A();
B* dataSetB = new [] B();
for ( int i = 0; i < 10; i++ ) {
    Foo(dataSetA[i]); // dataSetA is on the heap here
    Foo(dataSetB[i]); // dataSetB is on the heap here
} // this will be inefficient.

// To improve the efficiency above, put them into separate loops...

for (int i = 0; i < 10; i++ ) {
for (int i = 0; i < 10; i++ ) {
// This will be much more efficient than above.
// The code isn't perfect syntax, it's only psuedo code
// to illustrate a point.

This is what I was referring to by having separate implementations for stack variants versus heap variants. The algorithms themselves don't matter too much, it's the looping structures that you will use them in that do.

Convert a hexadecimal string to an integer efficiently in C?

#include "math.h"
#include "stdio.h"
//  The bits arg represents the bit say:8,16,32...                                                                                                              
volatile long Hex_To_Int(long Hex,char bits)
    long Hex_2_Int;
    char byte;


    return Hex_2_Int;

void main (void)
    int Dec;   
    char Hex=0xFA;
    Dec= Hex_To_Int(Hex,8);  //convert an 8-bis hexadecimal value to a number in base 10
    printf("the number is %d",Dec);

Improve INSERT-per-second performance of SQLite

I coudn't get any gain from transactions until I raised cache_size to a higher value i.e. PRAGMA cache_size=10000;

What is the runtime performance cost of a Docker container?

Docker isn't virtualization, as such -- instead, it's an abstraction on top of the kernel's support for different process namespaces, device namespaces, etc.; one namespace isn't inherently more expensive or inefficient than another, so what actually makes Docker have a performance impact is a matter of what's actually in those namespaces.

Docker's choices in terms of how it configures namespaces for its containers have costs, but those costs are all directly associated with benefits -- you can give them up, but in doing so you also give up the associated benefit:

  • Layered filesystems are expensive -- exactly what the costs are vary with each one (and Docker supports multiple backends), and with your usage patterns (merging multiple large directories, or merging a very deep set of filesystems will be particularly expensive), but they're not free. On the other hand, a great deal of Docker's functionality -- being able to build guests off other guests in a copy-on-write manner, and getting the storage advantages implicit in same -- ride on paying this cost.
  • DNAT gets expensive at scale -- but gives you the benefit of being able to configure your guest's networking independently of your host's and have a convenient interface for forwarding only the ports you want between them. You can replace this with a bridge to a physical interface, but again, lose the benefit.
  • Being able to run each software stack with its dependencies installed in the most convenient manner -- independent of the host's distro, libc, and other library versions -- is a great benefit, but needing to load shared libraries more than once (when their versions differ) has the cost you'd expect.

And so forth. How much these costs actually impact you in your environment -- with your network access patterns, your memory constraints, etc -- is an item for which it's difficult to provide a generic answer.

What's the fastest way to read a text file line-by-line?

While File.ReadAllLines() is one of the simplest ways to read a file, it is also one of the slowest.

If you're just wanting to read lines in a file without doing much, according to these benchmarks, the fastest way to read a file is the age old method of:

using (StreamReader sr = File.OpenText(fileName))
        string s = String.Empty;
        while ((s = sr.ReadLine()) != null)
               //do minimal amount of work here

However, if you have to do a lot with each line, then this article concludes that the best way is the following (and it's faster to pre-allocate a string[] if you know how many lines you're going to read) :

AllLines = new string[MAX]; //only allocate memory here

using (StreamReader sr = File.OpenText(fileName))
        int x = 0;
        while (!sr.EndOfStream)
               AllLines[x] = sr.ReadLine();
               x += 1;
} //Finished. Close the file

//Now parallel process each line in the file
Parallel.For(0, AllLines.Length, x =>
    DoYourStuff(AllLines[x]); //do your work here

Find out how much memory is being used by an object in Python

This must be used with care because an override on the objects __sizeof__ might be misleading.

Using the bregman.suite, some tests with sys.getsizeof output a copy of an array object (data) in an object instance as being bigger than the object itself (mfcc).

>>> mfcc = MelFrequencyCepstrum(filepath, params)
>>> data = mfcc.X[:]
>>> sys.getsizeof(mfcc)
>>> sys.getsizeof(mfcc.X)
>>> sys.getsizeof(data)
>>> mfcc
<bregman.features.MelFrequencyCepstrum object at 0x104ad3e90>

Why is printing "B" dramatically slower than printing "#"?

Yes the culprit is definitely word-wrapping. When I tested your two programs, NetBeans IDE 8.2 gave me the following result.

  1. First Matrix: O and # = 6.03 seconds
  2. Second Matrix: O and B = 50.97 seconds

Looking at your code closely you have used a line break at the end of first loop. But you didn't use any line break in second loop. So you are going to print a word with 1000 characters in the second loop. That causes a word-wrapping problem. If we use a non-word character " " after B, it takes only 5.35 seconds to compile the program. And If we use a line break in the second loop after passing 100 values or 50 values, it takes only 8.56 seconds and 7.05 seconds respectively.

Random r = new Random();
for (int i = 0; i < 1000; i++) {
    for (int j = 0; j < 1000; j++) {
        if(r.nextInt(4) == 0) {
        } else {
        if(j%100==0){               //Adding a line break in second loop      

Another advice is that to change settings of NetBeans IDE. First of all, go to NetBeans Tools and click Options. After that click Editor and go to Formatting tab. Then select Anywhere in Line Wrap Option. It will take almost 6.24% less time to compile the program.

NetBeans Editor Settings

C++ performance vs. Java/C#

I don't know Java programs are always slow. :-) I've never really noticed C# programs being particularly slow, though.

Why is "1000000000000000 in range(1000000000000001)" so fast in Python 3?

Use the source, Luke!

In CPython, range(...).__contains__ (a method wrapper) will eventually delegate to a simple calculation which checks if the value can possibly be in the range. The reason for the speed here is we're using mathematical reasoning about the bounds, rather than a direct iteration of the range object. To explain the logic used:

  1. Check that the number is between start and stop, and
  2. Check that the stride value doesn't "step over" our number.

For example, 994 is in range(4, 1000, 2) because:

  1. 4 <= 994 < 1000, and
  2. (994 - 4) % 2 == 0.

The full C code is included below, which is a bit more verbose because of memory management and reference counting details, but the basic idea is there:

static int
range_contains_long(rangeobject *r, PyObject *ob)
    int cmp1, cmp2, cmp3;
    PyObject *tmp1 = NULL;
    PyObject *tmp2 = NULL;
    PyObject *zero = NULL;
    int result = -1;

    zero = PyLong_FromLong(0);
    if (zero == NULL) /* MemoryError in int(0) */
        goto end;

    /* Check if the value can possibly be in the range. */

    cmp1 = PyObject_RichCompareBool(r->step, zero, Py_GT);
    if (cmp1 == -1)
        goto end;
    if (cmp1 == 1) { /* positive steps: start <= ob < stop */
        cmp2 = PyObject_RichCompareBool(r->start, ob, Py_LE);
        cmp3 = PyObject_RichCompareBool(ob, r->stop, Py_LT);
    else { /* negative steps: stop < ob <= start */
        cmp2 = PyObject_RichCompareBool(ob, r->start, Py_LE);
        cmp3 = PyObject_RichCompareBool(r->stop, ob, Py_LT);

    if (cmp2 == -1 || cmp3 == -1) /* TypeError */
        goto end;
    if (cmp2 == 0 || cmp3 == 0) { /* ob outside of range */
        result = 0;
        goto end;

    /* Check that the stride does not invalidate ob's membership. */
    tmp1 = PyNumber_Subtract(ob, r->start);
    if (tmp1 == NULL)
        goto end;
    tmp2 = PyNumber_Remainder(tmp1, r->step);
    if (tmp2 == NULL)
        goto end;
    /* result = ((int(ob) - start) % step) == 0 */
    result = PyObject_RichCompareBool(tmp2, zero, Py_EQ);
    return result;

static int
range_contains(rangeobject *r, PyObject *ob)
    if (PyLong_CheckExact(ob) || PyBool_Check(ob))
        return range_contains_long(r, ob);

    return (int)_PySequence_IterSearch((PyObject*)r, ob,

The "meat" of the idea is mentioned in the line:

/* result = ((int(ob) - start) % step) == 0 */ 

As a final note - look at the range_contains function at the bottom of the code snippet. If the exact type check fails then we don't use the clever algorithm described, instead falling back to a dumb iteration search of the range using _PySequence_IterSearch! You can check this behaviour in the interpreter (I'm using v3.5.0 here):

>>> x, r = 1000000000000000, range(1000000000000001)
>>> class MyInt(int):
...     pass
>>> x_ = MyInt(x)
>>> x in r  # calculates immediately :) 
>>> x_ in r  # iterates for ages.. :( 
^\Quit (core dumped)

Fast query runs slow in SSRS

In my case, I just had to disconnect and connect the SSMS. I profiled the query and the duration of execution was showing 1 minute even though the query itself runs under 2 seconds. Restarted the connection and ran again, this time the duration showed the correct execution time.

Fastest method to escape HTML tags as HTML entities?

I'll add XMLSerializer to the pile. It provides the fastest result without using any object caching (not on the serializer, nor on the Text node).

function serializeTextNode(text) {
  return new XMLSerializer().serializeToString(document.createTextNode(text));

The added bonus is that it supports attributes which is serialized differently than text nodes:

function serializeAttributeValue(value) {
  const attr = document.createAttribute('a');
  attr.value = value;
  return new XMLSerializer().serializeToString(attr);

You can see what it's actually replacing by checking the spec, both for text nodes and for attribute values. The full documentation has more node types, but the concept is the same.

As for performance, it's the fastest when not cached. When you do allow caching, then calling innerHTML on an HTMLElement with a child Text node is fastest. Regex would be slowest (as proven by other comments). Of course, XMLSerializer could be faster on other browsers, but in my (limited) testing, a innerHTML is fastest.

Fastest single line:

new XMLSerializer().serializeToString(document.createTextNode(text));

Fastest with caching:

const cachedElementParent = document.createElement('div');
const cachedChildTextNode = document.createTextNode('');

function serializeTextNode(text) {
  cachedChildTextNode.nodeValue = text;
  return cachedElementParent.innerHTML;

Measure the time it takes to execute a t-sql query

Another way is using a SQL Server built-in feature named Client Statistics which is accessible through Menu > Query > Include Client Statistics.

You can run each query in separated query window and compare the results which is given in Client Statistics tab just beside the Messages tab.

For example in image below it shows that the average time elapsed to get the server reply for one of my queries is 39 milliseconds.


You can read all 3 ways for acquiring execution time in here. You may even need to display Estimated Execution Plan ctrlL for further investigation about your query.

Using varchar(MAX) vs TEXT on SQL Server

You can't search a text field without converting it from text to varchar.

declare @table table (a text)
insert into @table values ('a')
insert into @table values ('a')
insert into @table values ('b')
insert into @table values ('c')
insert into @table values ('d')

select *
from @table
where a ='a'

This give an error:

The data types text and varchar are incompatible in the equal to operator.

Wheras this does not:

declare @table table (a varchar(max))

Interestingly, LIKE still works, i.e.

where a like '%a%'

What is a "cache-friendly" code?


On modern computers, only the lowest level memory structures (the registers) can move data around in single clock cycles. However, registers are very expensive and most computer cores have less than a few dozen registers. At the other end of the memory spectrum (DRAM), the memory is very cheap (i.e. literally millions of times cheaper) but takes hundreds of cycles after a request to receive the data. To bridge this gap between super fast and expensive and super slow and cheap are the cache memories, named L1, L2, L3 in decreasing speed and cost. The idea is that most of the executing code will be hitting a small set of variables often, and the rest (a much larger set of variables) infrequently. If the processor can't find the data in L1 cache, then it looks in L2 cache. If not there, then L3 cache, and if not there, main memory. Each of these "misses" is expensive in time.

(The analogy is cache memory is to system memory, as system memory is too hard disk storage. Hard disk storage is super cheap but very slow).

Caching is one of the main methods to reduce the impact of latency. To paraphrase Herb Sutter (cfr. links below): increasing bandwidth is easy, but we can't buy our way out of latency.

Data is always retrieved through the memory hierarchy (smallest == fastest to slowest). A cache hit/miss usually refers to a hit/miss in the highest level of cache in the CPU -- by highest level I mean the largest == slowest. The cache hit rate is crucial for performance since every cache miss results in fetching data from RAM (or worse ...) which takes a lot of time (hundreds of cycles for RAM, tens of millions of cycles for HDD). In comparison, reading data from the (highest level) cache typically takes only a handful of cycles.

In modern computer architectures, the performance bottleneck is leaving the CPU die (e.g. accessing RAM or higher). This will only get worse over time. The increase in processor frequency is currently no longer relevant to increase performance. The problem is memory access. Hardware design efforts in CPUs therefore currently focus heavily on optimizing caches, prefetching, pipelines and concurrency. For instance, modern CPUs spend around 85% of die on caches and up to 99% for storing/moving data!

There is quite a lot to be said on the subject. Here are a few great references about caches, memory hierarchies and proper programming:

Main concepts for cache-friendly code

A very important aspect of cache-friendly code is all about the principle of locality, the goal of which is to place related data close in memory to allow efficient caching. In terms of the CPU cache, it's important to be aware of cache lines to understand how this works: How do cache lines work?

The following particular aspects are of high importance to optimize caching:

  1. Temporal locality: when a given memory location was accessed, it is likely that the same location is accessed again in the near future. Ideally, this information will still be cached at that point.
  2. Spatial locality: this refers to placing related data close to each other. Caching happens on many levels, not just in the CPU. For example, when you read from RAM, typically a larger chunk of memory is fetched than what was specifically asked for because very often the program will require that data soon. HDD caches follow the same line of thought. Specifically for CPU caches, the notion of cache lines is important.

Use appropriate containers

A simple example of cache-friendly versus cache-unfriendly is 's std::vector versus std::list. Elements of a std::vector are stored in contiguous memory, and as such accessing them is much more cache-friendly than accessing elements in a std::list, which stores its content all over the place. This is due to spatial locality.

A very nice illustration of this is given by Bjarne Stroustrup in this youtube clip (thanks to @Mohammad Ali Baydoun for the link!).

Don't neglect the cache in data structure and algorithm design

Whenever possible, try to adapt your data structures and order of computations in a way that allows maximum use of the cache. A common technique in this regard is cache blocking ( version), which is of extreme importance in high-performance computing (cfr. for example ATLAS).

Know and exploit the implicit structure of data

Another simple example, which many people in the field sometimes forget is column-major (ex. ,) vs. row-major ordering (ex. ,) for storing two dimensional arrays. For example, consider the following matrix:

1 2
3 4

In row-major ordering, this is stored in memory as 1 2 3 4; in column-major ordering, this would be stored as 1 3 2 4. It is easy to see that implementations which do not exploit this ordering will quickly run into (easily avoidable!) cache issues. Unfortunately, I see stuff like this very often in my domain (machine learning). @MatteoItalia showed this example in more detail in his answer.

When fetching a certain element of a matrix from memory, elements near it will be fetched as well and stored in a cache line. If the ordering is exploited, this will result in fewer memory accesses (because the next few values which are needed for subsequent computations are already in a cache line).

For simplicity, assume the cache comprises a single cache line which can contain 2 matrix elements and that when a given element is fetched from memory, the next one is too. Say we want to take the sum over all elements in the example 2x2 matrix above (lets call it M):

Exploiting the ordering (e.g. changing column index first in ):

M[0][0] (memory) + M[0][1] (cached) + M[1][0] (memory) + M[1][1] (cached)
= 1 + 2 + 3 + 4
--> 2 cache hits, 2 memory accesses

Not exploiting the ordering (e.g. changing row index first in ):

M[0][0] (memory) + M[1][0] (memory) + M[0][1] (memory) + M[1][1] (memory)
= 1 + 3 + 2 + 4
--> 0 cache hits, 4 memory accesses

In this simple example, exploiting the ordering approximately doubles execution speed (since memory access requires much more cycles than computing the sums). In practice, the performance difference can be much larger.

Avoid unpredictable branches

Modern architectures feature pipelines and compilers are becoming very good at reordering code to minimize delays due to memory access. When your critical code contains (unpredictable) branches, it is hard or impossible to prefetch data. This will indirectly lead to more cache misses.

This is explained very well here (thanks to @0x90 for the link): Why is processing a sorted array faster than processing an unsorted array?

Avoid virtual functions

In the context of , virtual methods represent a controversial issue with regard to cache misses (a general consensus exists that they should be avoided when possible in terms of performance). Virtual functions can induce cache misses during look up, but this only happens if the specific function is not called often (otherwise it would likely be cached), so this is regarded as a non-issue by some. For reference about this issue, check out: What is the performance cost of having a virtual method in a C++ class?

Common problems

A common problem in modern architectures with multiprocessor caches is called false sharing. This occurs when each individual processor is attempting to use data in another memory region and attempts to store it in the same cache line. This causes the cache line -- which contains data another processor can use -- to be overwritten again and again. Effectively, different threads make each other wait by inducing cache misses in this situation. See also (thanks to @Matt for the link): How and when to align to cache line size?

An extreme symptom of poor caching in RAM memory (which is probably not what you mean in this context) is so-called thrashing. This occurs when the process continuously generates page faults (e.g. accesses memory which is not in the current page) which require disk access.

Prepend text to beginning of string

If you want to use the version of Javascript called ES 2015 (aka ES6) or later, you can use template strings introduced by ES 2015 and recommended by some guidelines (like Airbnb's style guide):

const after = "test";
const mystr = `This is: ${after}`;

Replace values in list using Python

Riffing on a side question asked by the OP in a comment, i.e.:

what if I had a generator that yields the values from range(11) instead of a list. Would it be possible to replace values in the generator?

Sure, it's trivially easy...:

def replaceiniter(it, predicate, replacement=None):
  for item in it:
    if predicate(item): yield replacement
    else: yield item

Just pass any iterable (including the result of calling a generator) as the first arg, the predicate to decide if a value must be replaced as the second arg, and let 'er rip.

For example:

>>> list(replaceiniter(xrange(11), lambda x: x%2))
[0, None, 2, None, 4, None, 6, None, 8, None, 10]

Should try...catch go inside or outside a loop?

I'll put my $0.02 in. Sometimes you wind up needing to add a "finally" later on in your code (because who ever writes their code perfectly the first time?). In those cases, suddenly it makes more sense to have the try/catch outside the loop. For example:

try {
    for(int i = 0; i < max; i++) {
        String myString = ...;
        float myNum = Float.parseFloat(myString);
} catch (NumberFormatException ex) {
    return null;
} finally {
    dbConnection.release();  // Always release DB connection, even if transaction fails.

Because if you get an error, or not, you only want to release your database connection (or pick your favorite type of other resource...) once.

Is it better to use std::memcpy() or std::copy() in terms to performance?

If you really need maximum copying performance (which you might not), use neither of them.

There's a lot that can be done to optimize memory copying - even more if you're willing to use multiple threads/cores for it. See, for example:

What's missing/sub-optimal in this memcpy implementation?

both the question and some of the answers have suggested implementations or links to implementations.

Check if a string contains an element from a list (of strings)

If speed is critical, you might want to look for the Aho-Corasick algorithm for sets of patterns.

It's a trie with failure links, that is, complexity is O(n+m+k), where n is the length of the input text, m the cumulative length of the patterns and k the number of matches. You just have to modify the algorithm to terminate after the first match is found.

Performance of Arrays vs. Lists

if you are just getting a single value out of either (not in a loop) then both do bounds checking (you're in managed code remember) it's just the list does it twice. See the notes later for why this is likely not a big deal.

If you are using your own for(int int i = 0; i < x.[Length/Count];i++) then the key difference is as follows:

  • Array:
    • bounds checking is removed
  • Lists
    • bounds checking is performed

If you are using foreach then the key difference is as follows:

  • Array:
    • no object is allocated to manage the iteration
    • bounds checking is removed
  • List via a variable known to be List.
    • the iteration management variable is stack allocated
    • bounds checking is performed
  • List via a variable known to be IList.
    • the iteration management variable is heap allocated
    • bounds checking is performed also Lists values may not be altered during the foreach whereas the array's can be.

The bounds checking is often no big deal (especially if you are on a cpu with a deep pipeline and branch prediction - the norm for most these days) but only your own profiling can tell you if that is an issue. If you are in parts of your code where you are avoiding heap allocations (good examples are libraries or in hashcode implementations) then ensuring the variable is typed as List not IList will avoid that pitfall. As always profile if it matters.

Why is C so fast, and why aren't other languages as fast or faster?

1) As others have said, C does less for you. No initializing variables, no array bounds checking, no memory management, etc. Those features in other languages cost memory and CPU cycles that C doesn't spend.

2) Answers saying that C is less abstracted and therefore faster are only half correct I think. Technically speaking, if you had a "sufficiently advanced compiler" for language X, then language X could approach or equal the speed of C. The difference with C is that since it maps so obviously (if you've taken an architecture course) and directly to assembly language that even a naive compiler can do a decent job. For something like Python, you need a very advanced compiler to predict the probable types of objects and generate machine code on the fly -- C's semantics are simple enough that a simple compiler can do well.

android start activity from service

one cannot use the Context of the Service; was able to get the (package) Context alike:

Intent intent = new Intent(getApplicationContext(), SomeActivity.class);

Testing for empty or nil-value string

The second clause does not need a !variable.nil? check—if evaluation reaches that point, variable.nil is guaranteed to be false (because of short-circuiting).

This should be sufficient:

variable = id if variable.nil? || variable.empty?

If you're working with Ruby on Rails, Object.blank? solves this exact problem:

An object is blank if it’s false, empty, or a whitespace string. For example, "", " ", nil, [], and {} are all blank.

Count number of lines in a git repository

The answer by Carl Norum assumes there are no files with spaces, one of the characters of IFS with the others being tab and newline. The solution would be to terminate the line with a NULL byte.

 git ls-files -z | xargs -0 cat | wc -l

What does 'x packages are looking for funding' mean when running `npm install`?

You can skip fund using:

npm install --no-fund YOUR PACKAGE NAME

For example :

npm install --no-fund core-js

Fade In Fade Out Android Animation in Java

Another alternative:

No need to define 2 animation for fadeIn and fadeOut. fadeOut is reverse of fadeIn.

So you can do this with Animation.REVERSE like this:

AlphaAnimation alphaAnimation = new AlphaAnimation(0.0f, 1.0f);

then onAnimationEnd:

alphaAnimation.setAnimationListener(new Animation.AnimationListener() {
        public void onAnimationStart(Animation animation) {
            //TODO: Run when animation start

        public void onAnimationEnd(Animation animation) {
            //TODO: Run when animation end

        public void onAnimationRepeat(Animation animation) {
            //TODO: Run when animation repeat

Change SQLite database mode to read-write

If using Android.

Make sure you have added the permission to write to your EXTERNAL_STORAGE to your AndroidManifest.xml.

Add this line to your AndroidManifest.xml file above and outside your <application> tag.

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

This will allow your application to write to the sdcard. This will help if your EXTERNAL_STORAGE is where you have stored your database on the device.

CSS file not refreshing in browser

The Ctrl + F5 solusion didn't work for me in Chrome.

But I found How to Clear Chrome Cache for Specific Website Only (3 Steps):

  1. As the page is loaded, open Chrome Developer Tools (Right-Click > Inspect) or (Menu > More Tools > Developer Tools)
  2. Next, go to the Refresh button in Chrome browser, and Right-Click the Refresh button.
  3. Select "Empty Cache and Hard Refresh".

Hope this answer helps someone!

Emulator: ERROR: x86 emulation currently requires hardware acceleration

I solved this Issue by enabling virtualization technology from system Settings.

Just followed these steps

  • Restart my Computer
  • Continuously press Esc and then F10 to enter BIOS setup
  • configuration
  • Check Virtualization technology

Your system settings may be changed According to your Computer. You can google (how to enable virtualizatino for YOUR_PC_NAME).

I hope it helps.

converting a base 64 string to an image and saving it

In a similar scenario what worked for me was the following:

byte[] bytes = Convert.FromBase64String(Base64String);    
ImageTagId.ImageUrl = "data:image/jpeg;base64," + Convert.ToBase64String(bytes);

ImageTagId is the ID of the ASP image tag.

Convert Java object to XML string

Testing And working Java code to convert java object to XML:

import java.util.ArrayList;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.XmlElement;
import javax.xml.bind.annotation.XmlRootElement;

public class Customer {

    String name;
    int age;
    int id;
    String desc;
    ArrayList<String> list;

    public ArrayList<String> getList()
        return list;

    public void setList(ArrayList<String> list)
        this.list = list;

    public String getDesc()
        return desc;

    public void setDesc(String desc)
        this.desc = desc;

    public String getName() {
        return name;

    public void setName(String name) { = name;

    public int getAge() {
        return age;

    public void setAge(int age) {
        this.age = age;

    public int getId() {
        return id;

    public void setId(int id) { = id;

import java.util.ArrayList;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;

public class createXML {

    public static void main(String args[]) throws Exception
        ArrayList<String> list = new ArrayList<String>();
        Customer c = new Customer();
        c.setDesc("some desc ");
        JAXBContext jaxbContext = JAXBContext.newInstance(Customer.class);
        Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
        jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        StringWriter sw = new StringWriter();
        jaxbMarshaller.marshal(c, sw);
        String xmlString = sw.toString();


How to access environment variable values?

The original question (first part) was "how to check environment variables in Python."

Here's how to check if $FOO is set:

except KeyError: 
   print "Please set the environment variable FOO"

Cannot connect to the Docker daemon at unix:/var/run/docker.sock. Is the docker daemon running?

I installed docker from snap repository. So I also had to start from snap (running Ubuntu).

sudo snap start docker

Otherwise you can also install it from their repositories.

Fatal error: Call to a member function prepare() on null

You can try/catch PDOExceptions (your configs could differ but the important part is the try/catch):

try {
        $dbh = new PDO(
            DB_TYPE . ':host=' . DB_HOST . ';dbname=' . DB_NAME . ';charset=' . DB_CHARSET,
                PDO::ATTR_PERSISTENT            => true,
                PDO::ATTR_ERRMODE               => PDO::ERRMODE_EXCEPTION,

    } catch ( PDOException $e ) {
        echo 'ERROR!';
        print_r( $e );

The print_r( $e ); line will show you everything you need, for example I had a recent case where the error message was like unknown database 'my_db'.

Can (a== 1 && a ==2 && a==3) ever evaluate to true?

If you take advantage of how == works, you could simply create an object with a custom toString (or valueOf) function that changes what it returns each time it is used such that it satisfies all three conditions.

const a = {_x000D_
  i: 1,_x000D_
  toString: function () {_x000D_
    return a.i++;_x000D_
if(a == 1 && a == 2 && a == 3) {_x000D_
  console.log('Hello World!');_x000D_

The reason this works is due to the use of the loose equality operator. When using loose equality, if one of the operands is of a different type than the other, the engine will attempt to convert one to the other. In the case of an object on the left and a number on the right, it will attempt to convert the object to a number by first calling valueOf if it is callable, and failing that, it will call toString. I used toString in this case simply because it's what came to mind, valueOf would make more sense. If I instead returned a string from toString, the engine would have then attempted to convert the string to a number giving us the same end result, though with a slightly longer path.

How to add font-awesome to Angular 2 + CLI project

There are many good answers here. But if you tried all of them and still getting squares instead fontawesome icons, check your css rules. In my case I had the following rule:

* {
  font-family: Roboto-Light, Roboto, 'Helvetica Neue', sans-serif !important;

And it overrides fontawesome fonts. Just replacing * selector to body solved my problem:

body {
  font-family: Roboto-Light, Roboto, 'Helvetica Neue', sans-serif !important;

EditorFor() and html properties

As at MVC 5, if you wish to add any attributes you can simply do

 @Html.EditorFor(m => m.Name, new { htmlAttributes = new { @required = "true", @anotherAttribute = "whatever" } })

Information found from this blog

Eclipse/Maven error: "No compiler is provided in this environment"

When I was runing mvn compile, I was getting below error in console:

[INFO] Changes detected - recompiling the module!
[WARNING] File encoding has not been set, using platform encoding Cp1252, i.e. build is platform dependent!
[INFO] Compiling 1087 source files to C:\Code\DevVNextComplete\Development_vNext\Source\JARS\target\classes
[INFO] -------------------------------------------------------------
[INFO] -------------------------------------------------------------
[ERROR] No compiler is provided in this environment. Perhaps you are running on a JRE rather than a JDK?
[INFO] 1 error


I had added, the JAVA_HOME variable in my environment variables and then it worked.

enter image description here

Login failed for user 'DOMAIN\MACHINENAME$'

Appreciate there are a few good answers here, but as I've just lost time working this out, hopefully this can help someone.

In my case, everything had been working fine, then stopped for no apparent reason with the error stated in the question.

IIS was running as Network service and Network Service had been set up on SQL Server previously (see other answers to this post). Server roles and user mappings looked correct.

The issue was; for absolutely no apparent reason; Network Service had switched to 'Deny' Login rights in the database.

To fix:

  1. Open SSMS > Security > Logins.
  2. Right click 'NT AUTHORITY\NETWORK SERVICE' and Click Properties.
  3. Go to 'Status' tab and set Permission to Connect To Database Engine To 'Grant'.

Network Service Allowed

Is there a JSON equivalent of XQuery/XPath?

I know the OP tagged the question with javascript but in my case I was looking exactly the same thing but from a Java backend (with Camel).

An interesting thing that can also be useful if you are using an integration framework like Camel, jsonPath is also supported by a specific Camel Component since Camel 2.13.

Example from the Camel doc above:

    .when().jsonpath("$[?(@.price < 10)]")
    .when().jsonpath("$[?(@.price < 30)]")

which is quite straightforward to use.

How to cherry pick a range of commits and merge into another branch?

All the above options will prompt you to resolve merge conflicts. If you are merging changes committed for a team, it is difficult to get resolved the merge conflicts from developers and proceed. However, "git merge" will do the merge in one shot but you can not pass a range of revisions as argument. we have to use "git diff" and "git apply" commands to do the merge range of revs. I have observed that "git apply" will fail if the patch file has diff for too many file, so we have to create a patch per file and then apply. Note that the script will not be able to delete the files that are deleted in source branch. This is a rare case, you can manually delete such files from target branch. The exit status of "git apply" is not zero if it is not able to apply the patch, however if you use -3way option it will fall back to 3 way merge and you don't have to worry about this failure.

Below is the script.

enter code here


    # This script will merge the diff between two git revisions to checked out branch
    # Make sure to cd to git source area and checkout the target branch
    # Make sure that checked out branch is clean run "git reset --hard HEAD"


    echo Start version: $START
    echo End version: $END

    mkdir -p ~/temp
    echo > /tmp/status
    #get files
    git --no-pager  diff  --name-only ${START}..${END} > ~/temp/files
    echo > ~/temp/error.log
    # merge every file
    for file in `cat  ~/temp/files`
      git --no-pager diff --binary ${START}..${END} $file > ~/temp/git-diff
      if [ $? -ne 0 ]
#      Diff usually fail if the file got deleted 
        echo Skipping the merge: git diff command failed for $file >> ~/temp/error.log
        echo Skipping the merge: git diff command failed for $file
        echo "STATUS: FAILED $file" >>  /tmp/status
        echo "STATUS: FAILED $file"
    # skip the merge for this file and continue the merge for others
        rm -f ~/temp/git-diff

      git apply  --ignore-space-change --ignore-whitespace  --3way --allow-binary-replacement ~/temp/git-diff

      if [ $? -ne 0 ]
#  apply failed, but it will fall back to 3-way merge, you can ignore this failure
         echo "git apply command filed for $file"
       STATUS=`git status -s $file`

       if [ ! "$STATUS" ]
#   status is null if the merged diffs are already present in the target file
         echo "STATUS:NOT_MERGED $file"
         echo "STATUS: NOT_MERGED $file$"  >>  /tmp/status
#     3 way merge is successful
         echo STATUS: $STATUS
         echo "STATUS: $STATUS"  >>  /tmp/status

    echo GIT merge failed for below listed files

    cat ~/temp/error.log

    echo "Git merge status per file is available in /tmp/status"

SQL to Query text in access with an apostrophe in it

When you include a string literal in a query, you can enclose the string in either single or double quotes; Access' database engine will accept either. So double quotes will avoid the problem with a string which contains a single quote.

SELECT * FROM tblStudents WHERE [name] Like "Daniel O'Neal";

If you want to keep the single quotes around your string, you can double up the single quote within it, as mentioned in other answers.

SELECT * FROM tblStudents WHERE [name] Like 'Daniel O''Neal';

Notice the square brackets surrounding name. I used the brackets to lessen the chance of confusing the database engine because name is a reserved word.

It's not clear why you're using the Like comparison in your query. Based on what you've shown, this should work instead.

SELECT * FROM tblStudents WHERE [name] = "Daniel O'Neal";

How to wait until an element exists?

I think that still there isnt any answer here with easy and readable working example. Use MutationObserver interface to detect DOM changes, like this:

var observer = new MutationObserver(function(mutations) {_x000D_
    if ($("p").length) {_x000D_
        console.log("Exist, lets do something");_x000D_
        observer.disconnect(); _x000D_
        //We can disconnect observer once the element exist if we dont want observe more changes in the DOM_x000D_
// Start observing_x000D_
observer.observe(document.body, { //document.body is node target to observe_x000D_
    childList: true, //This is a must have for the observer with subtree_x000D_
    subtree: true //Set to true if changes must also be observed in descendants._x000D_
$(document).ready(function() {_x000D_
    $("button").on("click", function() {_x000D_
        setTimeout(function() {_x000D_
            $("#newContent").append("<p>New element</p>");_x000D_
        }, 2000);_x000D_
<script src=""></script>_x000D_
<button>New content</button>_x000D_
<div id="newContent"></div>

Note: Spanish Mozilla docs about MutationObserver are more detailed if you want more information.

Maven2 property that indicates the parent directory

The following small profile worked for me. I needed such a configuration for CheckStyle, which I put into the config directory in the root of the project, so I can run it from the main module and from submodules.


It won't work for nested modules, but I'm sure it can be modified for that using several profiles with different exists's. (I have no idea why there should be "../.." in the verification tag and just ".." in the overriden property itself, but it works only in that way.)

Generating random numbers with normal distribution in Excel

Use the NORMINV function together with RAND():


To keep your set of random values from changing, select all the values, copy them, and then paste (special) the values back into the same range.

Sample output (column A), 500 numbers generated with this formula:

enter image description here

Negative matching using grep (match lines that do not contain foo)

You can also use awk for these purposes, since it allows you to perform more complex checks in a clearer way:

Lines not containing foo:

awk '!/foo/'

Lines containing neither foo nor bar:

awk '!/foo/ && !/bar/'

Lines containing neither foo nor bar which contain either foo2 or bar2:

awk '!/foo/ && !/bar/ && (/foo2/ || /bar2/)'

And so on.

Remove 'standalone="yes"' from generated XML

In case you are getting property exception, add the following configuration:

              "<?xml version=\"1.0\" encoding=\"UTF-8\"?>");
jaxbMarshaller.setProperty("com.sun.xml.internal.bind.xmlDeclaration", Boolean.FALSE);
jaxbMarshaller.setProperty(Marshaller.JAXB_FRAGMENT, Boolean.TRUE);  

How to correctly use "section" tag in HTML5?

You can definitely use the section tag as a container. It is there to group content in a more semantically significant way than with a div or as the html5 spec says:

The section element represents a generic section of a document or application. A section, in this context, is a thematic grouping of content, typically with a heading.

TypeScript: casting HTMLElement

You always can hack type system using:

var script = (<HTMLScriptElement[]><any>document.getElementsByName(id))[0];

What's the difference between eval, exec, and compile?

exec is for statement and does not return anything. eval is for expression and returns value of expression.

expression means "something" while statement means "do something".

NSDate get year/month/day

i do in this way ....

NSDate * mydate = [NSDate date];

NSCalendar * mycalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];

NSCalendarUnit units = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay;

NSDateComponents * myComponents  = [mycalendar components:units fromDate:mydate];


How to change the text on the action bar

Update: Latest ActionBar (Title) pattern:

FYI, ActionBar was introduced in API Level 11. ActionBar is a window feature at the top of the Activity that may display the activity title, navigation modes, and other interactive items like search.

I exactly remember about customizing title bar and making it consistent through the application. So I can make a comparison with the earlier days and can list some of the advantages of using ActionBar:

  1. It offers your users a familiar interface across applications that the system gracefully adapts for different screen configurations.
  2. Developers don't need to write much code for displaying the Activity Title, icons and navigation modes because ActionBar is already ready with top level abstraction.

For example:

enter image description here

enter image description here

=> Normal way,

getActionBar().setTitle("Hello world App");   
getSupportActionBar().setTitle("Hello world App");  // provide compatibility to all the versions

=> Customizing Action Bar,

For example:

public void setActionBar(String heading) {
    // TODO Auto-generated method stub actionBar = getSupportActionBar();
    actionBar.setBackgroundDrawable(new ColorDrawable(getResources().getColor(R.color.title_bar_gray)));


Styling the Action Bar:

The ActionBar provides you with basic and familiar looks, navigation modes and other quick actions to perform. But that doesn't mean it looks the same in every app. You can customize it as per your UI and design requirements. You just have to define and write styles and themes.

Read more at: Styling the Action Bar

And if you want to generate styles for ActionBar then this Style Generator tool can help you out.


Old: Earlier days:

=> Normal way,

you can Change the Title of each screen (i.e. Activity) by setting their Android:label

   <activity android:name=".Hello_World"
                  android:label="This is the Hello World Application">

=> Custom - Title - bar

But if you want to Customize title-bar in your own way, i.e. Want to put Image icon and custom-text, then the following code works for me:


<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android=""


<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android=""

<ImageView android:id="@+id/ImageView01" 


  android:text="This is my new title" 

public class TitleBar extends Activity {

    public void onCreate(Bundle savedInstanceState) {
        final boolean customTitleSupported = 
        if (customTitleSupported) {
        final TextView myTitleText = (TextView) findViewById(;
        if (myTitleText != null) {
            myTitleText.setText("NEW TITLE");
            // user can also set color using "Color" and then
            // "Color value constant"
            // myTitleText.setBackgroundColor(Color.GREEN);


The strings.xml file is defined under the values folder.

<?xml version="1.0" encoding="utf-8"?>
    <string name="hello">Hello World, Set_Text_TitleBar!</string>
    <string name="app_name">Set_Text_TitleBar</string>
    <color name="titlebackgroundcolor">#3232CD</color>
    <color name="titletextcolor">#FFFF00</color>

Run an OLS regression with Pandas Data Frame

This would require me to reformat the data into lists inside lists, which seems to defeat the purpose of using pandas in the first place.

No it doesn't, just convert to a NumPy array:

>>> data = np.asarray(df)

This takes constant time because it just creates a view on your data. Then feed it to scikit-learn:

>>> from sklearn.linear_model import LinearRegression
>>> lr = LinearRegression()
>>> X, y = data[:, 1:], data[:, 0]
>>>, y)
LinearRegression(copy_X=True, fit_intercept=True, normalize=False)
>>> lr.coef_
array([  4.01182386e-01,   3.51587361e-04])
>>> lr.intercept_

How to change an Eclipse default project into a Java project

Depending on the Eclipse in question the required WTP packages may be found with different names. For example in Eclipse Luna I found it easiest to search with "Tools" and choose one that mentioned Tools for Java EE development. That added the project facet functionality. Searching with "WTP" wasn't of much help.

How to determine if a list of polygon points are in clockwise order?

Here's a simple Python 3 implementation based on this answer (which, in turn, is based on the solution proposed in the accepted answer)

def is_clockwise(points):
    # points is your list (or array) of 2d points.
    assert len(points) > 0
    s = 0.0
    for p1, p2 in zip(points, points[1:] + [points[0]]):
        s += (p2[0] - p1[0]) * (p2[1] + p1[1])
    return s > 0.0

RecyclerView inside ScrollView is not working

I used CustomLayoutManager to disable RecyclerView Scrolling. Also don't use Recycler View as WrapContent, use it as 0dp, Weight=1

public class CustomLayoutManager extends LinearLayoutManager {
    private boolean isScrollEnabled;

    // orientation should be LinearLayoutManager.VERTICAL or HORIZONTAL
    public CustomLayoutManager(Context context, int orientation, boolean isScrollEnabled) {
        super(context, orientation, false);
        this.isScrollEnabled = isScrollEnabled;

    public boolean canScrollVertically() {
        //Similarly you can customize "canScrollHorizontally()" for managing horizontal scroll
        return isScrollEnabled && super.canScrollVertically();

Use CustomLayoutManager in RecyclerView:

CustomLayoutManager mLayoutManager = new CustomLayoutManager(getBaseActivity(), CustomLayoutManager.VERTICAL, false);
        ((DefaultItemAnimator) recyclerView.getItemAnimator()).setSupportsChangeAnimations(false); 


<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android=""










How do I set up access control in SVN?

@Stephen Bailey

To complete your answer, you can also delegate the user rights to the project manager, through a plain text file in your repository.

To do that, you set up your SVN database with a default authz file containing the following:

# The content of this file always precedes the content of the
# $REPOS/admin/acl_descriptions.txt file.
# It describes the immutable permissions on main folders.
svnadmins = xxx,yyy,....

@svnadmins = rw
* = r
@svnadmins = rw
@projadmins = r
* =

@projadmins = rw

This default authz file authorizes the SVN administrators to modify a visible plain text file within your SVN repository, called '/admin/acl_descriptions.txt', in which the SVN administrators or project managers will modify and register the users.

Then you set up a pre-commit hook which will detect if the revision is composed of that file (and only that file).

If it is, this hook's script will validate the content of your plain text file and check if each line is compliant with the SVN syntax.

Then a post-commit hook will update the \conf\authz file with the concatenation of:

  • the TEMPLATE authz file presented above
  • the plain text file /admin/acl_descriptions.txt

The first iteration is done by the SVN administrator, who adds:

projadmins = zzzz

He commits his modification, and that updates the authz file.

Then the project manager 'zzzz' can add, remove or declare any group of users and any users he wants. He commits the file and the authz file is updated.

That way, the SVN administrator does not have to individually manage any and all users for all SVN repositories.

How to add elements of a Java8 stream into an existing List

As far as I can see, all other answers so far used a collector to add elements to an existing stream. However, there's a shorter solution, and it works for both sequential and parallel streams. You can simply use the method forEachOrdered in combination with a method reference.

List<String> source = ...;
List<Integer> target = ...;

The only restriction is, that source and target are different lists, because you are not allowed to make changes to the source of a stream as long as it is processed.

Note that this solution works for both sequential and parallel streams. However, it does not benefit from concurrency. The method reference passed to forEachOrdered will always be executed sequentially.

React - changing an uncontrolled input

Simple solution to resolve this problem is to set an empty value by default :

<input name='myInput' value={this.state.myInput || ''} onChange={this.handleChange} />

Quickest way to clear all sheet contents VBA

The .Cells range isn't limited to ones that are being used, so your code is clearing the content of 1,048,576 rows and 16,384 columns - 17,179,869,184 total cells. That's going to take a while. Just clear the UsedRange instead:


Alternately, you can delete the sheet and re-add it:

Application.DisplayAlerts = False
Application.DisplayAlerts = True
Dim sheet As Worksheet
Set sheet = Sheets.Add
sheet.Name = "Zeros"

How can I update npm on Windows?

this is best tool to maintain version of NODE.Js i NVM

Node Version Manager (nvm) for Windows but for Windows, with an installer. Download Now! This has always been a node version manager, not an io.js manager, so there is no back-support for io.js. However, node 4+ is supported. enter image description here

Tricks to manage the available memory in an R session

I really appreciate some of the answers above, following @hadley and @Dirk that suggest closing R and issuing source and using command line I come up with a solution that worked very well for me. I had to deal with hundreds of mass spectras, each occupies around 20 Mb of memory so I used two R scripts, as follows:

First a wrapper:

#!/usr/bin/Rscript --vanilla --default-packages=utils

for(l in 1:length(fdir)) {

   for(k in 1:length(fds)) {
     system(paste("Rscript runConsensus.r", l, k))

with this script I basically control what my main script do runConsensus.r, and I write the data answer for the output. With this, each time the wrapper calls the script it seems the R is reopened and the memory is freed.

Hope it helps.

Threading Example in Android

Here is a simple threading example for Android. It's very basic but it should help you to get a perspective.

Android code -


import android.os.Bundle;
import android.widget.TextView;

public class Test12Activity extends Activity {

    public void onCreate(Bundle savedInstanceState) {
        final TextView txt1 = (TextView) findViewById(;

        new Thread(new Runnable() { 
            public void run(){        


Android application xml - main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android=""

    android:id = "@+id/sm"


Where can I find the .apk file on my device, when I download any app and install?

There is an app in google play known as MyAppSharer. Open the app, search for the app that you have installed, check apk and select share. The app would take some time and build the apk. You can then close the app. The apk of the file is located in /sdcard/MyAppSharer

This does not require rooting your phone and works only for apps that are currently installed on your phone

Creating dummy variables in pandas for python

You can create dummy variables to handle the categorical data

# Creating dummy variables for categorical datatypes
trainDfDummies = pd.get_dummies(trainDf, columns=['Col1', 'Col2', 'Col3', 'Col4'])

This will drop the original columns in trainDf and append the column with dummy variables at the end of the trainDfDummies dataframe.

It automatically creates the column names by appending the values at the end of the original column name.

Comparing boxed Long values 127 and 128

num1 and num2 are Long objects. You should be using equals() to compare them. == comparison might work sometimes because of the way JVM boxes primitives, but don't depend on it.

if (num1.equals(num1))

Android OnClickListener - identify a button

Another way of doing it is a single listener from activity , like this:

public class MyActivity extends Activity implements OnClickListener {
    .......  code

    //my listener
    public void onClick(View v) {
        if (v.getId() == { 

        if (v.getId() == { 

I Like to do it with single IF instead of switch-else, but if you prefer that, then you should do:

//my listener
public void onClick(View v) {
    switch(v.getId()) {


Select first 10 distinct rows in mysql

FROM    people
WHERE   names = 'Smith'

Grep characters before and after match?

With gawk , you can use match function:

    x="hey there how are you"
    echo "$x" |awk --re-interval '{match($0,/(.{4})how(.{4})/,a);print a[1],a[2]}'
    ere   are

If you are ok with perl, more flexible solution : Following will print three characters before the pattern followed by actual pattern and then 5 character after the pattern.

echo hey there how are you |perl -lne 'print "$1$2$3" if /(.{3})(there)(.{5})/'
ey there how

This can also be applied to words instead of just characters.Following will print one word before the actual matching string.

echo hey there how are you |perl -lne 'print $1 if /(\w+) there/'

Following will print one word after the pattern:

echo hey there how are you |perl -lne 'print $2 if /(\w+) there (\w+)/'

Following will print one word before the pattern , then the actual word and then one word after the pattern:

echo hey there how are you |perl -lne 'print "$1$2$3" if /(\w+)( there )(\w+)/'
hey there how

Embedding JavaScript engine into .NET

There is an implementation of an ActiveX Scripting Engine Host in C# available here: parse and execute JS by C#

It allows to use Javascript (or VBScript) directly from C#, in native 32-bit or 64-bit processes. The full source is ~500 lines of C# code. It only has an implicit dependency on the installed JScript (or VBScript) engine DLL.

For example, the following code:

Console.WriteLine(ScriptEngine.Eval("jscript", "1+2/3"));

will display 1.66666666666667

Provisioning Profiles menu item missing from Xcode 5

After searching a few times in google, i found one software for provisioning profiles.

Install this iPhone configuration utility software and manage your all provisioning profiles in MAC.

Verifying a specific parameter with Moq

A simpler way would be to do:

    a => a.Execute(
        It.Is<Params>(p => p.Id == 7)

"Access is denied" JavaScript error when trying to access the document object of a programmatically-created <iframe> (IE-only)

For me I found the better answer was to check the file permissons that access is being denied to.

I just update to jQuery-1.8.0.js and was getting the Access Denied error in IE9.

From Windows Explorer

  • I right clicked on the file selected the Properties
  • Selected the Security Tab
  • Clicked the Advanced Button
  • Selected the Owner Tab
  • Clicked on Edit Button
  • Selected Administrators(MachineName\Administrators)
  • Clicked Apply
  • Closed all the windows.

Tested the site. No more issue.

I had to do the same for the the jQuery-UI script I had just updated as well

IPython Notebook save location

  1. Based on my experience, if you use "ipython.exe" in python-2.7.5/Scripts from other directory, as typing at the comamnd prompt as ipython notebook, *.ipynb files will be loaded and saved in the current directory. ("notebook" is just a comamnd line parameter) I think the "ipython notebook.exe" in Winpython top directory is not relevant for your request. As for me, I added the ipython.exe directory to the Path only.
  2. If you want to make your profile in user directory, see below:

How to give ASP.NET access to a private key in a certificate in the certificate store?

If you are trying to load a cert from a .pfx file in IIS the solution may be as simple as enabling this option for the Application Pool.

Right click on the App Pool and select Advanced Settings.

Then enable Load User Profile

enter image description here

What is the best free SQL GUI for Linux for various DBMS systems

For Oracle, I highly recommend the free Oracle SQL Developer

The doucmentation states it also works with non-oracle databases - i've never tried that feature myself, but I do know that it works really well with Oracle

How do I prompt for Yes/No/Cancel input in a Linux shell script?

Use the read command:

echo Would you like to install? "(Y or N)"

read x

# now check if $x is "y"
if [ "$x" = "y" ]; then
    # do something here!

and then all of the other stuff you need

How to spyOn a value property (rather than a method) with Jasmine

I'm a bit late to the party here i know but,

You could directly access the calls object, which can give you the variables for each call


Cannot make file No such file or directory

You may want to use Apache Commons IO's FileUtils.openOutputStream(File) method. It has good Exception messages when something went wrong and also creates necessary parent dirs. If everything was right then you directly get your OutputStream - very neat.

If you just want to touch the file then use FileUtils.touch(File) instead.

How to update (append to) an href in jquery?

$("a.directions-link").attr("href", $("a.directions-link").attr("href")+"...your additions...");

No Creators, like default construct, exist): cannot deserialize from Object value (no delegate- or property-based Creator

I solved this issue by adding a no argument constractor. If you are using Lombok, you only need to add @NoArgsConstructor annotation:

public class User {
    private Long userId;
    private String shortName;

How to schedule a task to run when shutting down windows

What I can suggest doing is creating a shortcut to the .bat file (for example on your desktop) and a when you want to shutdown your computer (and run the .bat file) click on the shortcut you created. After doing this, edit the .bat file and add this line of code to the end or where needed:

c:\windows\system32\shutdown -s -f -t 00

What this does it is

  1. Runs the shutdown process
  2. Displays a alert
  3. Forces all running processes to stop
  4. Executes immediately

Can we rely on String.isEmpty for checking null condition on a String in Java?

You can't use String.isEmpty() if it is null. Best is to have your own method to check null or empty.

public static boolean isBlankOrNull(String str) {
    return (str == null || "".equals(str.trim()));

How to find longest string in the table column data

This was the first result on "longest string in postgres" google search so I'll put my answer here for those looking for a postgres solution.

SELECT max(char_length(column)) AS Max_Length_String FROM table

postgres docs:

Volatile vs. Interlocked vs. lock

EDIT: As noted in comments, these days I'm happy to use Interlocked for the cases of a single variable where it's obviously okay. When it gets more complicated, I'll still revert to locking...

Using volatile won't help when you need to increment - because the read and the write are separate instructions. Another thread could change the value after you've read but before you write back.

Personally I almost always just lock - it's easier to get right in a way which is obviously right than either volatility or Interlocked.Increment. As far as I'm concerned, lock-free multi-threading is for real threading experts, of which I'm not one. If Joe Duffy and his team build nice libraries which will parallelise things without as much locking as something I'd build, that's fabulous, and I'll use it in a heartbeat - but when I'm doing the threading myself, I try to keep it simple.

How do I get elapsed time in milliseconds in Ruby?

The answer is something like:

t_start =
# time-consuming operation
t_end =

milliseconds = (t_start - t_end) * 1000.0

However, the approach risks to be inaccurate. I found this post by Luca Guidi:

system clock is constantly floating and it doesn't move only forwards. If your calculation of elapsed time is based on it, you're very likely to run into calculation errors or even outages.

So, it is recommended to use Process.clock_gettime instead. Something like:

def measure_time
  start_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
  end_time = Process.clock_gettime(Process::CLOCK_MONOTONIC)
  elapsed_time = end_time - start_time


elapsed = measure_time do
    # your time-consuming task here:
    sleep 2.2321

=> 2.232

Add UIPickerView & a Button in Action sheet - How?

For those guys who are tying to find the DatePickerDoneClick function... here is the simple code to dismiss the Action Sheet. Obviously aac should be an ivar (the one which goes in your implmentation .h file)

- (void)DatePickerDoneClick:(id)sender{
    [aac dismissWithClickedButtonIndex:0 animated:YES];

Sql select rows containing part of string

You can use the LIKE operator to compare the content of a T-SQL string, e.g.

SELECT * FROM [table] WHERE [field] LIKE '%stringtosearchfor%'.

The percent character '%' is a wild card- in this case it says return any records where [field] at least contains the value "stringtosearchfor".

How can I iterate over files in a given directory?

Since Python 3.5, things are much easier with os.scandir()

with os.scandir(path) as it:
    for entry in it:
        if".asm") and entry.is_file():
            print(, entry.path)

Using scandir() instead of listdir() can significantly increase the performance of code that also needs file type or file attribute information, because os.DirEntry objects expose this information if the operating system provides it when scanning a directory. All os.DirEntry methods may perform a system call, but is_dir() and is_file() usually only require a system call for symbolic links; os.DirEntry.stat() always requires a system call on Unix but only requires one for symbolic links on Windows.

Can I force a page break in HTML printing?

Let's say you have a blog with articles like this:

<div class="article"> ... </div>

Just adding this to the CSS worked for me:

@media print {
  .article { page-break-after: always; }

(tested and working on Chrome 69 and Firefox 62).


sudo service mongodb restart gives "unrecognized service error" in ubuntu 14.0.4

Original Source -

If you're on Ubuntu 16.04 and face the unrecognized service error, these instructions will fix it for you:-

  1. Open a terminal window.
  2. Issue the command sudo apt-key adv —keyserver hkp:// —recv EA312927
  3. Issue the command sudo touch /etc/apt/sources.list.d/mongodb-org.list
  4. Issue the command sudo gedit /etc/apt/sources.list.d/mongodb-org.list
  5. Copy and paste one of the following lines from below (depending upon your release) into the open file. For 12.04: deb precise/mongodb-org/3.6 multiverse For 14.04: deb trusty/mongodb-org/3.6 multiverse For 16.04: deb xenial/mongodb-org/3.6 multiverse

Make sure to edit the version number with the appropriate latest version and save the file.

Installation Open a terminal window and issue command sudo apt-get update && sudo apt-get install -y mongodb-org

Let the installation complete.

Running MongoDB To start the database, issue the command sudo service mongodb start. You should now be able to issue the command to see that MongoDB is running: systemctl status mongodb

Ubuntu 16.04 solution If you are using Ubuntu 16.04, you may run into an issue where you see the error mongodb: unrecognized service due to the switch from upstart to systemd. To get around this, you have to follow these steps.

  • If you added the /etc/apt/sources.list.d/mongodb-org.list, remove it with the command sudo rm /etc/apt/sources.list.d/mongodb-org.list

  • Update apt with the command sudo apt-get update

  • Install the official MongoDB version from the standard repositories with the command sudo apt-get install mongodb in order to get the service set up properly

  • Remove what you just installed with the command sudo apt-get remove mongodb && sudo apt-get autoremove

Now follow steps 1 through 5 listed above to install MongoDB; this should re-install the latest version of MongoDB with the systemd services already in place. When you issue the command systemctl status mongodb you should see that the server is active.

I mostly copy pasted the above (with minor modifications and typo fixes) from here -

Maximum concurrent connections to MySQL

I can assure you that raw speed ultimately lies in the non-standard use of Indexes for blazing speed using large tables.

How to insert spaces/tabs in text using HTML/CSS

Types of Spaces in HTML

Creates four spaces between the text- &emsp;

Creates two spaces between the text - &ensp;

Creates a regular space between the text - &nbsp;

creates a narrow space ( similar to regular space but slight difference - "&thinsp";

spacing between sentences - "</br>"

This link might help you. Check out []

hash function for string

Wikipedia shows a nice string hash function called Jenkins One At A Time Hash. It also quotes improved versions of this hash.

uint32_t jenkins_one_at_a_time_hash(char *key, size_t len)
    uint32_t hash, i;
    for(hash = i = 0; i < len; ++i)
        hash += key[i];
        hash += (hash << 10);
        hash ^= (hash >> 6);
    hash += (hash << 3);
    hash ^= (hash >> 11);
    hash += (hash << 15);
    return hash;

Dynamically adding elements to ArrayList in Groovy

What you actually created with:

MyType[] list = []

Was fixed size array (not list) with size of 0. You can create fixed size array of size for example 4 with:

MyType[] array = new MyType[4]

But there's no add method of course.

If you create list with def it's something like creating this instance with Object (You can read more about def here). And [] creates empty ArrayList in this case.

So using def list = [] you can then append new items with add() method of ArrayList

list.add(new MyType())

Or more groovy way with overloaded left shift operator:

list << new MyType() 

Convert from enum ordinal to enum type

I agree with most people that using ordinal is probably a bad idea. I usually solve this problem by giving the enum a private constructor that can take for example a DB value then create a static fromDbValue function similar to the one in Jan's answer.

public enum ReportTypeEnum {

    private static Logger log = LoggerFactory.getLogger(ReportEnumType.class);  
    private static Map<Integer, ReportTypeEnum> lookup;
    private Integer dbValue;

    private ReportTypeEnum(Integer dbValue) {
        this.dbValue = dbValue;

    static {
        try {
            ReportTypeEnum[] vals = ReportTypeEnum.values();
            lookup = new HashMap<Integer, ReportTypeEnum>(vals.length);

            for (ReportTypeEnum  rpt: vals)
                lookup.put(rpt.getDbValue(), rpt);
         catch (Exception e) {
             // Careful, if any exception is thrown out of a static block, the class
             // won't be initialized
             log.error("Unexpected exception initializing " + ReportTypeEnum.class, e);

    public static ReportTypeEnum fromDbValue(Integer dbValue) {
        return lookup.get(dbValue);

    public Integer getDbValue() {
        return this.dbValue;


Now you can change the order without changing the lookup and vice versa.

Padding zeros to the left in postgreSQL

The to_char() function is there to format numbers:

select to_char(column_1, 'fm000') as column_2
from some_table;

The fm prefix ("fill mode") avoids leading spaces in the resulting varchar. The 000 simply defines the number of digits you want to have.

psql (9.3.5)
Type "help" for help.

postgres=> with sample_numbers (nr) as (
postgres(>     values (1),(11),(100)
postgres(> )
postgres-> select to_char(nr, 'fm000')
postgres-> from sample_numbers;
(3 rows)


For more details on the format picture, please see the manual:

Add Items to ListView - Android

ListView myListView = (ListView) rootView.findViewById(;
ArrayList<String> myStringArray1 = new ArrayList<String>();
adapter = new CustomAdapter(getActivity(), R.layout.row, myStringArray1);

Try it like this

public OnClickListener moreListener = new OnClickListener() {

    public void onClick(View v) {
        adapter = null;
        adapter = new CustomAdapter(getActivity(), R.layout.row, myStringArray1);

Visual Studio "Could not copy" .... during build

This question was the first result when looking for the following error:

Could not copy the file "..." because it was not found.

when building in Visual Studio 2013 (Update 3).

Solution: Uninstalling the "Productivity Power Tools" in Visual Studio 2013.

google map API zoom range

Available Zoom Levels

Zoom level 0 is the most zoomed out zoom level available and each integer step in zoom level halves the X and Y extents of the view and doubles the linear resolution.

Google Maps was built on a 256x256 pixel tile system where zoom level 0 was a 256x256 pixel image of the whole earth. A 256x256 tile for zoom level 1 enlarges a 128x128 pixel region from zoom level 0.

As correctly stated by bkaid, the available zoom range depends on where you are looking and the kind of map you are using:

  • Road maps - seem to go up to zoom level 22 everywhere
  • Hybrid and satellite maps - the max available zoom levels depend on location. Here are some examples:
  • Remote regions of Antarctica: 13
  • Gobi Desert: 17
  • Much of the U.S. and Europe: 21
  • "Deep zoom" locations: 22-23 (see bkaid's link)

Note that these values are for the Google Static Maps API which seems to give one more zoom level than the Javascript API. It appears that the extra zoom level available for Static Maps is just an upsampled version of the max-resolution image from the Javascript API.

Map Scale at Various Zoom Levels

Google Maps uses a Mercator projection so the scale varies substantially with latitude. A formula for calculating the correct scale based on latitude is:

meters_per_pixel = 156543.03392 * Math.cos( * Math.PI / 180) / Math.pow(2, zoom)

Formula is from Chris Broadfoot's comment.

Google Maps basics

Zoom Level - zoom

0 - 19

0 lowest zoom (whole world)

19 highest zoom (individual buildings, if available) Retrieve current zoom level using mapObject.getZoom()

What you're looking for are the scales for each zoom level. Use these:

20 : 1128.497220
19 : 2256.994440
18 : 4513.988880
17 : 9027.977761
16 : 18055.955520
15 : 36111.911040
14 : 72223.822090
13 : 144447.644200
12 : 288895.288400
11 : 577790.576700
10 : 1155581.153000
9  : 2311162.307000
8  : 4622324.614000
7  : 9244649.227000
6  : 18489298.450000
5  : 36978596.910000
4  : 73957193.820000
3  : 147914387.600000
2  : 295828775.300000
1  : 591657550.500000

Reduce git repository size

Thanks for your replies. Here's what I did:

git gc
git gc --aggressive
git prune

That seemed to have done the trick. I started with around 10.5MB and now it's little more than 980KBs.

Center fixed div with dynamic width (CSS)

This works regardless of the size of its contents

.centered {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);


Using variables inside a bash heredoc

As a late corolloary to the earlier answers here, you probably end up in situations where you want some but not all variables to be interpolated. You can solve that by using backslashes to escape dollar signs and backticks; or you can put the static text in a variable.

Name='Rich Ba$tard'
cat <<____HERE
$Name, you can win a lot of $dough this week!
Notice that \`backticks' need escaping if you want
literal text, not `pwd`, just like in variables like
\$HOME (current value: $HOME)


Note that any of the quoting mechanisms -- \____HERE or "____HERE" or '____HERE' -- will disable all variable interpolation, and turn the here-document into a piece of literal text.

A common task is to combine local variables with script which should be evaluated by a different shell, programming language, or remote host.

ssh -t remote <<:
    echo "$local is the value from the host which ran the ssh command"
    # Prevent here doc from expanding locally; remote won't see backslash
    # Same here
    echo "\$remote is the value from the host we ssh:ed to"

How can I get a value from a map?"key") throws exception if missing key

If k does not match the key of any element in the container, the function throws an out_of_range exception.

Add column to SQL query results

why dont you add a "source" column to each of the queries with a static value like

select 'source 1' as Source, column1, column2...
from table1


select 'source 2' as Source, column1, column2...
from table2

How do I count occurrence of duplicate items in array

    $input = [1,2,1,3,2,4,10];
    //if give string
    //$input = "hello hello how are you how hello";
    //$array = explode(' ',$input);
    $count_val = [];
    foreach($array as $val){
//output ( [1] => 2 [2] => 2 [3] => 1 [4] => 1 [10] => 1 )

How to query the permissions on an Oracle directory?

This should give you the roles, users and permissions granted on a directory:

  FROM all_tab_privs 
 WHERE table_name = 'your_directory';  --> needs to be upper case

And yes, it IS in the all_TAB_privs view ;-) A better name for that view would be something like "ALL_OBJECT_PRIVS", since it also includes PL/SQL objects and their execute permissions as well.

Which HTTP methods match up to which CRUD methods?

Right now (2016) the latest HTTP verbs are GET, POST, PATCH, PUT and DELETE


  • HTTP GET - SELECT/Request
  • HTTP PATCH - When PUTting a complete resource representation is cumbersome and utilizes more bandwidth, e.g.: when you have to update partially a column

Hope this helps!

If you are interested on designing REST APIs this is an ansewome reading to have! website online version github repository

Define global constants

You can make a class for your global variable and then export this class like this:

export class CONSTANT {
    public static message2 = [
        { "NAME_REQUIRED": "Name is required" }

    public static message = {
        "NAME_REQUIRED": "Name is required",

After creating and exporting your CONSTANT class, you should import this class in that class where you want to use, like this:

import { Component, OnInit                       } from '@angular/core';
import { CONSTANT                                } from '../../constants/dash-constant';

  selector   : 'team-component',
  templateUrl: `../app/modules/dashboard/dashComponents/teamComponents/team.component.html`,

export class TeamComponent implements OnInit {
  constructor() {

  ngOnInit() {

You can use this either in constructor or ngOnInit(){}, or in any predefine methods.

How to create jobs in SQL Server Express edition

SQL Server Express doesn't include SQL Server Agent, so it's not possible to just create SQL Agent jobs.

What you can do is:
You can create jobs "manually" by creating batch files and SQL script files, and running them via Windows Task Scheduler.
For example, you can backup your database with two files like this:


sqlcmd -i backup.sql


backup database TeamCity to disk = 'c:\backups\MyBackup.bak'

Just put both files into the same folder and exeute the batch file via Windows Task Scheduler.

The first file is just a Windows batch file which calls the sqlcmd utility and passes a SQL script file.
The SQL script file contains T-SQL. In my example, it's just one line to backup a database, but you can put any T-SQL inside. For example, you could do some UPDATE queries instead.

If the jobs you want to create are for backups, index maintenance or integrity checks, you could also use the excellent Maintenance Solution by Ola Hallengren.

It consists of a bunch of stored procedures (and SQL Agent jobs for non-Express editions of SQL Server), and in the FAQ there’s a section about how to run the jobs on SQL Server Express:

How do I get started with the SQL Server Maintenance Solution on SQL Server Express?

SQL Server Express has no SQL Server Agent. Therefore, the execution of the stored procedures must be scheduled by using cmd files and Windows Scheduled Tasks. Follow these steps.

SQL Server Express has no SQL Server Agent. Therefore, the execution of the stored procedures must be scheduled by using cmd files and Windows Scheduled Tasks. Follow these steps.

  1. Download MaintenanceSolution.sql.

  2. Execute MaintenanceSolution.sql. This script creates the stored procedures that you need.

  3. Create cmd files to execute the stored procedures; for example:
    sqlcmd -E -S .\SQLEXPRESS -d master -Q "EXECUTE dbo.DatabaseBackup @Databases = 'USER_DATABASES', @Directory = N'C:\Backup', @BackupType = 'FULL'" -b -o C:\Log\DatabaseBackup.txt

  4. In Windows Scheduled Tasks, create tasks to call the cmd files.

  5. Schedule the tasks.

  6. Start the tasks and verify that they are completing successfully.

Changing Font Size For UITableView Section Headers

Swift 3:

Simplest way to adjust only size:

func tableView(_ tableView: UITableView, willDisplayHeaderView view: UIView, forSection section: Int) {

    let header = view as! UITableViewHeaderFooterView

    if let textlabel = header.textLabel {
        textlabel.font = textlabel.font.withSize(15)

Auto-redirect to another HTML page

If you're using Apache and can use a .htaccess file you should use the following type of redirect. Add the following to an .htaccess file in the root of your website.

RewriteEngine On
RewriteRule ^/oldfile_path/file_name\.html$ /oldfile_path/file_name.html [R=301,L]

This has the advantage of being a very fast and immediate redirect. It also depends on your reason for the redirect. This is a more permanent method because it sends the HTTP 301 status code signifying that the file has moved permanently and causes many browsers to cache that request. You can change the code to something else like a 302 for temporary redirects.

Otherwise you can do a simple redirect using an HTML <meta> tag as suggested by others:

<meta http-equiv="refresh" content="5; url=">

By default the content="5" makes that redirect after 5 seconds. This will be slower and not all browsers support it. A redirect can also be done in the server language of your choice PHP, Node.js, etc.

Where is JAVA_HOME on macOS Mojave (10.14) to Lion (10.7)?

Update for Java 9 and some neat aliases.

In .bash_profile:

export JAVA_HOME8=`/usr/libexec/java_home --version 1.8`
export JAVA_HOME9=`/usr/libexec/java_home --version 9`

Note, that for the latest version it is 9 and not 1.9.

Set active Java:

export PATH=$JAVA_HOME/bin:$PATH

Some additional alias to switch between the different versions:

alias j8='export JAVA_HOME=$JAVA_HOME8; export PATH=$JAVA_HOME/bin:$PATH'
alias j9='export JAVA_HOME=$JAVA_HOME9; export PATH=$JAVA_HOME/bin:$PATH'

Test in terminal:

% j8
% java -version
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) 64-Bit Server VM (build 25.121-b13, mixed mode)
% j9
% java -version
java version "9"
Java(TM) SE Runtime Environment (build 9+181)
Java HotSpot(TM) 64-Bit Server VM (build 9+181, mixed mode)

EDIT: Update for Java 10

export JAVA_HOME10=`/usr/libexec/java_home --version 10`
alias j10='export JAVA_HOME=$JAVA_HOME10; export PATH=$JAVA_HOME/bin:$PATH'

EDIT: Update for Java 11

export JAVA_HOME11=`/usr/libexec/java_home --version 11`
alias j11='export JAVA_HOME=$JAVA_HOME11; export PATH=$JAVA_HOME/bin:$PATH'

How can I scan barcodes on iOS?

For a native iOS 7 bar code scanner take a look at my project on GitHub:

Calling a Function defined inside another function in Javascript

You can also try this.Here you are returning the function "inside" and invoking with the second set of parenthesis.

function outer() {
  return (function inside(){
    console.log("Inside inside function");


function outer2() {
    let inside = function inside(){
      console.log("Inside inside");
    return inside;

Download file inside WebView

If you don't want to use a download manager then you can use this code

webView.setDownloadListener(new DownloadListener() {

            public void onDownloadStart(String url, String userAgent, String contentDisposition
                    , String mimetype, long contentLength) {

                String fileName = URLUtil.guessFileName(url, contentDisposition, mimetype);

                try {
                    String address = Environment.getExternalStorageDirectory().getAbsolutePath() + "/"
                            + Environment.DIRECTORY_DOWNLOADS + "/" +
                    File file = new File(address);
                    boolean a = file.createNewFile();

                    URL link = new URL(url);
                    downloadFile(link, address);

                } catch (Exception e) {

 public void downloadFile(URL url, String outputFileName) throws IOException {

        try (InputStream in = url.openStream();
             ReadableByteChannel rbc = Channels.newChannel(in);
             FileOutputStream fos = new FileOutputStream(outputFileName)) {
            fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
           // do your work here


This will download files in the downloads folder in phone storage. You can use threads if you want to download that in the background (use thread.alive() and timer class to know the download is complete or not). This is useful when we download small files, as you can do the next task just after the download.

Ansible: How to delete files and folders inside a directory?

Below code will delete the entire contents of artifact_path

- name: Clean artifact path
    state: absent
    path: "{{ artifact_path }}/"

Note: this will delete the directory too.

How is an HTTP POST request made in node.js?

I like the simplicity of superagent ( Same API on both node and browser.

;(async function() {
  var response = await'', {age: 2})

There is also node-fetch (, which has an API that matches fetch from the browsers - however this requires manual query string encoding, does not automatically handle content types, or so any of the other work superagent does.

Automatically enter SSH password with script

This is basically an extension of abbotto's answer, with some additional steps (aimed at beginners) to make starting up your server, from your linux host, very easy:

  1. Write a simple bash script, e.g.:

sshpass -p "YOUR_PASSWORD" ssh -o StrictHostKeyChecking=no YOUR_USERNAME@SOME_SITE.COM
  1. Save the file, e.g. 'startMyServer', then make the file executable by running this in your terminal:
sudo chmod +x startMyServer
  1. Move the file to a folder which is in your 'PATH' variable (run 'echo $PATH' in your terminal to see those folders). So for example move it to '/usr/bin/'.

And voila, now you are able to get into your server by typing 'startMyServer' into your terminal.

P.S. (1) this is not very secure, look into ssh keys for better security.

P.S. (2) SMshrimant answer is quite similar and might be more elegant to some. But I personally prefer to work in bash scripts.

Maven: How to run a .java file from command line passing arguments

You could run: mvn exec:exec -Dexec.args="arg1".

This will pass the argument arg1 to your program.

You should specify the main class fully qualified, for example, a that is in a package test would need

mvn exec:java  -Dexec.mainClass=test.Main

By using the -f parameter, as decribed here, you can also run it from other directories.

mvn exec:java -Dexec.mainClass=test.Main -f folder/pom.xm

For multiple arguments, simply separate them with a space as you would at the command line.

mvn exec:java -Dexec.mainClass=test.Main -Dexec.args="arg1 arg2 arg3"

For arguments separated with a space, you can group using 'argument separated with space' inside the quotation marks.

mvn exec:java -Dexec.mainClass=test.Main -Dexec.args="'argument separated with space' 'another one'"

Object comparison in JavaScript

The following algorithm will deal with self-referential data structures, numbers, strings, dates, and of course plain nested javascript objects:

Objects are considered equivalent when

  • They are exactly equal per === (String and Number are unwrapped first to ensure 42 is equivalent to Number(42))
  • or they are both dates and have the same valueOf()
  • or they are both of the same type and not null and...
    • they are not objects and are equal per == (catches numbers/strings/booleans)
    • or, ignoring properties with undefined value they have the same properties all of which are considered recursively equivalent.

Functions are not considered identical by function text. This test is insufficient because functions may have differing closures. Functions are only considered equal if === says so (but you could easily extend that equivalent relation should you choose to do so).

Infinite loops, potentially caused by circular datastructures, are avoided. When areEquivalent attempts to disprove equality and recurses into an object's properties to do so, it keeps track of the objects for which this sub-comparison is needed. If equality can be disproved, then some reachable property path differs between the objects, and then there must be a shortest such reachable path, and that shortest reachable path cannot contain cycles present in both paths; i.e. it is OK to assume equality when recursively comparing objects. The assumption is stored in a property areEquivalent_Eq_91_2_34, which is deleted after use, but if the object graph already contains such a property, behavior is undefined. The use of such a marker property is necessary because javascript doesn't support dictionaries using arbitrary objects as keys.

function unwrapStringOrNumber(obj) {
    return (obj instanceof Number || obj instanceof String 
            ? obj.valueOf() 
            : obj);
function areEquivalent(a, b) {
    a = unwrapStringOrNumber(a);
    b = unwrapStringOrNumber(b);
    if (a === b) return true; //e.g. a and b both null
    if (a === null || b === null || typeof (a) !== typeof (b)) return false;
    if (a instanceof Date) 
        return b instanceof Date && a.valueOf() === b.valueOf();
    if (typeof (a) !== "object") 
        return a == b; //for boolean, number, string, xml

    var newA = (a.areEquivalent_Eq_91_2_34 === undefined),
        newB = (b.areEquivalent_Eq_91_2_34 === undefined);
    try {
        if (newA) a.areEquivalent_Eq_91_2_34 = [];
        else if (a.areEquivalent_Eq_91_2_34.some(
            function (other) { return other === b; })) return true;
        if (newB) b.areEquivalent_Eq_91_2_34 = [];
        else if (b.areEquivalent_Eq_91_2_34.some(
            function (other) { return other === a; })) return true;

        var tmp = {};
        for (var prop in a) 
            if(prop != "areEquivalent_Eq_91_2_34") 
                tmp[prop] = null;
        for (var prop in b) 
            if (prop != "areEquivalent_Eq_91_2_34") 
                tmp[prop] = null;

        for (var prop in tmp) 
            if (!areEquivalent(a[prop], b[prop]))
                return false;
        return true;
    } finally {
        if (newA) delete a.areEquivalent_Eq_91_2_34;
        if (newB) delete b.areEquivalent_Eq_91_2_34;

Chrome disable SSL checking for sites?

Mac Users please execute the below command from terminal to disable the certificate warning.

/Applications/Google\\ Chrome --ignore-certificate-errors --ignore-urlfetcher-cert-requests &> /dev/null

Note that this will also have Google Chrome mark all HTTPS sites as insecure in the URL bar.

Using SQL LOADER in Oracle to import CSV file

-- Step 1: Create temp table. create table Billing ( TAP_ID char(10), ACCT_NUM char(10));


-- Step 2: Create Control file.

load data infile IN_DATA.txt into table Billing fields terminated by ',' (TAP_ID, ACCT_NUM)

-- Step 3: Create input data file. IN_DATA.txt file content: 100,15678966

-- Step 4: Execute command from run: .. client\bin>sqlldr username@db-sis__id/password control='Billing.ctl'

What is a provisioning profile used for when developing iPhone applications?

A Quote from : iPhone Developer Program (~8MB PDF)

A provisioning profile is a collection of digital entities that uniquely ties developers and devices to an authorized iPhone Development Team and enables a device to be used for testing. A Development Provisioning Profile must be installed on each device on which you wish to run your application code. Each Development Provisioning Profile will contain a set of iPhone Development Certificates, Unique Device Identifiers and an App ID. Devices specified within the provisioning profile can be used for testing only by those individuals whose iPhone Development Certificates are included in the profile. A single device can contain multiple provisioning profiles.

How do I return multiple values from a function?

Python's tuples, dicts, and objects offer the programmer a smooth tradeoff between formality and convenience for small data structures ("things"). For me, the choice of how to represent a thing is dictated mainly by how I'm going to use the structure. In C++, it's a common convention to use struct for data-only items and class for objects with methods, even though you can legally put methods on a struct; my habit is similar in Python, with dict and tuple in place of struct.

For coordinate sets, I'll use a tuple rather than a point class or a dict (and note that you can use a tuple as a dictionary key, so dicts make great sparse multidimensional arrays).

If I'm going to be iterating over a list of things, I prefer unpacking tuples on the iteration:

for score,id,name in scoreAllTheThings():
    if score > goodScoreThreshold:
        print "%6.3f #%6d %s"%(score,id,name) the object version is more cluttered to read:

for entry in scoreAllTheThings():
    if entry.score > goodScoreThreshold:
        print "%6.3f #%6d %s"%(entry.score,,

...let alone the dict.

for entry in scoreAllTheThings():
    if entry['score'] > goodScoreThreshold:
        print "%6.3f #%6d %s"%(entry['score'],entry['id'],entry['name'])

If the thing is widely used, and you find yourself doing similar non-trivial operations on it in multiple places in the code, then it's usually worthwhile to make it a class object with appropriate methods.

Finally, if I'm going to be exchanging data with non-Python system components, I'll most often keep them in a dict because that's best suited to JSON serialization.

Sorting a vector of custom objects

In your class, you may overload the "<" operator.

class MyClass
  bool operator <(const MyClass& rhs)
    return this->key < rhs.key;

MVVM Passing EventArgs As Command Parameter

Here is a version of @adabyron's answer that prevents the leaky EventArgs abstraction.

First, the modified EventToCommandBehavior class (now a generic abstract class and formatted with ReSharper code cleanup). Note the new GetCommandParameter virtual method and its default implementation:

public abstract class EventToCommandBehavior<TEventArgs> : Behavior<FrameworkElement>
    where TEventArgs : EventArgs
    public static readonly DependencyProperty EventProperty = DependencyProperty.Register("Event", typeof(string), typeof(EventToCommandBehavior<TEventArgs>), new PropertyMetadata(null, OnEventChanged));
    public static readonly DependencyProperty CommandProperty = DependencyProperty.Register("Command", typeof(ICommand), typeof(EventToCommandBehavior<TEventArgs>), new PropertyMetadata(null));
    public static readonly DependencyProperty PassArgumentsProperty = DependencyProperty.Register("PassArguments", typeof(bool), typeof(EventToCommandBehavior<TEventArgs>), new PropertyMetadata(false));
    private Delegate _handler;
    private EventInfo _oldEvent;

    public string Event
        get { return (string)GetValue(EventProperty); }
        set { SetValue(EventProperty, value); }

    public ICommand Command
        get { return (ICommand)GetValue(CommandProperty); }
        set { SetValue(CommandProperty, value); }

    public bool PassArguments
        get { return (bool)GetValue(PassArgumentsProperty); }
        set { SetValue(PassArgumentsProperty, value); }

    protected override void OnAttached()

    protected virtual object GetCommandParameter(TEventArgs e)
        return e;

    private void AttachHandler(string eventName)
        _oldEvent?.RemoveEventHandler(AssociatedObject, _handler);

        if (string.IsNullOrEmpty(eventName))

        EventInfo eventInfo = AssociatedObject.GetType().GetEvent(eventName);

        if (eventInfo != null)
            MethodInfo methodInfo = typeof(EventToCommandBehavior<TEventArgs>).GetMethod("ExecuteCommand", BindingFlags.Instance | BindingFlags.NonPublic);

            _handler = Delegate.CreateDelegate(eventInfo.EventHandlerType, this, methodInfo);
            eventInfo.AddEventHandler(AssociatedObject, _handler);
            _oldEvent = eventInfo;
            throw new ArgumentException($"The event '{eventName}' was not found on type '{AssociatedObject.GetType().FullName}'.");

    private static void OnEventChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        var behavior = (EventToCommandBehavior<TEventArgs>)d;

        if (behavior.AssociatedObject != null)

    // ReSharper disable once UnusedMember.Local
    // ReSharper disable once UnusedParameter.Local
    private void ExecuteCommand(object sender, TEventArgs e)
        object parameter = PassArguments ? GetCommandParameter(e) : null;

        if (Command?.CanExecute(parameter) == true)

Next, an example derived class that hides DragCompletedEventArgs. Some people expressed concern about leaking the EventArgs abstraction into their view model assembly. To prevent this, I created an interface that represents the values we care about. The interface can live in the view model assembly with the private implementation in the UI assembly:

// UI assembly
public class DragCompletedBehavior : EventToCommandBehavior<DragCompletedEventArgs>
    protected override object GetCommandParameter(DragCompletedEventArgs e)
        return new DragCompletedArgs(e);

    private class DragCompletedArgs : IDragCompletedArgs
        public DragCompletedArgs(DragCompletedEventArgs e)
            Canceled = e.Canceled;
            HorizontalChange = e.HorizontalChange;
            VerticalChange = e.VerticalChange;

        public bool Canceled { get; }
        public double HorizontalChange { get; }
        public double VerticalChange { get; }

// View model assembly
public interface IDragCompletedArgs
    bool Canceled { get; }
    double HorizontalChange { get; }
    double VerticalChange { get; }

Cast the command parameter to IDragCompletedArgs, similar to @adabyron's answer.

Difference between F5, Ctrl + F5 and click on refresh button?

F5 is a standard page reload.


Ctrl + F5 refreshes the page by clearing the cached content of the page.

Having the cursor in the address field and pressing Enter will also do the same as Ctrl + F5.

Should __init__() call the parent class's __init__()?

Yes, you should always call base class __init__ explicitly as a good coding practice. Forgetting to do this can cause subtle issues or run time errors. This is true even if __init__ doesn't take any parameters. This is unlike other languages where compiler would implicitly call base class constructor for you. Python doesn't do that!

The main reason for always calling base class _init__ is that base class may typically create member variable and initialize them to defaults. So if you don't call base class init, none of that code would be executed and you would end up with base class that has no member variables.


class Base:
  def __init__(self):
    print('base init')

class Derived1(Base):
  def __init__(self):
    print('derived1 init')

class Derived2(Base):
  def __init__(self):
    super(Derived2, self).__init__()
    print('derived2 init')

print('Creating Derived1...')
d1 = Derived1()
print('Creating Derived2...')
d2 = Derived2()

This prints..

Creating Derived1...
derived1 init
Creating Derived2...
base init
derived2 init

Run this code.

How to add noise (Gaussian/salt and pepper etc) to image in Python with OpenCV

I don't know is there any method in Python API.But you can use this simple code to add Salt-and-Pepper noise to an image.

import numpy as np
import random
import cv2

def sp_noise(image,prob):
    Add salt and pepper noise to image
    prob: Probability of the noise
    output = np.zeros(image.shape,np.uint8)
    thres = 1 - prob 
    for i in range(image.shape[0]):
        for j in range(image.shape[1]):
            rdn = random.random()
            if rdn < prob:
                output[i][j] = 0
            elif rdn > thres:
                output[i][j] = 255
                output[i][j] = image[i][j]
    return output

image = cv2.imread('image.jpg',0) # Only for grayscale image
noise_img = sp_noise(image,0.05)
cv2.imwrite('sp_noise.jpg', noise_img)

How to check if a Docker image with a specific tag exist locally?

I usually test the result of docker images -q (as in this script):

if [[ "$(docker images -q myimage:mytag 2> /dev/null)" == "" ]]; then
  # do something

But since docker images only takes REPOSITORY as parameter, you would need to grep on tag, without using -q.

docker images takes tags now (docker 1.8+) [REPOSITORY[:TAG]]

The other approach mentioned below is to use docker inspect.
But with docker 17+, the syntax for images is: docker image inspect (on an non-existent image, the exit status will be non-0)

As noted by iTayb in the comments:

  • The docker images -q method can get really slow on a machine with lots of images. It takes 44s to run on a 6,500 images machine.
  • The docker image inspect returns immediately.

Handling the TAB character in Java

Or you could just perform a trim() on the string to handle the case when people use spaces instead of tabs (unless you are reading makefiles)

Array to Collection: Optimized code

You can use:


Java math function to convert positive int to negative and negative to positive?

The easiest thing to do is 0- the value

for instance if int i = 5;

0-i would give you -5

and if i was -6;

0- i would give you 6

How to find if element with specific id exists or not

You can simply use if(yourElement)

var a = document.getElementById("elemA");_x000D_
var b = document.getElementById("elemB");_x000D_
  console.log("elemA exists");_x000D_
  console.log("elemA does not exist");_x000D_
  console.log("elemB exists");_x000D_
  console.log("elemB does not exist");
<div id="elemA"></div>

How do I center an SVG in a div?

Having read above that svg is inline by default, I just added the following to the div:

<div style="text-align:center;">

and it did the trick for me.

Purists may not like it (it’s an image, not text) but in my opinion HTML and CSS screwed up over centring, so I think it’s justified.

How can I rollback an UPDATE query in SQL server 2005?

You need this tool and you can find the transaction and reverse it.

ApexSQL Log

What's the difference between .bashrc, .bash_profile, and .environment?

A good place to look at is the man page of bash. Here's an online version. Look for "INVOCATION" section.

How to initialize an array in Java?


 Datatype[] variable = new Datatype[] { value1,value2.... }

 Datatype variable[]  = new Datatype[] { value1,value2.... }

Example :

int [] points = new int[]{ 1,2,3,4 };

Technically what is the main difference between Oracle JDK and OpenJDK?

Technical differences are a consequence of the goal of each one (OpenJDK is meant to be the reference implementation, open to the community, while Oracle is meant to be a commercial one)

They both have "almost" the same code of the classes in the Java API; but the code for the virtual machine itself is actually different, and when it comes to libraries, OpenJDK tends to use open libraries while Oracle tends to use closed ones; for instance, the font library.

Angular 4 HttpClient Query Parameters

You can pass it like this

let param: any = {'userId': 2};
this.http.get(`${ApiUrl}`, {params: param})

How to change button color with tkinter

Another way to change color of a button if you want to do multiple operations along with color change. Using the Tk().after method and binding a change method allows you to change color and do other operations.

Label.destroy is another example of the after method.

    def export_win():
        //Some Operation
        orig_color = export_finding_graph.cget("background")
        export_finding_graph.configure(background = "green")

        tt = "Exported"
        label = Label(tab1_closed_observations, text=tt, font=("Helvetica", 12))

        def change(orig_color):
            export_finding_graph.configure(background = orig_color)

        tab1_closed_observations.after(1000, lambda: change(orig_color))
        tab1_closed_observations.after(500, label.destroy)

    export_finding_graph = Button(tab1_closed_observations, text='Export', command=export_win)

You can also revert to the original color.

java.lang.RuntimeException: Unable to start activity ComponentInfo

Dear You have used two Intent launcher in your Manifest. Make only one Activity as launcher: Your manifest activity is :

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android=""
    android:versionName="1.0" >
    <uses-sdk android:minSdkVersion="8" />
        android:label="@string/app_name" >
            android:label="@string/app_name" >
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.LAUNCHER" />
            android:label="@string/app_name" >
                <action android:name="android.intent.action.MAIN" />
                <category android:name="android.intent.category.ALTERNATIVE" />

now write code will be ( i have made your 'MyActivityBook' your default activity launcher. Copy and paste it on your manifest.

<?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android=""
        android:versionName="1.0" >
        <uses-sdk android:minSdkVersion="8" />
            android:label="@string/app_name" >
                android:label="@string/app_name" >

                android:label="@string/app_name" >
                    <action android:name="android.intent.action.MAIN" />
                    <category android:name="android.intent.category.LAUNCHER" />

and Second error may be if you copy paste old code then please update com.example.packagename.FILE_NAME

hope this will work !

how to change language for DataTable

for Arabic language

 var table = $('#my_table')
                            "sProcessing": "???? ???????...",
                            "sLengthMenu": "???? _MENU_ ??????",
                            "sZeroRecords": "?? ???? ??? ??? ?????",
                            "sInfo": "????? _START_ ??? _END_ ?? ??? _TOTAL_ ????",
                            "sInfoEmpty": "???? 0 ??? 0 ?? ??? 0 ???",
                            "sInfoFiltered": "(?????? ?? ????? _MAX_ ?????)",
                            "sInfoPostFix": "",
                            "sSearch": "????:",
                            "sUrl": "",
                            "oPaginate": {
                                "sFirst": "?????",
                                "sPrevious": "??????",
                                "sNext": "??????",
                                "sLast": "??????"


Author: Ossama Khayat

Accessing the web page's HTTP Headers in JavaScript

If we're talking about Request headers, you can create your own headers when doing XmlHttpRequests.

var request = new XMLHttpRequest();
request.setRequestHeader("X-Requested-With", "XMLHttpRequest");"GET", path, true);

Color text in discord

Discord doesn't allow colored text. Though, currently, you have two options to "mimic" colored text.

Option #1 (Markdown code-blocks)

Discord supports Markdown and uses highlight.js to highlight code-blocks. Some programming languages have specific color outputs from highlight.js and can be used to mimic colored output.

To use code-blocks, send a normal message in this format (Which follows Markdown's standard format).


Languages that currently reproduce nice colors: prolog (red/orange), css (yellow).

Option #2 (Embeds)

Discord now supports Embeds and Webhooks, which can be used to display colored blocks, they also support markdown. For documentation on how to use Embeds, please read your lib's documentation.

(Embed Cheat-sheet)
Embed Cheat-sheet

Check if table exists

 * Method that checks if all tables exist
 * If a table doesnt exist it creates the table
public void checkTables() {
    try {
        startConn();// method that connects with mysql database
        String useDatabase = "USE " + getDatabase() + ";";
        String[] tables = {"Patients", "Procedures", "Payments", "Procedurables"};//thats table names that I need to create if not exists
        DatabaseMetaData metadata = conn.getMetaData();

        for(int i=0; i< tables.length; i++) {
            ResultSet rs = metadata.getTables(null, null, tables[i], null);
            if(! {
                System.out.println("Table " + tables[i] + " created");
    } catch(SQLException e) {
        System.out.println("checkTables() " + e.getMessage());
    closeConn();// Close connection with mysql database

How can I check whether Google Maps is fully loaded?

If you're using web components, then they have this as an example:

map.addEventListener('google-map-ready', function(e) {
   alert('Map loaded!');

Call removeView() on the child's parent first

What I was doing wrong so I got this error is I wasn't instantiating dynamic layout and adding childs to it so got this error

Could not resolve Spring property placeholder

Ensure 'idm.url' is set in property file and the property file is loaded

HTML CSS How to stop a table cell from expanding

Simply set the max-width attribute to 280px like this:

<td align="left" valign="top" style="overflow:hidden;" nowrap="nowrap" max-width="280px" width="280px">

This will solve your problem.

Can't run Curl command inside my Docker Container

If you are using an Alpine based image, you have to

... \
apk add --no-cache curl \
curl ...

Finding repeated words on a string and counting the repetitions

public class Counter {

private static final int COMMA_AND_SPACE_PLACE = 2;

private String mTextToCount;
private ArrayList<String> mSeparateWordsList;

public Counter(String mTextToCount) {
    this.mTextToCount = mTextToCount;

    mSeparateWordsList = cutStringIntoSeparateWords(mTextToCount);

private ArrayList<String> cutStringIntoSeparateWords(String text)
    ArrayList<String> returnedArrayList = new ArrayList<>();

    if(text.indexOf(',') == -1)
        return returnedArrayList;

    int position1 = 0;
    int position2 = 0;

    while(position2 < text.length())
        char c = ',';
        if(text.toCharArray()[position2] == c)
            String tmp = text.substring(position1, position2);
            position1 += tmp.length() + COMMA_AND_SPACE_PLACE;

    if(position1 < position2)
        returnedArrayList.add(text.substring(position1, position2));

    return returnedArrayList;

public int[] countWords()
    if(mSeparateWordsList == null) return null;

    HashMap<String, Integer> wordsMap = new HashMap<>();

    for(String s: mSeparateWordsList)
        int cnt;

            cnt = wordsMap.get(s);
        } else {
            cnt = 1;
        wordsMap.put(s, cnt);
    return printCounterResults(wordsMap);

private int[] printCounterResults(HashMap<String, Integer> m)
    int index = 0;
    int[] returnedIntArray = new int[m.size()];

    for(int i: m.values())
        returnedIntArray[index] = i;

    return returnedIntArray;



How to get the directory of the currently running file?

EDIT: As of Go 1.8 (Released February 2017) the recommended way of doing this is with os.Executable:

func Executable() (string, error)

Executable returns the path name for the executable that started the current process. There is no guarantee that the path is still pointing to the correct executable. If a symlink was used to start the process, depending on the operating system, the result might be the symlink or the path it pointed to. If a stable result is needed, path/filepath.EvalSymlinks might help.

To get just the directory of the executable you can use path/filepath.Dir.


package main

import (

func main() {
    ex, err := os.Executable()
    if err != nil {
    exPath := filepath.Dir(ex)


You should be able to use os.Getwd

func Getwd() (pwd string, err error)

Getwd returns a rooted path name corresponding to the current directory. If the current directory can be reached via multiple paths (due to symbolic links), Getwd may return any one of them.

For example:

package main

import (

func main() {
    pwd, err := os.Getwd()
    if err != nil {

What is a magic number, and why is it bad?

I assume this is a response to my answer to your earlier question. In programming, a magic number is an embedded numerical constant that appears without explanation. If it appears in two distinct locations, it can lead to circumstances where one instance is changed and not another. For both these reasons, it's important to isolate and define the numerical constants outside the places where they're used.

How to Flatten a Multidimensional Array?

If you have an array of objects and want to flatten it with a node, just use this function:

function objectArray_flatten($array,$childField) {
    $result = array();
    foreach ($array as $node)
        $result[] = $node;
            $result = array_merge(

    return $result;

How to access the GET parameters after "?" in Express?

const express = require('express')
const bodyParser = require('body-parser')
const { usersNdJobs, userByJob, addUser , addUserToCompany } = require ('./db/db.js')

const app = express()
app.set('view engine', 'pug')
app.use(bodyParser.urlencoded({ extended: false }))

app.get('/', (req, res) => {
    .then((users) => {
      res.render('users', { users })

app.get('/api/company/users', (req, res) => {
  const companyname = req.query.companyName
    .then((users) => {
      res.render('job', { users })
})'/api/users/add', (req, res) => {
  const userName = req.body.userName
  const jobName = req.body.jobName
  console.log("user name = "+userName+", job name : "+jobName)
  addUser(userName, jobName)
    .then((result) => {
    .catch((error) => {
      res.status(404).json({ 'message': error.toString() })
})'/users/add', (request, response) => {
  const { userName, job } = request.body
  addTeam(userName, job)
  .then((user) => {
      "city": user.job
  .catch((err) => {
    request.status(400).json({"message": err})
})'/api/user/company/add', (req, res) => {
  const userName = req.body.userName
  const companyName = req.body.companyName
  console.log(userName, companyName)
  addUserToCompany(userName, companyName)
  .then((result) => {

app.get('/api/company/user', (req, res) => {
 const companyname = req.query.companyName
 .then((users) => {
   res.render('jobs', { users })

app.listen(3000, () =>
  console.log('Example app listening on port 3000!')

What does map(&:name) mean in Ruby?

It's equivalent to

def tag_names
  @tag_names || { |tag| }.join(' ')

How to create a user in Django?

The correct way to create a user in Django is to use the create_user function. This will handle the hashing of the password, etc..

from django.contrib.auth.models import User
user = User.objects.create_user(username='john',
                                 email='[email protected]',
                                 password='glass onion')

Convert String XML fragment to Document Node in Java

Element node =  DocumentBuilderFactory
    .parse(new ByteArrayInputStream("<node>value</node>".getBytes()))

If you can decode JWT, how are they secure?

Only JWT's privateKey, which is on your server will decrypt the encrypted JWT. Those who know the privateKey will be able to decrypt the encrypted JWT.

Hide the privateKey in a secure location in your server and never tell anyone the privateKey.

window.onload vs $(document).ready()

window.onload: A normal JavaScript event.

document.ready: A specific jQuery event when the entire HTML has been loaded.

Are SSL certificates bound to the servers ip address?

Most SSL certificates are bound to the hostname of the machine and not the ip address.

You might get a better answer if you ask this question on

Why does Eclipse Java Package Explorer show question mark on some classes?

In a enabled project the small question mark (?) indicates that your file is not yet added to the SVN repository.

The small orange rectangle is an indication that your file is committed in the repository.

An asterisk (*) indicates a local change.

Returning unique_ptr from functions

One thing that i didn't see in other answers is To clarify another answers that there is a difference between returning std::unique_ptr that has been created within a function, and one that has been given to that function.

The example could be like this:

class Test
{int i;};
std::unique_ptr<Test> foo1()
    std::unique_ptr<Test> res(new Test);
    return res;
std::unique_ptr<Test> foo2(std::unique_ptr<Test>&& t)
    // return t;  // this will produce an error!
    return std::move(t);

auto test1=foo1();
auto test2=foo2(std::unique_ptr<Test>(new Test));

How to execute python file in linux

You have to add a shebang. A shebang is the first line of the file. Its what the system is looking for in order to execute a file.

It should look like that :

#!/usr/bin/env python

or the real path


You should also check the file have the right to be execute. chmod +x

As Fabian said, take a look to Wikipedia : Wikipedia - Shebang (en)

Managing jQuery plugin dependency in webpack

You've mixed different approaches how to include legacy vendor modules. This is how I'd tackle it:

1. Prefer unminified CommonJS/AMD over dist

Most modules link the dist version in the main field of their package.json. While this is useful for most developers, for webpack it is better to alias the src version because this way webpack is able to optimize dependencies better (e.g. when using the DedupePlugin).

// webpack.config.js

module.exports = {
    resolve: {
        alias: {
            jquery: "jquery/src/jquery"

However, in most cases the dist version works just fine as well.

2. Use the ProvidePlugin to inject implicit globals

Most legacy modules rely on the presence of specific globals, like jQuery plugins do on $ or jQuery. In this scenario you can configure webpack, to prepend var $ = require("jquery") everytime it encounters the global $ identifier.

var webpack = require("webpack");


    plugins: [
        new webpack.ProvidePlugin({
            $: "jquery",
            jQuery: "jquery"

3. Use the imports-loader to configure this

Some legacy modules rely on this being the window object. This becomes a problem when the module is executed in a CommonJS context where this equals module.exports. In this case you can override this with the imports-loader.

Run npm i imports-loader --save-dev and then

module: {
    loaders: [
            test: /[\/\\]node_modules[\/\\]some-module[\/\\]index\.js$/,
            loader: "imports-loader?this=>window"

The imports-loader can also be used to manually inject variables of all kinds. But most of the time the ProvidePlugin is more useful when it comes to implicit globals.

4. Use the imports-loader to disable AMD

There are modules that support different module styles, like AMD, CommonJS and legacy. However, most of the time they first check for define and then use some quirky code to export properties. In these cases, it could help to force the CommonJS path by setting define = false.

module: {
    loaders: [
            test: /[\/\\]node_modules[\/\\]some-module[\/\\]index\.js$/,
            loader: "imports-loader?define=>false"

5. Use the script-loader to globally import scripts

If you don't care about global variables and just want legacy scripts to work, you can also use the script-loader. It executes the module in a global context, just as if you had included them via the <script> tag.

6. Use noParse to include large dists

When there is no AMD/CommonJS version of the module and you want to include the dist, you can flag this module as noParse. Then webpack will just include the module without parsing it, which can be used to improve the build time. This means that any feature requiring the AST, like the ProvidePlugin, will not work.

module: {
    noParse: [

IFrame: This content cannot be displayed in a frame

use <meta http-equiv="X-Frame-Options" content="allow"> in the one to show in the iframe to allow it.

Hide Button After Click (With Existing Form on Page)

CSS code:



Html code:

<button onclick="block_none()">Check Availability</button>

Javascript Code:

function block_none(){

JQuery: if div is visible

You can use .is(':visible')

Selects all elements that are visible.

For example:


Also, you can get the div which is visible by:


Live example:

#selectDiv {_x000D_
  display: none;  _x000D_
<script src=""></script>_x000D_
<div id="selectDiv"></div>_x000D_
<div id="visibleDiv"></div>

What's the best practice using a settings file in Python?

Yaml and Json are the simplest and most commonly used file formats to store settings/config. PyYaml can be used to parse yaml. Json is already part of python from 2.5. Yaml is a superset of Json. Json will solve most uses cases except multi line strings where escaping is required. Yaml takes care of these cases too.

>>> import json
>>> config = {'handler' : '', 'timeoutsec' : 5 }
>>> json.dump(config, open('/tmp/config.json', 'w'))
>>> json.load(open('/tmp/config.json'))   
{u'handler': u'', u'timeoutsec': 5}

mysqldump exports only one table

Quoting this link:

  • Exporting the Table

To export the table run the following command from the command line:

mysqldump -p --user=username dbname tableName > tableName.sql

This will export the tableName to the file tableName.sql.

  • Importing the Table

To import the table run the following command from the command line:

mysql -u username -p -D dbname < tableName.sql

The path to the tableName.sql needs to be prepended with the absolute path to that file. At this point the table will be imported into the DB.

Streaming a video file to an html5 video player with Node.js so that the video controls continue to work?

The accepted answer to this question is awesome and should remain the accepted answer. However I ran into an issue with the code where the read stream was not always being ended/closed. Part of the solution was to send autoClose: true along with start:start, end:end in the second createReadStream arg.

The other part of the solution was to limit the max chunksize being sent in the response. The other answer set end like so:

var end = positions[1] ? parseInt(positions[1], 10) : total - 1;

...which has the effect of sending the rest of the file from the requested start position through its last byte, no matter how many bytes that may be. However the client browser has the option to only read a portion of that stream, and will, if it doesn't need all of the bytes yet. This will cause the stream read to get blocked until the browser decides it's time to get more data (for example a user action like seek/scrub, or just by playing the stream).

I needed this stream to be closed because I was displaying the <video> element on a page that allowed the user to delete the video file. However the file was not being removed from the filesystem until the client (or server) closed the connection, because that is the only way the stream was getting ended/closed.

My solution was just to set a maxChunk configuration variable, set it to 1MB, and never pipe a read a stream of more than 1MB at a time to the response.

// same code as accepted answer
var end = positions[1] ? parseInt(positions[1], 10) : total - 1;
var chunksize = (end - start) + 1;

// poor hack to send smaller chunks to the browser
var maxChunk = 1024 * 1024; // 1MB at a time
if (chunksize > maxChunk) {
  end = start + maxChunk - 1;
  chunksize = (end - start) + 1;

This has the effect of making sure that the read stream is ended/closed after each request, and not kept alive by the browser.

I also wrote a separate StackOverflow question and answer covering this issue.

iPhone hide Navigation Bar only on first page

One slight tweak I had to make on the other answers is to only unhide the bar in viewWillDisappear if the reason it is disappearing is due to a navigation item being pushed on it. This is because the view can disappear for other reasons.

So I only unhide the bar if this view is no longer the topmost view:

- (void) viewWillDisappear:(BOOL)animated
    if (self.navigationController.topViewController != self)
        [self.navigationController setNavigationBarHidden:NO animated:animated];

    [super viewWillDisappear:animated];

Mongoimport of json file

Run the import command in another terminal. (not inside mongo shell.)

mongoimport --db test --collection user --drop --file ~/downloads/user.json

How do I pass an object from one activity to another on Android?

Maybe it's an unpopular answer, but in the past I've simply used a class that has a static reference to the object I want to persist through activities. So,

public class PersonHelper
    public static Person person;

I tried going down the Parcelable interface path, but ran into a number of issues with it and the overhead in your code was unappealing to me.

Basic http file downloading and saving to disk in python?

Four methods using wget, urllib and request.

import requests
from StringIO import StringIO
from PIL import Image
import profile as profile
import urllib
import wget

url = ''

def testRequest():
    image_name = 'test1.jpg'
    r = requests.get(url, stream=True)
    with open(image_name, 'wb') as f:
        for chunk in r.iter_content():

def testRequest2():
    image_name = 'test2.jpg'
    r = requests.get(url)
    i =

def testUrllib():
    image_name = 'test3.jpg'
    testfile = urllib.URLopener()
    testfile.retrieve(url, image_name)

def testwget():
    image_name = 'test4.jpg', image_name)

if __name__ == '__main__':'testRequest()')'testRequest2()')'testUrllib()')'testwget()')

testRequest - 4469882 function calls (4469842 primitive calls) in 20.236 seconds

testRequest2 - 8580 function calls (8574 primitive calls) in 0.072 seconds

testUrllib - 3810 function calls (3775 primitive calls) in 0.036 seconds

testwget - 3489 function calls in 0.020 seconds

How can I add spaces between two <input> lines using CSS?

You can also wrap your text in label fields, so your form will be more self-explainable semantically.

Just remember to float labels and inputs to the left and to add a specific width to them, and the containing form. Then you can add margins to both of them, to adjust the spacing between the lines (you understand, of course, that this is a pretty minimal markup that expects content to be as big as to some limit).

That way you wont have to add any more elements, just the label-input pairs, all of them wrapped in a form element.

For example:

<label for="txtName">Name</label>
<input id"txtName" type="text">
<label for="txtEmail">Email</label>
<input id"txtEmail" type="text">
<label for="txtAddress">Address</label>
<input id"txtAddress" type="text">
<input type="submit" value="Submit The Form">

And the css will be:

float:left; /*to clear the floats of inner elements,usefull if you wanna add a border or background image*/
margin-bottom:10px; /*or whatever you want the spacing to be*/
margin-bottom:10px; /*or whatever you want the spacing to be*/

Display the current time and date in an Android application

Calendar c = Calendar.getInstance();
int month=c.get(Calendar.MONTH)+1;
String sDate = c.get(Calendar.YEAR) + "-" + month+ "-" + c.get(Calendar.DAY_OF_MONTH) +
"T" + c.get(Calendar.HOUR_OF_DAY)+":"+c.get(Calendar.MINUTE)+":"+c.get(Calendar.SECOND);

This will give date time format like 2010-05-24T18:13:00

Creating a UITableView Programmatically

- (void)viewDidLoad {
     [super viewDidLoad];
     arr=[[NSArray alloc]initWithObjects:@"ABC",@"XYZ", nil];
     tableview = [[UITableView alloc]initWithFrame:tableFrame style:UITableViewStylePlain];   
     tableview.delegate = self;
     tableview.dataSource = self;
     [self.view addSubview:tableview];

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
    return arr.count;

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"MyCell"];

    if(cell == nil)
        cell = [[UITableViewCell alloc]initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"MyCell"];

    cell.textLabel.text=[arr objectAtIndex:indexPath.row];

    return cell;

How to convert from java.sql.Timestamp to java.util.Date?


Instant instant = myResultSet.getObject( … , Instant.class ) ;

…or, if your JDBC driver does not support the optional Instant, it is required to support OffsetDateTime:

OffsetDateTime odt = myResultSet.getObject( … , OffsetDateTime.class ) ;

Avoid both java.util.Date & java.sql.Timestamp. They have been replaced by the java.time classes. Specifically, the Instant class representing a moment on the timeline in UTC with a resolution of nanoseconds (up to nine (9) digits of a decimal fraction).

Different Values ? Unverified Problem

To address the main part of the Question: "Why different dates between java.util.Date and java.sql.Timestamp objects when one is derived from the other?"

There must be a problem with your code. You did not post your code, so we cannot pinpoint the problem.

First, that string value you show for value of java.util.Date did not come from its default toString method, so you obviously were doing additional operations.

Secondly, when I run similar code I do indeed get exact same date-time values.

First create a java.sql.Timestamp object.

// Timestamp
long millis1 =  new java.util.Date().getTime();
java.sql.Timestamp ts = new java.sql.Timestamp(millis1);

Now extract the count-of-milliseconds-since-epoch to instantiate a java.util.Date object.

// Date
long millis2 = ts.getTime();
java.util.Date date = new java.util.Date( millis2 );

Dump values to console.

System.out.println("millis1 = " + millis1 );
System.out.println("ts = " + ts );
System.out.println("millis2 = " + millis2 );
System.out.println("date = " + date );

When run.

millis1 = 1434666385642
ts = 2015-06-18 15:26:25.642
millis2 = 1434666385642
date = Thu Jun 18 15:26:25 PDT 2015

So the code shown in the Question is indeed a valid way to convert from java.sql.Timestamp to java.util.Date, though you will lose any nanoseconds data.

java.util.Date someDate = new Date( someJUTimestamp.getTime() ); 

Different Formats Of String Output

Note that the output of the toString methods is a different format, as documented. The java.sql.Timestamp follows SQL format, similar to ISO 8601 format but without the T in middle.

Ignore Inheritance

As discussed on comments on other Answers and the Question, you should ignore the fact that java.sql.Timestamp inherits from java.util.Date. The j.s.Timestamp doc clearly states that you should not view one as a sub-type of the other: (emphasis mine)

Due to the differences between the Timestamp class and the java.util.Date class mentioned above, it is recommended that code not view Timestamp values generically as an instance of java.util.Date. The inheritance relationship between Timestamp and java.util.Date really denotes implementation inheritance, and not type inheritance.

If you ignore the Java team’s advice and take such a view, one critical problem is that you will lose data: any microsecond or nanosecond part of a second that may be coming from the database is lost as a Date has only millisecond resolution.

Basically, all the old date-time classes from early Java are a big mess: java.util.Date, j.u.Calendar, java.text.SimpleDateFormat, java.sql.Timestamp/.Date/.Time. They were one of the first valiant efforts at a date-time framework in the industry, but ultimately they fail. Specifically here, java.sql.Timestamp is a java.util.Date with nanoseconds tacked on; this is a hack, not good design.


Avoid the old date-time classes bundled with early versions of Java.

Instead use the java.time package (Tutorial) built into Java 8 and later whenever possible.

Basics of java.time… An Instant is a moment on the timeline in UTC. Apply a time zone (ZoneId) to get a ZonedDateTime.

Example code using java.time as of Java 8. With a JDBC driver supporting JDBC 4.2 and later, you can directly exchange java.time classes with your database; no need for the legacy classes.

Instant instant = myResultSet.getObject( … , Instant.class) ;  // Instant is the raw underlying data, an instantaneous point on the time-line stored as a count of nanoseconds since epoch.

You may want to adjust into a time zone other than UTC.

ZoneId z = ZoneId.of( "America/Montreal" );  // Always make time zone explicit rather than relying implicitly on the JVM’s current default time zone being applied.
ZonedDateTime zdt = instant.atZone( z ) ;

Perform your business logic. Here we simply add a day.

ZonedDateTime zdtNextDay = zdt.plusDays( 1 ); // Add a day to get "day after".

At the last stage, if absolutely needed, convert to a java.util.Date for interoperability.

java.util.Date dateNextDay = Date.from( zdtNextDay.toInstant( ) );  // WARNING: Losing data (the nanoseconds resolution).

Dump to console.

System.out.println( "instant = " + instant );
System.out.println( "zdt = " + zdt );
System.out.println( "zdtNextDay = " + zdtNextDay );
System.out.println( "dateNextDay = " + dateNextDay );

When run.

instant = 2015-06-18T16:44:13.123456789Z
zdt = 2015-06-18T19:44:13.123456789-04:00[America/Montreal]
zdtNextDay = 2015-06-19T19:44:13.123456789-04:00[America/Montreal]
dateNextDay = Fri Jun 19 16:44:13 PDT 2015


If you must use the legacy types to interface with old code not yet updated for java.time, you may convert. Use new methods added to the old java.util.Date and java.sql.* classes for conversion.

Instant instant = myJavaSqlTimestamp.toInstant() ;


java.sql.Timestamp ts = java.sql.Timestamp.from( instant ) ;

See the Tutorial chapter, Legacy Date-Time Code, for more info on conversions.

Fractional Second

Be aware of the resolution of the fractional second. Conversions from nanoseconds to milliseconds means potentially losing some data.

About java.time

The java.time framework is built into Java 8 and later. These classes supplant the troublesome old legacy date-time classes such as java.util.Date, Calendar, & SimpleDateFormat.

The Joda-Time project, now in maintenance mode, advises migration to the java.time classes.

To learn more, see the Oracle Tutorial. And search Stack Overflow for many examples and explanations. Specification is JSR 310.

You may exchange java.time objects directly with your database. Use a JDBC driver compliant with JDBC 4.2 or later. No need for strings, no need for java.sql.* classes.

Where to obtain the java.time classes?

The ThreeTen-Extra project extends java.time with additional classes. This project is a proving ground for possible future additions to java.time. You may find some useful classes here such as Interval, YearWeek, YearQuarter, and more.

VBA error 1004 - select method of range class failed

You have to select the sheet before you can select the range.

I've simplified the example to isolate the problem. Try this:

Option Explicit

Sub RangeError()

    Dim sourceBook As Workbook
    Dim sourceSheet As Worksheet
    Dim sourceSheetSum As Worksheet

    Set sourceBook = ActiveWorkbook
    Set sourceSheet = sourceBook.Sheets("Sheet1")
    Set sourceSheetSum = sourceBook.Sheets("Sheet2")


    sourceSheetSum.Range("C3").Select           'THIS IS THE PROBLEM LINE

End Sub

Replace Sheet1 and Sheet2 with your sheet names.

IMPORTANT NOTE: Using Variants is dangerous and can lead to difficult-to-kill bugs. Use them only if you have a very specific reason for doing so.

Calculate execution time of a SQL query?

You can use


Displays the number of milliseconds required to parse, compile, and execute each statement

When SET STATISTICS TIME is ON, the time statistics for a statement are displayed. When OFF, the time statistics are not displayed

USE AdventureWorks2012;  
SELECT ProductID, StartDate, EndDate, StandardCost   
FROM Production.ProductCostHistory  
WHERE StandardCost < 500.00;  

Bootstrap 4 - Inline List?


Remove a list’s bullets and apply some light margin with a combination of two classes, .list-inline and .list-inline-item.

<ul class="list-inline">
   <li class="list-inline-item"><a class="social-icon text-xs-center" target="_blank" href="#">FB</a></li>
   <li class="list-inline-item"><a class="social-icon text-xs-center" target="_blank" href="#">G+</a></li>
   <li class="list-inline-item"><a class="social-icon text-xs-center" target="_blank" href="#">T</a></li>

How can I find out if I have Xcode commandline tools installed?

For macOS catalina try this : open Xcode. if not existing. download from App store (about 11GB) then open Xcode>open developer tool>more developer tool and used my apple id to download a compatible command line tool. Then, after downloading, I opened Xcode>Preferences>Locations>Command Line Tool and selected the newly downloaded command line tool from downloads.

Why does this AttributeError in python occur?

AttributeError is raised when attribute of the object is not available.

An attribute reference is a primary followed by a period and a name:

attributeref ::= primary "." identifier

To return a list of valid attributes for that object, use dir(), e.g.:


So probably you need to do simply: import scipy.sparse

How to convert .crt to .pem

You can do this conversion with the OpenSSL library

Windows binaries can be found here:

Once you have the library installed, the command you need to issue is:

openssl x509 -in mycert.crt -out mycert.pem -outform PEM

Unable to auto-detect email address

it's pretty simple but tricky at the first time.
For example:
If my email is [email protected] type:

git config --global [email protected]


If my username is mrsuicidesheep type:

git config mrsuicidesheep

ASP.NET Background image

Use this Code in code behind

Div_Card.Style["background-image"] = Page.ResolveUrl(Session["Img_Path"].ToString());

JDBC connection to MSSQL server in windows authentication mode

After struggling a lot, I finally found a solution, here we go -

Download the file jtds-1.3.1.jar and ntlmauth.dll and save it in Program File -> Java -> JDK -> jre -> bin.

Then use the following code -

String pPSSDBDriverName = "";
conn = DriverManager.getConnection("jdbc:jtds:sqlserver://<ur_server:port>;UseNTLMv2=true;Domain=AD;Trusted_Connection=yes");
stmt = conn.createStatement();
String sql = " DELETE FROM <data> where <condition>;

The ALTER TABLE statement conflicted with the FOREIGN KEY constraint

You should see if your tables has any data on the rows. If "yes" then you should truncate the table(s) or else you can make them to have the same number of data at tblDomare.PersNR to tblBana.BanNR and vise-verse.

Is there a way to list all resources in AWS

You can run advanced queries via AWS Config (and from the CLI for Config), that will list all resources. If you define an aggregator that covers all reasons (and perhaps multiple accounts), you can get a very comprehensive view . . . As simple as "SELECT *"

Return index of highest value in an array

My solution is:

$maxs = array_keys($array, max($array))

this way you can retrieve every key related to a given max value.

If you are interested only in one key among all simply use $maxs[0]

How can I get this ASP.NET MVC SelectList to work?

Using the constructor that accepts items, dataValueField, dataTextField, selectedValue as parameters :

ViewData["myList"] = 
                new SelectList(new[] { "10", "15", "25", "50", "100", "1000" }
                .Select(x => new {value = x, text = x}), 
                "value", "text", "15");

Then in your view :

<%=Html.DropDownList("myList") %>

How to pass parameters to a Script tag?

Create an attribute that contains a list of the parameters, like so:

<script src="http://path/to/widget.js" data-params="1, 3"></script>

Then, in your JavaScript, get the parameters as an array:

var script = document.currentScript || 

var params = (script.getAttribute('data-params') || '').split(/, */);

params[0]; // -> 1
params[1]; // -> 3

How to run a program automatically as admin on Windows 7 at startup?

You can do this by installing the task while running as administrator via the TaskSchedler library. I'm making the assumption here that .NET/C# is a suitable platform/language given your related questions.

This library gives you granular access to the Task Scheduler API, so you can adjust settings that you cannot otherwise set via the command line by calling schtasks, such as the priority of the startup. Being a parental control application, you'll want it to have a startup priority of 0 (maximum), which schtasks will create by default a priority of 7.

Below is a code example of installing a properly configured startup task to run the desired application as administrator indefinitely at logon. This code will install a task for the very process that it's running from.

Copyright © 2017 Jesse Nicholson  
This Source Code Form is subject to the terms of the Mozilla Public
License, v. 2.0. If a copy of the MPL was not distributed with this
file, You can obtain one at

/// <summary>
/// Used for synchronization when creating run at startup task.
/// </summary>
private ReaderWriterLockSlim m_runAtStartupLock = new ReaderWriterLockSlim();

public void EnsureStarupTaskExists()

        using(var ts = new Microsoft.Win32.TaskScheduler.TaskService())
            // Start off by deleting existing tasks always. Ensure we have a clean/current install of the task.
            ts.RootFolder.DeleteTask(Process.GetCurrentProcess().ProcessName, false);

            // Create a new task definition and assign properties
            using(var td = ts.NewTask())
                td.Principal.RunLevel = Microsoft.Win32.TaskScheduler.TaskRunLevel.Highest;
                // This is not normally necessary. RealTime is the highest priority that
                // there is.
                td.Settings.Priority = ProcessPriorityClass.RealTime;
                td.Settings.DisallowStartIfOnBatteries = false;
                td.Settings.StopIfGoingOnBatteries = false;
                td.Settings.WakeToRun = false;
                td.Settings.AllowDemandStart = false;
                td.Settings.IdleSettings.RestartOnIdle = false;                    
                td.Settings.IdleSettings.StopOnIdleEnd = false;
                td.Settings.RestartCount = 0;                    
                td.Settings.AllowHardTerminate = false;
                td.Settings.Hidden = true;
                td.Settings.Volatile = false;
                td.Settings.Enabled = true;
                td.Settings.Compatibility = Microsoft.Win32.TaskScheduler.TaskCompatibility.V2;
                td.Settings.ExecutionTimeLimit = TimeSpan.Zero;

                td.RegistrationInfo.Description = "Runs the content filter at startup.";

                // Create a trigger that will fire the task at this time every other day
                var logonTrigger = new Microsoft.Win32.TaskScheduler.LogonTrigger();
                logonTrigger.Enabled = true;                    
                logonTrigger.Repetition.StopAtDurationEnd = false;
                logonTrigger.ExecutionTimeLimit = TimeSpan.Zero;

                // Create an action that will launch Notepad whenever the trigger fires
                td.Actions.Add(new Microsoft.Win32.TaskScheduler.ExecAction(Process.GetCurrentProcess().MainModule.FileName, "/StartMinimized", null));

                // Register the task in the root folder
                ts.RootFolder.RegisterTaskDefinition(Process.GetCurrentProcess().ProcessName, td);

How to create EditText accepts Alphabets only in android?

Through Xml you can do easily as type following code in xml (editText)...


only characters will be accepted...

Bridged networking not working in Virtualbox under Windows 10

In Case some one is looking and none of the above resolves your issue :

placing the WIFI as the first adapter [MTDesktop, AllowALL] and the LAN WIRED [MTServer,AllowAll] as the second adapter. In the Guest machine I disable the First Adapter in Adapter Settings. I can then ping internal, external whatever.

How to make layout with rounded corners..?

Here's a copy of a XML file to create a drawable with a white background, black border and rounded corners:

 <?xml version="1.0" encoding="UTF-8"?> 
    <shape xmlns:android=""> 
        <solid android:color="#ffffffff"/>    

        <stroke android:width="3dp"

        <padding android:left="1dp"

        <corners android:bottomRightRadius="7dp" android:bottomLeftRadius="7dp" 
         android:topLeftRadius="7dp" android:topRightRadius="7dp"/> 

save it as a xml file in the drawable directory, Use it like you would use any drawable background(icon or resource file) using its resource name (R.drawable.your_xml_name)

I want my android application to be only run in portrait mode?

I use


It is helpful if you do not want to support up side down portrait mode.

Simplest way to form a union of two lists

The easiest way is to use LINQ's Union method:

var aUb = A.Union(B).ToList();

I can't delete a remote master branch on git

To answer the question literally (since GitHub is not in the question title), also be aware of this post over on superuser. EDIT: Answer copied here in relevant part, slightly modified for clarity in square brackets:

You're getting rejected because you're trying to delete the branch that your origin has currently "checked out".

If you have direct access to the repo, you can just open up a shell [in the bare repo] directory and use good old git branch to see what branch origin is currently on. To change it to another branch, you have to use git symbolic-ref HEAD refs/heads/another-branch.

how to get current month and year

    public string GetCurrentYear()
        string CurrentYear = DateTime.Now.Year.ToString();

        return CurrentYear;

    public string GetCurrentMonth()
        string CurrentMonth = DateTime.Now.Month.ToString();

        return CurrentMonth;

How can I programmatically invoke an onclick() event from a anchor tag while keeping the ‘this’ reference in the onclick function?

Granted, OP stated very similarly that this didn't work, but it did for me. Based on the notes in my source, it seems it was implemented around the time, or after, OP's post. Perhaps it's more standard now.


In my case, my button didn't have an ID. If your element has an id, preferably use the following (untested).


I originally tried this method and it didn't work. After Googling I came back and realized my element was by name, and didn't have an ID. Double check you're calling the right attribute.


How to make sure docker's time syncs with that of the host?

This will reset the time in the docker server:

docker run --rm --privileged alpine hwclock -s

Next time you create a container the clock should be correct.


Merging arrays with the same keys

Two entries in an array can't share a key, you'll need to change the key for the duplicate

Java Thread Example?

There is no guarantee that your threads are executing simultaneously regardless of any trivial example anyone else posts. If your OS only gives the java process one processor to work on, your java threads will still be scheduled for each time slice in a round robin fashion. Meaning, no two will ever be executing simultaneously, but the work they do will be interleaved. You can use monitoring tools like Java's Visual VM (standard in the JDK) to observe the threads executing in a Java process.

Using variable in SQL LIKE statement

This works for me on the Northwind sample DB, note that SearchLetter has 2 characters to it and SearchLetter also has to be declared for this to run:

declare @SearchLetter2 char(2)
declare @SearchLetter char(1)
Set @SearchLetter = 'A'
Set @SearchLetter2 = @SearchLetter+'%'
select * from Customers where ContactName like @SearchLetter2 and Region='WY'

Undefined index with $_POST

In PHP, a variable or array element which has never been set is different from one whose value is null; attempting to access such an unset value is a runtime error.

That's what you're running into: the array $_POST does not have any element at the key "username", so the interpreter aborts your program before it ever gets to the nullity test.

Fortunately, you can test for the existence of a variable or array element without actually trying to access it; that's what the special operator isset does:

if (isset($_POST["username"]))
  $user = $_POST["username"];
  echo $user;
  echo " is your username";
  $user = null;
  echo "no username supplied";

This looks like it will blow up in exactly the same way as your code, when PHP tries to get the value of $_POST["username"] to pass as an argument to the function isset(). However, isset() is not really a function at all, but special syntax recognized before the evaluation stage, so the PHP interpreter checks for the existence of the value without actually trying to retrieve it.

It's also worth mentioning that as runtime errors go, a missing array element is considered a minor one (assigned the E_NOTICE level). If you change the error_reporting level so that notices are ignored, your original code will actually work as written, with the attempted array access returning null. But that's considered bad practice, especially for production code.

Side note: PHP does string interpolation, so the echo statements in the if block can be combined into one:

echo "$user is your username";

How to Navigate from one View Controller to another using Swift

In my experience navigationController was nil so I changed my code to this:

let next = self.storyboard?.instantiateViewControllerWithIdentifier("DashboardController") as! DashboardController
self.presentViewController(next, animated: true, completion: nil)

Don't forget to set ViewController StoryBoard Id in StoryBoard -> identity inspector

SyntaxError: multiple statements found while compiling a single statement

A (partial) practical work-around is to put things into a throw-away function.


x = 1
x += 1

results in

>>> x = 1
x += 1
  File "<stdin>", line 1
    x += 1

SyntaxError: multiple statements found while compiling a single statement

However, pasting

def abc():
  x = 1
  x += 1


>>> def abc():
  x = 1
  x += 1
>>> abc()

Of course, this is OK for a quick one-off, won't work for everything you might want to do, etc. But then, going to ipython / jupyter qtconsole is probably the next simplest option.

When is it appropriate to use C# partial classes?

Here is a list of some of the advantages of partial classes.

You can separate UI design code and business logic code so that it is easy to read and understand. For example you are developing an web application using Visual Studio and add a new web form then there are two source files, "aspx.cs" and "aspx.designer.cs" . These two files have the same class with the partial keyword. The ".aspx.cs" class has the business logic code while "aspx.designer.cs" has user interface control definition.

When working with automatically generated source, the code can be added to the class without having to recreate the source file. For example you are working with LINQ to SQL and create a DBML file. Now when you drag and drop a table it creates a partial class in designer.cs and all table columns have properties in the class. You need more columns in this table to bind on the UI grid but you don't want to add a new column to the database table so you can create a separate source file for this class that has a new property for that column and it will be a partial class. So that does affect the mapping between database table and DBML entity but you can easily get an extra field. It means you can write the code on your own without messing with the system generated code.

More than one developer can simultaneously write the code for the class.

You can maintain your application better by compacting large classes. Suppose you have a class that has multiple interfaces so you can create multiple source files depending on interface implements. It is easy to understand and maintain an interface implemented on which the source file has a partial class.

How to get value at a specific index of array In JavaScript?

You can use [];

var indexValue = Index[1];

Find common substring between two strings

A Trie data structure would work the best, better than DP. Here is the code.

class TrieNode:
    def __init__(self):
        self.child = [None]*26
        self.endWord = False

class Trie:

    def __init__(self):
        self.root = self.getNewNode()

    def getNewNode(self):
        return TrieNode()

    def insert(self,value):
        root = self.root

        for i,character in enumerate(value):
            index = ord(character) - ord('a')
            if not root.child[index]:
                root.child[index] = self.getNewNode()
            root = root.child[index]

        root.endWord = True

    def search(self,value):
        root = self.root

        for i,character in enumerate(value):
            index = ord(character) - ord('a')
            if not root.child[index]:
                return False
            root = root.child[index]
        return root.endWord

def main(): 

    # Input keys (use only 'a' through 'z' and lower case) 
    keys = ["the","anaswe"] 
    output = ["Not present in trie", 
            "Present in trie"] 

    # Trie object 
    t = Trie() 

    # Construct trie 
    for key in keys: 

    # Search for different keys 
    print("{} ---- {}".format("the",output["the")])) 
    print("{} ---- {}".format("these",output["these")])) 
    print("{} ---- {}".format("their",output["their")])) 
    print("{} ---- {}".format("thaw",output["thaw")])) 

if __name__ == '__main__': 

Let me know in case of doubts.

Applying function with multiple arguments to create a new pandas column

This solves the problem:

df['newcolumn'] = df.A * df.B

You could also do:

def fab(row):
  return row['A'] * row['B']

df['newcolumn'] = df.apply(fab, axis=1)

Dynamic LINQ OrderBy on IEnumerable<T> / IQueryable<T>

An alternate solution uses the following class/interface. It's not truly dynamic, but it works.

public interface IID
    int ID
        get; set;

public static class Utils
    public static int GetID<T>(ObjectQuery<T> items) where T:EntityObject, IID
        if (items.Count() == 0) return 1;
        return items.OrderByDescending(u => u.ID).FirstOrDefault().ID + 1;

How to change password using TortoiseSVN?

Password changes are handled by the subversion server administrator. As a user there is no password change option.

Check with your server admin.

If you are the admin, find your SVN Server installation. If you don't know where it is, it could be listed in Start->Programs, running under services in Start->Control Panel->Services or it could be listed under C:\Program Files.

The SVN Server should have an application to run to add/change/delete authentication and users.

Comparing two strings in C?

You are currently comparing the addresses of the two strings.

Use strcmp to compare the values of two char arrays

 if (strcmp(namet2, nameIt2) != 0)