Programs & Examples On #Stack

A stack is a last in, first out (LIFO) abstract data type and data structure. One common use of stacks is to store subroutine arguments and return addresses.

Which is faster: Stack allocation or Heap allocation

You can write a special heap allocator for specific sizes of objects that is very performant. However, the general heap allocator is not particularly performant.

Also I agree with Torbjörn Gyllebring about the expected lifetime of objects. Good point!

Stacking DIVs on top of each other?

If you mean by literally putting one on the top of the other, one on the top (Same X, Y positions, but different Z position), try using the z-index CSS attribute. This should work (untested)

    <div style='z-index: 1'>1</div>
    <div style='z-index: 2'>2</div>
    <div style='z-index: 3'>3</div>
    <div style='z-index: 4'>4</div>

This should show 4 on the top of 3, 3 on the top of 2, and so on. The higher the z-index is, the higher the element is positioned on the z-axis. I hope this helped you :)

What is the default stack size, can it grow, how does it work with garbage collection?

How much a stack can grow?

You can use a VM option named ss to adjust the maximum stack size. A VM option is usually passed using -X{option}. So you can use java -Xss1M to set the maximum of stack size to 1M.

Each thread has at least one stack. Some Java Virtual Machines(JVM) put Java stack(Java method calls) and native stack(Native method calls in VM) into one stack, and perform stack unwinding using a Managed to Native Frame, known as M2NFrame. Some JVMs keep two stacks separately. The Xss set the size of the Java Stack in most cases.

For many JVMs, they put different default values for stack size on different platforms.

Can we limit this growth?

When a method call occurs, a new stack frame will be created on the stack of that thread. The stack will contain local variables, parameters, return address, etc. In java, you can never put an object on stack, only object reference can be stored on stack. Since array is also an object in java, arrays are also not stored on stack. So, if you reduce the amount of your local primitive variables, parameters by grouping them into objects, you can reduce the space on stack. Actually, the fact that we cannot explicitly put objects on java stack affects the performance some time(cache miss).

Does stack has some default minimum value or default maximum value?

As I said before, different VMs are different, and may change over versions. See here.

how does garbage collection work on stack?

Garbage collections in Java is a hot topic. Garbage collection aims to collect unreachable objects in the heap. So that needs a definition of 'reachable.' Everything on the stack constitutes part of the root set references in GC. Everything that is reachable from every stack of every thread should be considered as live. There are some other root set references, like Thread objects and some class objects.

This is only a very vague use of stack on GC. Currently most JVMs are using a generational GC. This article gives brief introduction about Java GC. And recently I read a very good article talking about the GC on .net. The GC on oracle jvm is quite similar so I think that might also help you.

How to clone object in C++ ? Or Is there another solution?

The typical solution to this is to write your own function to clone an object. If you are able to provide copy constructors and copy assignement operators, this may be as far as you need to go.

class Foo
  Foo(const Foo& rhs) { /* copy construction from rhs*/ }
  Foo& operator=(const Foo& rhs) {};

// ...

Foo orig;
Foo copy = orig;  // clones orig if implemented correctly

Sometimes it is beneficial to provide an explicit clone() method, especially for polymorphic classes.

class Interface
  virtual Interface* clone() const = 0;

class Foo : public Interface
  Interface* clone() const { return new Foo(*this); }

class Bar : public Interface
  Interface* clone() const { return new Bar(*this); }

Interface* my_foo = /* somehow construct either a Foo or a Bar */;
Interface* copy = my_foo->clone();

EDIT: Since Stack has no member variables, there's nothing to do in the copy constructor or copy assignment operator to initialize Stack's members from the so-called "right hand side" (rhs). However, you still need to ensure that any base classes are given the opportunity to initialize their members.

You do this by calling the base class:

Stack(const Stack& rhs) 
: List(rhs)  // calls copy ctor of List class

Stack& operator=(const Stack& rhs) 
  return * this;

What is the function of the push / pop instructions used on registers in x86 assembly?

Here is how you push a register. I assume we are talking about x86.

push ebx
push eax

It is pushed on stack. The value of ESP register is decremented to size of pushed value as stack grows downwards in x86 systems.

It is needed to preserve the values. The general usage is

push eax           ;   preserve the value of eax
call some_method   ;   some method is called which will put return value in eax
mov  edx, eax      ;    move the return value to edx
pop  eax           ;    restore original eax

A push is a single instruction in x86, which does two things internally.

  1. Decrement the ESP register by the size of pushed value.
  2. Store the pushed value at current address of ESP register.

design a stack such that getMinimum( ) should be O(1)

I found a solution that satisfies all the constraints mentioned (constant time operations) and constant extra space.

The idea is to store the difference between min value and the input number, and update the min value if it is no longer the minimum.

The code is as follows:

public class MinStack {
    long min;
    Stack<Long> stack;

    public MinStack(){
        stack = new Stack<>();

    public void push(int x) {
        if (stack.isEmpty()) {
            min = x;
        } else {
            stack.push(x - min); //Could be negative if min value needs to change
            if (x < min) min = x;

    public int pop() {
        if (stack.isEmpty()) return;

        long pop = stack.pop();

        if (pop < 0) {
            long ret = min
            min = min - pop; //If negative, increase the min value
            return (int)ret;
        return (int)(pop + min);


    public int top() {
        long top = stack.peek();
        if (top < 0) {
            return (int)min;
        } else {
           return (int)(top + min);

    public int getMin() {
        return (int)min;

Credit goes to:

Parenthesis/Brackets Matching using Stack algorithm

Actually, there is no need to check any cases "manually". You can just run the following algorithm:

  1. Iterate over the given sequence. Start with an empty stack.

  2. If the current char is an opening bracket, just push it to the stack.

  3. If it's a closing bracket, check that the stack is not empty and the top element of the step is an appropriate opening bracket(that it is, matches this one). If it is not, report an error. Otherwise, pop the top element from the stack.

  4. In the end, the sequence is correct iff the stack is empty.

Why is it correct? Here is a sketch of a proof: if this algorithm reported that the sequence is corrected, it had found a matching pair of all brackets. Thus, the sequence is indeed correct by definition. If it has reported an error:

  1. If the stack was not empty in the end, the balance of opening and closing brackets is not zero. Thus, it is not a correct sequence.

  2. If the stack was empty when we had to pop an element, the balance is off again.

  3. If there was a wrong element on top of the stack, a pair of "wrong" brackets should match each other. It means that the sequence is not correct.

I have shown that:

  • If the algorithm has reported that the sequence is correct, it is correct.

  • If the algorithm has reported that the sequence is not correct, it is incorrect(note that I do not use the fact that there are no other cases except those that are mentioned in your question).

This two points imply that this algorithm works for all possible inputs.

What does "ulimit -s unlimited" do?

ulimit -s unlimited lets the stack grow unlimited.

This may prevent your program from crashing if you write programs by recursion, especially if your programs are not tail recursive (compilers can "optimize" those), and the depth of recursion is large.

How to increase the Java stack size?

It is hard to give a sensible solution since you are keen to avoid all sane approaches. Refactoring one line of code is the senible solution.

Note: Using -Xss sets the stack size of every thread and is a very bad idea.

Another approach is byte code manipulation to change the code as follows;

public static long fact(int n) { 
    return n < 2 ? n : n > 127 ? 0 : n * fact(n - 1); 

given every answer for n > 127 is 0. This avoid changing the source code.

What and where are the stack and heap?

What is a stack?

A stack is a pile of objects, typically one that is neatly arranged.

Enter image description here

Stacks in computing architectures are regions of memory where data is added or removed in a last-in-first-out manner.
In a multi-threaded application, each thread will have its own stack.

What is a heap?

A heap is an untidy collection of things piled up haphazardly.

Enter image description here

In computing architectures the heap is an area of dynamically-allocated memory that is managed automatically by the operating system or the memory manager library.
Memory on the heap is allocated, deallocated, and resized regularly during program execution, and this can lead to a problem called fragmentation.
Fragmentation occurs when memory objects are allocated with small spaces in between that are too small to hold additional memory objects.
The net result is a percentage of the heap space that is not usable for further memory allocations.

Both together

In a multi-threaded application, each thread will have its own stack. But, all the different threads will share the heap.
Because the different threads share the heap in a multi-threaded application, this also means that there has to be some coordination between the threads so that they don’t try to access and manipulate the same piece(s) of memory in the heap at the same time.

Which is faster – the stack or the heap? And why?

The stack is much faster than the heap.
This is because of the way that memory is allocated on the stack.
Allocating memory on the stack is as simple as moving the stack pointer up.

For people new to programming, it’s probably a good idea to use the stack since it’s easier.
Because the stack is small, you would want to use it when you know exactly how much memory you will need for your data, or if you know the size of your data is very small.
It’s better to use the heap when you know that you will need a lot of memory for your data, or you just are not sure how much memory you will need (like with a dynamic array).

Java Memory Model

Enter image description here

The stack is the area of memory where local variables (including method parameters) are stored. When it comes to object variables, these are merely references (pointers) to the actual objects on the heap.
Every time an object is instantiated, a chunk of heap memory is set aside to hold the data (state) of that object. Since objects can contain other objects, some of this data can in fact hold references to those nested objects.

Android: Clear the back stack

I found an interesting solution which might help. I did this in my onBackPressed() method.


FinishAffinity removes the connection of the existing activity to its stack. And then finish helps you exit that activity. Which will eventually exit the application.

Java balanced expressions check {[()]}

Please try this.

    import java.util.Stack;

    public class PatternMatcher {
        static String[] patterns = { "{([])}", "{}[]()", "(}{}]]", "{()", "{}" };
        static String openItems = "{([";

        boolean isOpen(String sy) {
            return openItems.contains(sy);

        String getOpenSymbol(String byCloseSymbol) {
            switch (byCloseSymbol) {
            case "}":
                return "{";
            case "]":
                return "[";
            case ")":
                return "(";

                return null;

        boolean isValid(String pattern) {

            if(pattern == null) {
                return false;

            Stack<String> stack = new Stack<String>();
            char[] symbols = pattern.toCharArray();

            if (symbols.length == 0 || symbols.length % 2 != 0) {
                return false;

            for (char c : symbols) {
                String symbol = Character.toString(c);
                if (isOpen(symbol)) {
                } else {
                    String openSymbol = getOpenSymbol(symbol);
                    if (stack.isEmpty() 
                            || openSymbol == null 
                            || !openSymbol.equals(stack.pop())) {
                        return false;
            return stack.isEmpty();

        public static void main(String[] args) {
            PatternMatcher patternMatcher = new PatternMatcher();

            for (String pattern : patterns) {
                boolean valid = patternMatcher.isValid(pattern);
                System.out.println(pattern + "\t" + valid);


What are SP (stack) and LR in ARM?

LR is link register used to hold the return address for a function call.

SP is stack pointer. The stack is generally used to hold "automatic" variables and context/parameters across function calls. Conceptually you can think of the "stack" as a place where you "pile" your data. You keep "stacking" one piece of data over the other and the stack pointer tells you how "high" your "stack" of data is. You can remove data from the "top" of the "stack" and make it shorter.

From the ARM architecture reference:

SP, the Stack Pointer

Register R13 is used as a pointer to the active stack.

In Thumb code, most instructions cannot access SP. The only instructions that can access SP are those designed to use SP as a stack pointer. The use of SP for any purpose other than as a stack pointer is deprecated. Note Using SP for any purpose other than as a stack pointer is likely to break the requirements of operating systems, debuggers, and other software systems, causing them to malfunction.

LR, the Link Register

Register R14 is used to store the return address from a subroutine. At other times, LR can be used for other purposes.

When a BL or BLX instruction performs a subroutine call, LR is set to the subroutine return address. To perform a subroutine return, copy LR back to the program counter. This is typically done in one of two ways, after entering the subroutine with a BL or BLX instruction:

• Return with a BX LR instruction.

• On subroutine entry, store LR to the stack with an instruction of the form: PUSH {,LR} and use a matching instruction to return: POP {,PC} ...

This link gives an example of a trivial subroutine.

Here is an example of how registers are saved on the stack prior to a call and then popped back to restore their content.

How do you implement a Stack and a Queue in JavaScript?

If you wanted to make your own data structures, you could build your own:

var Stack = function(){ = null;
  this.size = 0;

var Node = function(data){ = data;
  this.previous = null;

Stack.prototype.push = function(data) {
  var node = new Node(data);

  node.previous =; = node;
  this.size += 1;

Stack.prototype.pop = function() {
  temp =; =;
  this.size -= 1;
  return temp;

And for queue:

var Queue = function() {
  this.first = null;
  this.size = 0;

var Node = function(data) { = data; = null;

Queue.prototype.enqueue = function(data) {
  var node = new Node(data);

  if (!this.first){
    this.first = node;
  } else {
    n = this.first;
    while ( {
      n =;
    } = node;

  this.size += 1;
  return node;

Queue.prototype.dequeue = function() {
  temp = this.first;
  this.first =;
  this.size -= 1;
  return temp;

Stack smashing detected

Stack corruptions ususally caused by buffer overflows. You can defend against them by programming defensively.

Whenever you access an array, put an assert before it to ensure the access is not out of bounds. For example:

assert(i + 1 < N);
assert(i < N);
a[i + 1] = a[i];

This makes you think about array bounds and also makes you think about adding tests to trigger them if possible. If some of these asserts can fail during normal use turn them into a regular if.

C/C++ maximum stack size of program

(Added 26 Sept. 2020)

On 24 Oct. 2009, as @pixelbeat first pointed out here, Bruno Haible empirically discovered the following default thread stack sizes for several systems. He said that in a multithreaded program, "the default thread stack size is:"

- glibc i386, x86_64    7.4 MB
- Tru64 5.1             5.2 MB
- Cygwin                1.8 MB
- Solaris 7..10           1 MB
- MacOS X 10.5          460 KB
- AIX 5                  98 KB
- OpenBSD 4.0            64 KB
- HP-UX 11               16 KB

Note that the above units are all in MB and KB (base 1000 numbers), NOT MiB and KiB (base 1024 numbers). I've proven this to myself by verifying the 7.4 MB case.

He also stated that:

32 KB is more than you can safely allocate on the stack in a multithreaded program

And he said:

And the default stack size for sigaltstack, SIGSTKSZ, is

  • only 16 KB on some platforms: IRIX, OSF/1, Haiku.
  • only 8 KB on some platforms: glibc, NetBSD, OpenBSD, HP-UX, Solaris.
  • only 4 KB on some platforms: AIX.


He wrote the following simple Linux C program to empirically determine the above values. You can run it on your system today to quickly see what your maximum thread stack size is, or you can run it online on GDBOnline here:

Explanation: It simply creates a single new thread, so as to check the thread stack size and NOT the program stack size, in case they differ, then it has that thread repeatedly allocate 128 bytes of memory on the stack (NOT the heap), using the Linux alloca() call, after which it writes a 0 to the first byte of this new memory block, and then it prints out how many total bytes it has allocated. It repeats this process, allocating 128 more bytes on the stack each time, until the program crashes with a Segmentation fault (core dumped) error. The last value printed is the estimated maximum thread stack size allowed for your system.

Important note: alloca() allocates on the stack: even though this looks like dynamic memory allocation onto the heap, similar to a malloc() call, alloca() does NOT dynamically allocate onto the heap. Rather, alloca() is a specialized Linux function to "pseudo-dynamically" (I'm not sure what I'd call this, so that's the term I chose) allocate directly onto the stack as though it was statically-allocated memory. Stack memory used and returned by alloca() is scoped at the function-level, and is therefore "automatically freed when the function that called alloca() returns to its caller." That's why its static scope isn't exited and memory allocated by alloca() is NOT freed each time a for loop iteration is completed and the end of the for loop scope is reached. See man 3 alloca for details. Here's the pertinent quote (emphasis added):

The alloca() function allocates size bytes of space in the stack frame of the caller. This temporary space is automatically freed when the function that called alloca() returns to its caller.

The alloca() function returns a pointer to the beginning of the allocated space. If the allocation causes stack overflow, program behavior is undefined.

Here is Bruno Haible's program from 24 Oct. 2009, copied directly from the GNU mailing list here:

Again, you can run it live online here.

// By Bruno Haible
// 24 Oct. 2009
// Source:

// =============== Program for determining the default thread stack size =========

#include <alloca.h>
#include <pthread.h>
#include <stdio.h>
void* threadfunc (void*p) {
  int n = 0;
  for (;;) {
    printf("Allocated %d bytes\n", n);
    n += 128;
    *((volatile char *) alloca(128)) = 0;

int main()
  pthread_t thread;
  pthread_create(&thread, NULL, threadfunc, NULL);
  for (;;) {}

When I run it on GDBOnline using the link above, I get the exact same results each time I run it, as both a C and a C++17 program. It takes about 10 seconds or so to run. Here are the last several lines of the output:

Allocated 7449856 bytes
Allocated 7449984 bytes
Allocated 7450112 bytes
Allocated 7450240 bytes
Allocated 7450368 bytes
Allocated 7450496 bytes
Allocated 7450624 bytes
Allocated 7450752 bytes
Allocated 7450880 bytes
Segmentation fault (core dumped)

So, the thread stack size is ~7.45 MB for this system, as Bruno mentioned above (7.4 MB).

I've made a few changes to the program, mostly just for clarity, but also for efficiency, and a bit for learning.

Summary of my changes:

  1. [learning] I passed in BYTES_TO_ALLOCATE_EACH_LOOP as an argument to the threadfunc() just for practice passing in and using generic void* arguments in C.

  2. [efficiency] I made the main thread sleep instead of wastefully spinning.

  3. [clarity] I added more-verbose variable names, such as BYTES_TO_ALLOCATE_EACH_LOOP and bytes_allocated.

  4. [clarity] I changed this:

     *((volatile char *) alloca(128)) = 0;

    to this:

     volatile uint8_t * byte_buff = 
             (volatile uint8_t *)alloca(BYTES_TO_ALLOCATE_EACH_LOOP);
     byte_buff[0] = 0;

Here is my modified test program, which does exactly the same thing as Bruno's, and even has the same results:

You can run it online here, or download it from my repo here. If you choose to run it locally from my repo, here's the build and run commands I used for testing:

  1. Build and run it as a C program:

     mkdir -p bin && \
     gcc -Wall -Werror -g3 -O3 -std=c11 -pthread -o bin/tmp \
     onlinegdb--empirically_determine_max_thread_stack_size_GS_version.c && \
     time bin/tmp
  2. Build and run it as a C++ program:

     mkdir -p bin && \
     g++ -Wall -Werror -g3 -O3 -std=c++17 -pthread -o bin/tmp \
     onlinegdb--empirically_determine_max_thread_stack_size_GS_version.c && \
     time bin/tmp

It takes < 0.5 seconds to run locally on a fast computer with a thread stack size of ~7.4 MB.

Here's the program:

// =============== Program for determining the default thread stack size =========

// Modified by Gabriel Staples, 26 Sept. 2020

// Originally by Bruno Haible
// 24 Oct. 2009
// Source:

#include <alloca.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <unistd.h> // sleep

/// Thread function to repeatedly allocate memory within a thread, printing
/// the total memory allocated each time, until the program crashes. The last
/// value printed before the crash indicates how big a thread's stack size is.
void* threadfunc(void* bytes_to_allocate_each_loop)
    const uint32_t BYTES_TO_ALLOCATE_EACH_LOOP =

    uint32_t bytes_allocated = 0;
    while (true)
        printf("bytes_allocated = %u\n", bytes_allocated);
        // NB: it appears that you don't necessarily need `volatile` here,
        // but you DO definitely need to actually use (ex: write to) the
        // memory allocated by `alloca()`, as we do below, or else the
        // `alloca()` call does seem to get optimized out on some systems,
        // making this whole program just run infinitely forever without
        // ever hitting the expected segmentation fault.
        volatile uint8_t * byte_buff =
                (volatile uint8_t *)alloca(BYTES_TO_ALLOCATE_EACH_LOOP);
        byte_buff[0] = 0;
        bytes_allocated += BYTES_TO_ALLOCATE_EACH_LOOP;

int main()
    const uint32_t BYTES_TO_ALLOCATE_EACH_LOOP = 128;

    pthread_t thread;
    pthread_create(&thread, NULL, threadfunc,
    while (true)
        const unsigned int SLEEP_SEC = 10000;

    return 0;

Sample output (same results as Bruno Haible's original program):

bytes_allocated = 7450240                                                                                                                                                        
bytes_allocated = 7450368                                                                                                                                                        
bytes_allocated = 7450496                                                                                                                                                        
bytes_allocated = 7450624                                                                                                                                                        
bytes_allocated = 7450752                                                                                                                                                        
bytes_allocated = 7450880                                                                                                                                                        
Segmentation fault (core dumped) 

Android: Clear Activity Stack

Most of you are wrong. If you want to close existing activity stack regardless of what's in there and create new root, correct set of flags is the following:


From the doc:

public static final int FLAG_ACTIVITY_CLEAR_TASK
Added in API level 11

If set in an Intent passed to Context.startActivity(), this flag will cause any existing task that would be associated with the activity to be cleared before the activity is started. That is, the activity becomes the new root of an otherwise empty task, and any old activities are finished. This can only be used in conjunction with FLAG_ACTIVITY_NEW_TASK.

Object creation on the stack/heap?

C++ has Automatic variables - not Stack variables.

Automatic variable means that C++ compiler handles memory allocation / free by itself. C++ can automatically handle objects of any class - no matter whether it has dynamically allocated members or not. It's achieved by strong guarantee of C++ that object's destructor will be called automatically when execution is going out of scope where automatic variable was declared. Inside of a C++ object can be a lot of dynamic allocations with new in constructor, and when such an object is declared as an automatic variable - all dynamic allocations will be performed, and freed then in destructor.

Stack variables in C can't be dynamically allocated. Stack in C can store pointers, or fixed arrays or structs - all of fixed size, and these things are being allocated in memory in linear order. When a C program frees a stack variable - it just moves stack pointer back and nothing more.

Even though C++ programs can use Stack memory segment for storing primitive types, function's args, or other, - it's all decided by C++ compiler, not by program developer. Thus, it is conceptually wrong to equal C++ automatic variables and C stack variables.

Stack, Static, and Heap in C++

I'm sure one of the pedants will come up with a better answer shortly, but the main difference is speed and size.


Dramatically faster to allocate. It is done in O(1) since it is allocated when setting up the stack frame so it is essentially free. The drawback is that if you run out of stack space you are boned. You can adjust the stack size, but IIRC you have ~2MB to play with. Also, as soon as you exit the function everything on the stack is cleared. So it can be problematic to refer to it later. (Pointers to stack allocated objects leads to bugs.)


Dramatically slower to allocate. But you have GB to play with, and point to.

Garbage Collector

The garbage collector is some code that runs in the background and frees memory. When you allocate memory on the heap it is very easy to forget to free it, which is known as a memory leak. Over time, the memory your application consumes grows and grows until it crashes. Having a garbage collector periodically free the memory you no longer need helps eliminate this class of bugs. Of course this comes at a price, as the garbage collector slows things down.

Why is Java Vector (and Stack) class considered obsolete or deprecated?

Besides the already stated answers about using Vector, Vector also has a bunch of methods around enumeration and element retrieval which are different than the List interface, and developers (especially those who learned Java before 1.2) can tend to use them if they are in the code. Although Enumerations are faster, they don't check if the collection was modified during iteration, which can cause issues, and given that Vector might be chosen for its syncronization - with the attendant access from multiple threads, this makes it a particularly pernicious problem. Usage of these methods also couples a lot of code to Vector, such that it won't be easy to replace it with a different List implementation.

How does the stack work in assembly language?

What is Stack? A stack is a type of data structure -- a means of storing information in a computer. When a new object is entered in a stack, it is placed on top of all the previously entered objects. In other words, the stack data structure is just like a stack of cards, papers, credit card mailings, or any other real-world objects you can think of. When removing an object from a stack, the one on top gets removed first. This method is referred to as LIFO (last in, first out).

The term "stack" can also be short for a network protocol stack. In networking, connections between computers are made through a series of smaller connections. These connections, or layers, act like the stack data structure, in that they are built and disposed of in the same way.

How to implement a queue using two stacks?

A - How To Reverse A Stack

To understand how to construct a queue using two stacks, you should understand how to reverse a stack crystal clear. Remember how stack works, it is very similar to the dish stack on your kitchen. The last washed dish will be on the top of the clean stack, which is called as Last In First Out (LIFO) in computer science.

Lets imagine our stack like a bottle as below;

enter image description here

If we push integers 1,2,3 respectively, then 3 will be on the top of the stack. Because 1 will be pushed first, then 2 will be put on the top of 1. Lastly, 3 will be put on the top of the stack and latest state of our stack represented as a bottle will be as below;

enter image description here

Now we have our stack represented as a bottle is populated with values 3,2,1. And we want to reverse the stack so that the top element of the stack will be 1 and bottom element of the stack will be 3. What we can do ? We can take the bottle and hold it upside down so that all the values should reverse in order ?

enter image description here

Yes we can do that, but that's a bottle. To do the same process, we need to have a second stack that which is going to store the first stack elements in reverse order. Let's put our populated stack to the left and our new empty stack to the right. To reverse the order of the elements, we are going to pop each element from left stack, and push them to the right stack. You can see what happens as we do so on the image below;

enter image description here

So we know how to reverse a stack.

B - Using Two Stacks As A Queue

On previous part, I've explained how can we reverse the order of stack elements. This was important, because if we push and pop elements to the stack, the output will be exactly in reverse order of a queue. Thinking on an example, let's push the array of integers {1, 2, 3, 4, 5} to a stack. If we pop the elements and print them until the stack is empty, we will get the array in the reverse order of pushing order, which will be {5, 4, 3, 2, 1} Remember that for the same input, if we dequeue the queue until the queue is empty, the output will be {1, 2, 3, 4, 5}. So it is obvious that for the same input order of elements, output of the queue is exactly reverse of the output of a stack. As we know how to reverse a stack using an extra stack, we can construct a queue using two stacks.

Our queue model will consist of two stacks. One stack will be used for enqueue operation (stack #1 on the left, will be called as Input Stack), another stack will be used for the dequeue operation (stack #2 on the right, will be called as Output Stack). Check out the image below;

enter image description here

Our pseudo-code is as below;

Enqueue Operation

Push every input element to the Input Stack

Dequeue Operation

If ( Output Stack is Empty)
    pop every element in the Input Stack
    and push them to the Output Stack until Input Stack is Empty

pop from Output Stack

Let's enqueue the integers {1, 2, 3} respectively. Integers will be pushed on the Input Stack (Stack #1) which is located on the left;

enter image description here

Then what will happen if we execute a dequeue operation? Whenever a dequeue operation is executed, queue is going to check if the Output Stack is empty or not(see the pseudo-code above) If the Output Stack is empty, then the Input Stack is going to be extracted on the output so the elements of Input Stack will be reversed. Before returning a value, the state of the queue will be as below;

enter image description here

Check out the order of elements in the Output Stack (Stack #2). It's obvious that we can pop the elements from the Output Stack so that the output will be same as if we dequeued from a queue. Thus, if we execute two dequeue operations, first we will get {1, 2} respectively. Then element 3 will be the only element of the Output Stack, and the Input Stack will be empty. If we enqueue the elements 4 and 5, then the state of the queue will be as follows;

enter image description here

Now the Output Stack is not empty, and if we execute a dequeue operation, only 3 will be popped out from the Output Stack. Then the state will be seen as below;

enter image description here

Again, if we execute two more dequeue operations, on the first dequeue operation, queue will check if the Output Stack is empty, which is true. Then pop out the elements of the Input Stack and push them to the Output Stack unti the Input Stack is empty, then the state of the Queue will be as below;

enter image description here

Easy to see, the output of the two dequeue operations will be {4, 5}

C - Implementation Of Queue Constructed with Two Stacks

Here is an implementation in Java. I'm not going to use the existing implementation of Stack so the example here is going to reinvent the wheel;

C - 1) MyStack class : A Simple Stack Implementation

public class MyStack<T> {

    // inner generic Node class
    private class Node<T> {
        T data;
        Node<T> next;

        public Node(T data) {
   = data;

    private Node<T> head;
    private int size;

    public void push(T e) {
        Node<T> newElem = new Node(e);

        if(head == null) {
            head = newElem;
        } else {
   = head;
            head = newElem;     // new elem on the top of the stack


    public T pop() {
        if(head == null)
            return null;

        T elem =;
        head =;   // top of the stack is


        return elem;

    public int size() {
        return size;

    public boolean isEmpty() {
        return size == 0;

    public void printStack() {
        System.out.print("Stack: ");

        if(size == 0)
            System.out.print("Empty !");
            for(Node<T> temp = head; temp != null; temp =
                System.out.printf("%s ",;


C - 2) MyQueue class : Queue Implementation Using Two Stacks

public class MyQueue<T> {

    private MyStack<T> inputStack;      // for enqueue
    private MyStack<T> outputStack;     // for dequeue
    private int size;

    public MyQueue() {
        inputStack = new MyStack<>();
        outputStack = new MyStack<>();

    public void enqueue(T e) {

    public T dequeue() {
        // fill out all the Input if output stack is empty

        T temp = null;
        if(!outputStack.isEmpty()) {
            temp = outputStack.pop();

        return temp;

    public int size() {
        return size;

    public boolean isEmpty() {
        return size == 0;


C - 3) Demo Code

public class TestMyQueue {

    public static void main(String[] args) {
        MyQueue<Integer> queue = new MyQueue<>();

        // enqueue integers 1..3
        for(int i = 1; i <= 3; i++)

        // execute 2 dequeue operations 
        for(int i = 0; i < 2; i++)
            System.out.println("Dequeued: " + queue.dequeue());

        // enqueue integers 4..5
        for(int i = 4; i <= 5; i++)

        // dequeue the rest
            System.out.println("Dequeued: " + queue.dequeue());


C - 4) Sample Output

Dequeued: 1
Dequeued: 2
Dequeued: 3
Dequeued: 4
Dequeued: 5

What do Push and Pop mean for Stacks?

The rifle clip analogy posted by Oren A is pretty good, but I'll try another one and try to anticipate what the instructor was trying to get across.

A stack, as it's name suggests is an arrangement of "things" that has:

  • A top
  • A bottom
  • An ordering in between the top and bottom (e.g. second from the top, 3rd from the bottom).

(think of it as a literal stack of books on your desk and you can only take something from the top)

Pushing something on the stack means "placing it on top". Popping something from the stack means "taking the top 'thing'" off the stack.

A simple usage is for reversing the order of words. Say I want to reverse the word: "popcorn". I push each letter from left to right (all 7 letters), and then pop 7 letters and they'll end up in reverse order. It looks like this was what he was doing with those expressions.

push(p) push(o) push(p) push(c) push(o) push(r) push(n)

after pushing the entire word, the stack looks like:

   |  n   |  <- top
   |  r   |
   |  o   |
   |  c   |
   |  p   |
   |  o   |
   |  p   |  <- bottom (first "thing" pushed on an empty stack)

when I pop() seven times, I get the letters in this order:


conversion of infix/postfix/prefix is a pathological example in computer science when teaching stacks:

Infix to Postfix conversion.

Post fix conversion to an infix expression is pretty straight forward:

(scan expression from left to right)

  1. For every number (operand) push it on the stack.
  2. Every time you encounter an operator (+,-,/,*) pop twice from the stack and place the operator between them. Push that on the stack:

So if we have 53+2* we can convert that to infix in the following steps:

  1. Push 5.
  2. Push 3.
  3. Encountered +: pop 3, pop 5, push 5+3 on stack (be consistent with ordering of 5 and 3)
  4. Push 2.
  5. Encountered *: pop 2, pop (5+3), push (2 * (5+3)).

*When you reach the end of the expression, if it was formed correctly you stack should only contain one item.

By introducing 'x' and 'o' he may have been using them as temporary holders for the left and right operands of an infix expression: x + o, x - o, etc. (or order of x,o reversed).

There's a nice write up on wikipedia as well. I've left my answer as a wiki incase I've botched up any ordering of expressions.

Implement Stack using Two Queues

Here is very simple solution which uses one Queue and gives functionality like Stack.

public class CustomStack<T>
    Queue<T> que = new Queue<T>();

    public void push(T t) // STACK = LIFO / QUEUE = FIFO

        if( que.Count == 0)
            for (int i = 0; i < que.Count-1; i++)
                var data = que.Dequeue();



    public void pop()

        Console.WriteLine("\nStack Implementation:");
        foreach (var item in que)
            Console.Write("\n" + item.ToString() + "\t");

        var data = que.Dequeue();
        Console.Write("\n Dequeing :" + data);

    public void top()

        Console.Write("\n Top :" + que.Peek());


So in the above class named "CustomStack" what I am doing is just checking the queue for empty , if empty then insert one and from then on wards insert and then remove insert. By this logic first will come last. Example : In queue I inserted 1 and now trying to insert 2. Second time remove 1 and again insert so it becomes in reverse order.

Thank you.

Stack array using pop() and push()

Better solution for your Stack implementation

import java.util.List;
import java.util.ArrayList;
public class IntegerStack 


    private List<Integer> stack;

    public IntegerStack(int SIZE) 
        stack = new ArrayList<Integer>(SIZE);

    public void push(int i) 


     public int pop() 
           int i= stack.get(0);
           return i;
        } else{
           return -1;// Or any invalid value

     public int peek()
           return stack.get(0);
        } else{
           return -1;// Or any invalid value

     public boolean isEmpty() 


If you have to use Array... Here are problems in your code and possible solutions

import java.util.Arrays;
public class IntegerStack 

    private int stack [];
    private int top; 

    public IntegerStack(int SIZE) 
    stack = new int [SIZE];
    top = -1; // top should be 0. If you keep it as -1, problems will arise when SIZE is passed as 0. 
    // In your push method -1==0 will be false and your code will try to add the invalid element to Stack .. 
     /**Solution top=0; */

public void push(int i) 
    if (top == stack.length)

       stack[top]= i;


public int pop() 
    top --; // here you are reducing the top before giving the Object back 
      int value = stack[top];
       top --;
     return value; 
    } else{
      return -1;// OR invalid value
    return stack[top];

public int peek()
    return stack[top]; // Problem when stack is empty or size is 0
         return stack[top];
         return -1;// Or any invalid value


public boolean isEmpty() 
    if ( top == -1); // problem... we changed top to 0 above so here it need to check if its 0 and there should be no semicolon after the if statement
   /* Solution if(top==0) */
        return true;

private void extendStack()

    int [] copy = Arrays.copyOf(stack, stack.length); // The second parameter in Arrays.copyOf has no changes, so there will be no change in array length.
    stack=Arrays.copyOf(stack, stack.length+1); 


what is the basic difference between stack and queue?


Queue is a ordered collection of items.

Items are deleted at one end called ‘front’ end of the queue.

Items are inserted at other end called ‘rear’ of the queue.

The first item inserted is the first to be removed (FIFO).


Stack is a collection of items.

It allows access to only one data item: the last item inserted.

Items are inserted & deleted at one end called ‘Top of the stack’.

It is a dynamic & constantly changing object.

All the data items are put on top of the stack and taken off the top

This structure of accessing is known as Last in First out structure (LIFO)

What is a stack pointer used for in microprocessors?

Should you ever crave deeper understanding, I heartily recommend Patterson and Hennessy as an intro and Hennessy and Patterson as an intermediate to advanced text. They're pricey, but truly non-pareil; I just wish either or both were available when I got my Masters' degree and entered the workforce designing chips, systems, and parts of system software for them (but, alas!, that was WAY too long ago;-). Stack pointers are so crucial (and the distinction between a microprocessor and any other kind of CPU so utterly meaningful in this context... or, for that matter, in ANY other context, in the last few decades...!-) that I doubt anything but a couple of thorough from-the-ground-up refreshers can help!-)

Why is the use of alloca() not considered good practice?

Processes only have a limited amount of stack space available - far less than the amount of memory available to malloc().

By using alloca() you dramatically increase your chances of getting a Stack Overflow error (if you're lucky, or an inexplicable crash if you're not).

Java ArrayList how to add elements at the beginning

Take this example :-

List<String> element1 = new ArrayList<>();
List<String> element2 = new ArrayList<>();

SQL providerName in web.config


'gulp' is not recognized as an internal or external command

I was having the same exception with node v12.13.1,

Downgraded node to v10.15.3 and it works fine now.

Why does 2 mod 4 = 2?

For a visual way to think about it, picture a clock face that, in your particular example, only goes to 4 instead of 12. If you start at 4 on the clock (which is like starting at zero) and go around it clockwise for 2 "hours", you land on 2, just like going around it clockwise for 6 "hours" would also land you on 2 (6 mod 4 == 2 just like 2 mod 4 == 2).

How can I make my layout scroll both horizontally and vertically?

Since other solutions are old and either poorly-working or not working at all, I've modified NestedScrollView, which is stable, modern and it has all you expect from a scroll view. Except for horizontal scrolling.

Here's the repo:

I've made no changes, no "improvements" to the original NestedScrollView expect for what was absolutely necessary. The code is based on androidx.core:core:1.3.0, which is the latest stable version at the time of writing.

All of the following works:

  • Lift on scroll (since it's basically a NestedScrollView)
  • Edge effects in both dimensions
  • Fill viewport in both dimensions

How to check if a string is a valid JSON string in JavaScript without using Try/Catch

I think I know why you want to avoid that. But maybe try & catch !== try & catch. ;o) This came into my mind:

var json_verify = function(s){ try { JSON.parse(s); return true; } catch (e) { return false; }};

So you may also dirty clip to the JSON object, like:

JSON.verify = function(s){ try { JSON.parse(s); return true; } catch (e) { return false; }};

As this as encapsuled as possible, it may not break on error.

Java: how to import a jar file from command line

If you're running a jar file with java -jar, the -classpath argument is ignored. You need to set the classpath in the manifest file of your jar, like so:

Class-Path: jar1-name jar2-name directory-name/jar3-name

See the Java tutorials: Adding Classes to the JAR File's Classpath.

Edit: I see you already tried setting the class path in the manifest, but are you sure you used the correct syntax? If you skip the ':' after "Class-Path" like you showed, it would not work.

find path of current folder - cmd

for /f "delims=" %%i in ("%0") do set "curpath=%%~dpi"
echo "%curpath%"


echo "%cd%"

The double quotes are needed if the path contains any & characters.

How do I check/uncheck all checkboxes with a button using jQuery?

Below code will work if user select all checkboxs then check all-checkbox will be checked and if user unselect any one checkbox then check all-checkbox will be unchecked.

$("#checkall").change(function () {_x000D_
    $("input:checkbox").prop('checked', $(this).prop("checked"));_x000D_
$(".cb-element").change(function () {_x000D_
    $("#checkall").prop('checked', true);_x000D_
    $("#checkall").prop('checked', false);_x000D_
<script src=""></script>_x000D_
<input type="checkbox" name="all" id="checkall" />Check All</br>_x000D_
<input type="checkbox" class="cb-element" /> Checkbox  1</br>_x000D_
<input type="checkbox" class="cb-element" /> Checkbox  2</br>_x000D_
<input type="checkbox" class="cb-element" /> Checkbox  3

Django set field value after a form is initialized

in widget use 'value' attr. Example:

username = forms.CharField(
    widget=forms.TextInput(attrs={'readonly': True, 'value': 'CONSTANT_VALUE'}),

DLL and LIB files - what and why?

A DLL is a library of functions that are shared among other executable programs. Just look in your windows/system32 directory and you will find dozens of them. When your program creates a DLL it also normally creates a lib file so that the application *.exe program can resolve symbols that are declared in the DLL.

A .lib is a library of functions that are statically linked to a program -- they are NOT shared by other programs. Each program that links with a *.lib file has all the code in that file. If you have two programs A.exe and B.exe that link with C.lib then each A and B will both contain the code in C.lib.

How you create DLLs and libs depend on the compiler you use. Each compiler does it differently.

Read response body in JAX-RS client from a post request

I also had the same issue, trying to run a unit test calling code that uses readEntity. Can't use getEntity in production code because that just returns a ByteInputStream and not the content of the body and there is no way I am adding production code that is hit only in unit tests.

My solution was to create a response and then use a Mockito spy to mock out the readEntity method:

Response error = Response.serverError().build();
Response mockResponse = spy(error);

Note that you can't use the when(mockResponse.readEntity(String.class) option because that throws the same IllegalStateException.

Hope this helps!

Fastest way of finding differences between two files in unix?

Another option:

sort file1 file2 | uniq -u > file3

If you want to see just the duplicate entries use "uniq -d" option:

sort file1 file2 | uniq -d > file3

Editing in the Chrome debugger

Chrome DevTools has a Snippets panel where you can create and edit JavaScript code as you would in an editor, and execute it. Open DevTools, then select the Sources panel, then select the Snippets tab.

enter image description here

Understanding offsetWidth, clientWidth, scrollWidth and -Height, respectively

If you want to use scrollWidth to get the "REAL" CONTENT WIDTH/HEIGHT (as content can be BIGGER than the css-defined width/height-Box) the scrollWidth/Height is very UNRELIABLE as some browser seem to "MOVE" the paddingRIGHT & paddingBOTTOM if the content is to big. They then place the paddings at the RIGHT/BOTTOM of the "too broad/high content" (see picture below).

==> Therefore to get the REAL CONTENT WIDTH in some browsers you have to substract BOTH paddings from the scrollwidth and in some browsers you only have to substract the LEFT Padding.

I found a solution for this and wanted to add this as a comment, but was not allowed. So I took the picture and made it a bit clearer in the regard of the "moved paddings" and the "unreliable scrollWidth". In the BLUE AREA you find my solution on how to get the "REAL" CONTENT WIDTH!

Hope this helps to make things even clearer!

enter image description here

Setting Windows PATH for Postgres tools

All you need to do is to change the PATH variable to include the bin directory of your PostgreSQL installation.

An explanation on how to change environment variables is here:

To verify that the path is set correctly, you can use:

echo %PATH%

on the commandline.

How do I find the MySQL my.cnf location

You could always run find in a terminal.

find / -name my.cnf

java.nio.file.Path for a classpath resource

I wrote a small helper method to read Paths from your class resources. It is quite handy to use as it only needs a reference of the class you have stored your resources as well as the name of the resource itself.

public static Path getResourcePath(Class<?> resourceClass, String resourceName) throws URISyntaxException {
    URL url = resourceClass.getResource(resourceName);
    return Paths.get(url.toURI());

Difference between Inheritance and Composition

Inheritence means reusing the complete functionality of a class, Here my class have to use all the methods of the super class and my class will be titely coupled with the super class and code will be duplicated in both the classes in case of inheritence.

But we can overcome from all these problem when we use composition to talk with another class . composition is declaring an attribute of another class into my class to which we want to talk. and what functionality we want from that class we can get by using that attribute.

Can you delete multiple branches in one command with Git?

You might like this little item... It pulls the list and asks for confirmation of each item before finally deleting all selections...

git branch -d `git for-each-ref --format="%(refname:short)" refs/heads/\* | while read -r line; do read -p "remove branch: $line (y/N)?" answer </dev/tty; case "$answer" in y|Y) echo "$line";; esac; done`

Use -D to force deletions (like usual).

For readability, here is that broken up line by line...

git branch -d `git for-each-ref --format="%(refname:short)" refs/heads/\* |
    while read -r line; do 
        read -p "remove branch: $line (y/N)?" answer </dev/tty;
        case "$answer" in y|Y) echo "$line";; 

here is the xargs approach...

git for-each-ref --format="%(refname:short)" refs/heads/\* |
while read -r line; do 
    read -p "remove branch: $line (y/N)?" answer </dev/tty;
    case "$answer" in 
        y|Y) echo "$line";; 
done | xargs git branch -D

finally, I like to have this in my .bashrc

alias gitselect='git for-each-ref --format="%(refname:short)" refs/heads/\* | while read -r line; do read -p "select branch: $line (y/N)?" answer </dev/tty; case "$answer" in y|Y) echo "$line";; esac; done'

That way I can just say

gitSelect | xargs git branch -D.

Getting a timestamp for today at midnight?

$timestamp = strtotime('today midnight');

You might want to take a look what PHP has to offer:

How to search images from private 1.0 registry in docker?

Modifying the answer from @mre to get the list just from one command (valid at least for Docker Registry v2).

docker exec -it <your_registry_container_id> ls -a /var/lib/registry/docker/registry/v2/repositories/

How do I make a transparent canvas in html5?

I believe you are trying to do exactly what I just tried to do: I want two stacked canvases... the bottom one has a static image and the top one contains animated sprites. Because of the animation, you need to clear the background of the top layer to transparent at the start of rendering every new frame. I finally found the answer: it's not using globalAlpha, and it's not using a rgba() color. The simple, effective answer is:


Error:Conflict with dependency ''

When I added module: 'jsr305' as an additional exclude statement, it all worked out fine for me.

 androidTestCompile('', {
exclude group: '', module: 'support-annotations'
exclude module: 'jsr305'


Auto start print html page using javascript

<body onload="window.print()"> or window.onload = function() { window.print(); }

Is there a job scheduler library for node.js?

later.js is a pretty good JavaScript "scheduler" library. Can run on Node.js or in a web browser.

I can't delete a remote master branch on git

The quickest way is to switch default branch from master to another and you can remove master branch from the web interface.

Makefile - missing separator

You need to precede the lines starting with gcc and rm with a hard tab. Commands in make rules are required to start with a tab (unless they follow a semicolon on the same line). The result should look like this:

PROG = semsearch
all: $(PROG)
%: %.c
        gcc -o $@ $< -lpthread

        rm $(PROG)

Note that some editors may be configured to insert a sequence of spaces instead of a hard tab. If there are spaces at the start of these lines you'll also see the "missing separator" error. If you do have problems inserting hard tabs, use the semicolon way:

PROG = semsearch
all: $(PROG)
%: %.c ; gcc -o $@ $< -lpthread

clean: ; rm $(PROG)

Using css transform property in jQuery

If you're using jquery, jquery.transit is very simple and powerful lib that allows you to make your transformation while handling cross-browser compability for you. It can be as simple as this : $("#element").transition({x:'90px'}).

Take it from this link :

UnicodeEncodeError: 'ascii' codec can't encode character u'\xef' in position 0: ordinal not in range(128)

The problem according to your traceback is the print statement on line 136 of Unfortunately you didn't see fit to post that part of your code, but I'm going to guess it is just there for debugging. If you change it to:

print repr(ch)

then you should at least see what you are trying to print.

How can I get log4j to delete old rotating log files?

There is no default value to control deleting old log files created by DailyRollingFileAppender. But you can write your own custom Appender that deletes old log files in much the same way as setting maxBackupIndex does for RollingFileAppender.

Simple instructions found here

From 1:

If you are trying to use the Apache Log4J DailyRollingFileAppender for a daily log file, you may need to want to specify the maximum number of files which should be kept. Just like rolling RollingFileAppender supports maxBackupIndex. But the current version of Log4j (Apache log4j 1.2.16) does not provide any mechanism to delete old log files if you are using DailyRollingFileAppender. I tried to make small modifications in the original version of DailyRollingFileAppender to add maxBackupIndex property. So, it would be possible to clean up old log files which may not be required for future usage.

How to import a module in Python with importlib.import_module

For relative imports you have to:

  • a) use relative name
  • b) provide anchor explicitly

    importlib.import_module('.c', 'a.b')

Of course, you could also just do absolute import instead:


Python Socket Multiple Clients

Based on your question:

My question is, using the code below, how would you be able to have multiple clients connected? I've tried lists, but I just can't figure out the format for that. How can this be accomplished where multiple clients are connected at once and I am able to send a message to a specific client?

Using the code you gave, you can do this:

#!/usr/bin/python           # This is file                                                                                                                                                                           

import socket               # Import socket module
import thread

def on_new_client(clientsocket,addr):
    while True:
        msg = clientsocket.recv(1024)
        #do some checks and if msg == someWeirdSignal: break:
        print addr, ' >> ', msg
        msg = raw_input('SERVER >> ')
        #Maybe some code to compute the last digit of PI, play game or anything else can go here and when you are done.

s = socket.socket()         # Create a socket object
host = socket.gethostname() # Get local machine name
port = 50000                # Reserve a port for your service.

print 'Server started!'
print 'Waiting for clients...'

s.bind((host, port))        # Bind to the port
s.listen(5)                 # Now wait for client connection.

print 'Got connection from', addr
while True:
   c, addr = s.accept()     # Establish connection with client.
   #Note it's (addr,) not (addr) because second parameter is a tuple
   #Edit: (c,addr)
   #that's how you pass arguments to functions when creating new threads using thread module.

As Eli Bendersky mentioned, you can use processes instead of threads, you can also check python threading module or other async sockets framework. Note: checks are left for you to implement how you want and this is just a basic framework.

Select statement to find duplicates on certain fields

You mention "the first one", so I assume that you have some kind of ordering on your data. Let's assume that your data is ordered by some field ID.

This SQL should get you the duplicate entries except for the first one. It basically selects all rows for which another row with (a) the same fields and (b) a lower ID exists. Performance won't be great, but it might solve your problem.

SELECT A.ID, A.field1, A.field2, A.field3
  FROM myTable A
                 FROM myTable B
                WHERE B.field1 = A.field1
                  AND B.field2 = A.field2
                  AND B.field3 = A.field3
                  AND B.ID < A.ID)

Error executing command 'ant' on Mac OS X 10.9 Mavericks when building for Android with PhoneGap/Cordova

You can install ANT through macports or homebrew.

But if you want to do without 3rd party package managers, the problem can simply be fixed by downloading the binary release from the apache ANT web site and adding the binary to your system PATH.

For example, on Mountain Lion, in ~/.bash_profile and ~/.bashrc my path was setup like this:

export ANT_HOME="/usr/share/ant"
export PATH=$PATH:$ANT_HOME/bin

So after uncompressing apache-ant-1.9.2-bin.tar.bz2 I moved the resulting directory to /usr/share/ and renamed it ant.

Simple as that, the issue is fixed.

Note Don't forget to sudo chown -R root:wheel /usr/share/ant

Getting msbuild.exe without installing Visual Studio

The latest (as of Jan 2019) stand-alone MSBuild installers can be found here:

Scroll down to "Tools for Visual Studio 2019" and choose "Build Tools for Visual Studio 2019" (despite the name, it's for users who don't want the full IDE)

See this question for additional information.

How to rename JSON key

If you want to rename all occurrences of some key you can use a regex with the g option. For example:

var json = [{"_id":"1","email":"[email protected]","image":"some_image_url","name":"Name 1"},{"_id":"2","email":"[email protected]","image":"some_image_url","name":"Name 2"}];

str = JSON.stringify(json);

now we have the json in string format in str.

Replace all occurrences of "_id" to "id" using regex with the g option:

str = str.replace(/\"_id\":/g, "\"id\":");

and return to json format:

json = JSON.parse(str);

now we have our json with the wanted key name.

How to change language settings in R

For mac users, I found this on the R for Mac FAQ

If you use a non-standard setup (e.g. different language than formats), you can override the auto-detection performed by setting `force.LANG' defaults setting, such as for example

 defaults write org.R-project.R force.LANG en_US.UTF-8 

when run in Terminal it will enforce US-english setting regardless of the system setting. If you don't know what Terminal is you can use this R command instead:

 system("defaults write org.R-project.R force.LANG en_US.UTF-8") 

but do not forget to quit R and start again afterwards. Please note that you must always use `.UTF-8' version of the locale, otherwise will not work properly.

This helped me to change my console language from Chinese to English.

How do I import an existing Java keystore (.jks) file into a Java installation?

to load a KeyStore, you'll need to tell it the type of keystore it is (probably jceks), provide an inputstream, and a password. then, you can load it like so:

KeyStore ks  = KeyStore.getInstance(TYPE_OF_KEYSTORE);
ks.load(new FileInputStream(PATH_TO_KEYSTORE), PASSWORD);

this can throw a KeyStoreException, so you can surround in a try block if you like, or re-throw. Keep in mind a keystore can contain multiple keys, so you'll need to look up your key with an alias, here's an example with a symmetric key:

SecretKeyEntry entry = (KeyStore.SecretKeyEntry)ks.getEntry(SOME_ALIAS,new KeyStore.PasswordProtection(SOME_PASSWORD));
SecretKey someKey = entry.getSecretKey();

Difference between Subquery and Correlated Subquery

Above example is not Co-related Sub-Query. It is Derived Table / Inline-View since i.e, a Sub-query within FROM Clause.

A Corelated Sub-query should refer its parent(main Query) Table in it. For example See find the Nth max salary by Co-related Sub-query:

SELECT Salary 
FROM Employee E1
             FROM Employee E2
             WHERE E1.salary <E2.Salary) 

Co-Related Vs Nested-SubQueries.

Technical difference between Normal Sub-query and Co-related sub-query are:

1. Looping: Co-related sub-query loop under main-query; whereas nested not; therefore co-related sub-query executes on each iteration of main query. Whereas in case of Nested-query; subquery executes first then outer query executes next. Hence, the maximum no. of executes are NXM for correlated subquery and N+M for subquery.

2. Dependency(Inner to Outer vs Outer to Inner): In the case of co-related subquery, inner query depends on outer query for processing whereas in normal sub-query, Outer query depends on inner query.

3.Performance: Using Co-related sub-query performance decreases, since, it performs NXM iterations instead of N+M iterations. ¨ Co-related Sub-query Execution.

For more information with examples :

ASP.NET MVC Razor pass model to layout

Let's assume your model is a collection of objects (or maybe a single object). For each object in the model do the following.

1) Put the object you want to display in the ViewBag. For example:

  ViewBag.YourObject = yourObject;

2) Add a using statement at the top of _Layout.cshtml that contains the class definition for your objects. For example:

@using YourApplication.YourClasses;

3) When you reference yourObject in _Layout cast it. You can apply the cast because of what you did in (2).

Effects of the extern keyword on C functions

As far as I remember the standard, all function declarations are considered as "extern" by default, so there is no need to specify it explicitly.

That doesn't make this keyword useless since it can also be used with variables (and it that case - it's the only solution to solve linkage problems). But with the functions - yes, it's optional.

Update Item to Revision vs Revert to Revision

@BaltoStar update to revision syntax:

svn update -r30

Where 30 is revision number. Hope this help!

How to do this in Laravel, subquery where in

Have a look at the advanced wheres documentation for Fluent:

Here's an example of what you're trying to achieve:

    ->whereIn('id', function($query)
              ->whereRaw('orders.user_id =');

This will produce:

select * from users where id in (
    select 1 from orders where orders.user_id =

Examples of GoF Design Patterns in Java's core libraries

Even though I'm sort of a broken clock with this one, Java XML API uses Factory a lot. I mean just look at this:

Document doc = DocumentBuilderFactory.newInstance().newDocumentBuilder().parse(source);
String title = XPathFactory.newInstance().newXPath().evaluate("//title", doc);

...and so on and so forth.

Additionally various Buffers (StringBuffer, ByteBuffer, StringBuilder) use Builder.

What is the non-jQuery equivalent of '$(document).ready()'?

A little thing I put together


(function(exports, d) {
  function domReady(fn, context) {

    function onReady(event) {
      d.removeEventListener("DOMContentLoaded", onReady); || exports, event);

    function onReadyIe(event) {
      if (d.readyState === "complete") {
        d.detachEvent("onreadystatechange", onReadyIe); || exports, event);

    d.addEventListener && d.addEventListener("DOMContentLoaded", onReady) ||
    d.attachEvent      && d.attachEvent("onreadystatechange", onReadyIe);

  exports.domReady = domReady;
})(window, document);

How to use it

<script src="domready.js"></script>
  domReady(function(event) {
    alert("dom is ready!");

You can also change the context in which the callback runs by passing a second argument

function init(event) {
  alert("check the console");

domReady(init, console);

Laravel 5 not finding css files

Stop artisan serve

and try using

php -S localhost:8000 -t public

How do I format a number to a dollar amount in PHP

In PHP and C++ you can use the printf() function

printf("$%01.2f", $money);

Bubble Sort Homework

def merge_bubble(arr):
    k = len(arr)
    while k>2:
        for i in range(0,k-1):
            for j in range(0,k-1):
                if arr[j] > arr[j+1]:
                    arr[j],arr[j+1] = arr[j+1],arr[j]

        return arr
        if arr[0] > arr[1]:
            arr[0],arr[1] = arr[1],arr[0]
        return arr 

Right way to write JSON deserializer in Spring or extend it

I've searched a lot and the best way I've found so far is on this article:

Class to serialize

package net.sghill.example;

import net.sghill.example.UserDeserializer
import net.sghill.example.UserSerializer

@JsonDeserialize(using = UserDeserializer.class)
public class User {
    private ObjectId id;
    private String   username;
    private String   password;

    public User(ObjectId id, String username, String password) { = id;
        this.username = username;
        this.password = password;

    public ObjectId getId()       { return id; }
    public String   getUsername() { return username; }
    public String   getPassword() { return password; }

Deserializer class

package net.sghill.example;

import net.sghill.example.User;
import org.codehaus.jackson.JsonNode;
import org.codehaus.jackson.JsonParser;
import org.codehaus.jackson.ObjectCodec;


public class UserDeserializer extends JsonDeserializer<User> {

    public User deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException {
        ObjectCodec oc = jsonParser.getCodec();
        JsonNode node = oc.readTree(jsonParser);
        return new User(null, node.get("username").getTextValue(), node.get("password").getTextValue());

Edit: Alternatively you can look at this article which uses new versions of com.fasterxml.jackson.databind.JsonDeserializer.

MySQL select with CONCAT condition

There is an alternative to repeating the CONCAT expression or using subqueries. You can make use of the HAVING clause, which recognizes column aliases.

  neededfield, CONCAT(firstname, ' ', lastname) AS firstlast 
HAVING firstlast = "Bob Michael Jones"

Here is a working SQL Fiddle.

How to add a linked source folder in Android Studio?

If you're not using gradle (creating a project from an APK, for instance), this can be done through the Android Studio UI (as of version 3.3.2):

  • Right-click the project root directory, pick Open Module Settings
  • Hit the + Add Content Root button (center right)
  • Add your path and hit OK

In my experience (with native code), as long as your .so's are built with debug symbols and from the same absolute paths, breakpoints added in source files will be automatically recognized.

How to Apply Mask to Image in OpenCV?

You can use the mask to copy only the region of interest of an original image to a destination one:


where mask should be an 8-bit single channel array.

See more at the OpenCV docs

Access IP Camera in Python OpenCV

The easiest way to stream video via IP Camera !

I just edit your example. You must replace your IP and add /video on your link. And go ahead with your project

import cv2

cap = cv2.VideoCapture('')

    ret, frame =
    if cv2.waitKey(1) & 0xFF == ord('q'):

How To Set Up GUI On Amazon EC2 Ubuntu server

So I follow first answer, but my vnc viewer gives me grey screen when I connect to it. And I found this Ask Ubuntu link to solve that.

The only difference with previous answer is you need to install these extra packages:

apt-get install gnome-panel gnome-settings-daemon metacity nautilus gnome-terminal

And use this ~/.vnc/xstartup file:



[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &

gnome-panel &
gnome-settings-daemon &
metacity &
nautilus &
gnome-terminal &

Everything else is the same.

Tested on EC2 Ubuntu 14.04 LTS.

Returning IEnumerable<T> vs. IQueryable<T>

Yes, both will give you deferred execution.

The difference is that IQueryable<T> is the interface that allows LINQ-to-SQL (LINQ.-to-anything really) to work. So if you further refine your query on an IQueryable<T>, that query will be executed in the database, if possible.

For the IEnumerable<T> case, it will be LINQ-to-object, meaning that all objects matching the original query will have to be loaded into memory from the database.

In code:

IQueryable<Customer> custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);

That code will execute SQL to only select gold customers. The following code, on the other hand, will execute the original query in the database, then filtering out the non-gold customers in the memory:

IEnumerable<Customer> custs = ...;
// Later on...
var goldCustomers = custs.Where(c => c.IsGold);

This is quite an important difference, and working on IQueryable<T> can in many cases save you from returning too many rows from the database. Another prime example is doing paging: If you use Take and Skip on IQueryable, you will only get the number of rows requested; doing that on an IEnumerable<T> will cause all of your rows to be loaded in memory.

How can I change IIS Express port for a site

To specify a port for a Web application project that uses IIS Express

  1. In Solution Explorer, right-click the name of the application and then select Properties. Click the Web tab.

  2. In the Servers section, under Use Local IIS Web server, in the Project URL box change the port number.

  3. To the right of the Project URL box, click Create Virtual Directory, and then click OK.

  4. In the File menu, click Save Selected Items.

  5. To verify the change, press CTRL+F5 to run the project. The new port number appears in the address bar of the browser.

From How to: Specify a Port for the Development Server ( backup here).

Reading JSON from a file?

You can use pandas library to read the JSON file.

import pandas as pd
df = pd.read_json('strings.json',lines=True)

Deleting elements from std::set while iterating

C++20 will have "uniform container erasure", and you'll be able to write:

std::erase_if(numbers, [](int n){ return n % 2 == 0 });

And that will work for vector, set, deque, etc. See cppReference for more info.

String split on new line, tab and some number of spaces

Regex's aren't really the best tool for the job here. As others have said, using a combination of str.strip() and str.split() is the way to go. Here's a one liner to do it:

>>> data = '''\n\tName: John Smith
... \n\t  Home: Anytown USA
... \n\t    Phone: 555-555-555
... \n\t  Other Home: Somewhere Else
... \n\t Notes: Other data
... \n\tName: Jane Smith
... \n\t  Misc: Data with spaces'''
>>> {line.strip().split(': ')[0]:line.split(': ')[1] for line in data.splitlines() if line.strip() != ''}
{'Name': 'Jane Smith', 'Other Home': 'Somewhere Else', 'Notes': 'Other data', 'Misc': 'Data with spaces', 'Phone': '555-555-555', 'Home': 'Anytown USA'}


I need some extra operation based on the result of aggregate function. Finally I've found some solution for aggregate function and the operation based on the result in MongoDB. I've a collection Request with field request, source, status, requestDate.

Single Field Group By & Count:

    {"$group" : {_id:"$source", count:{$sum:1}}}

Multiple Fields Group By & Count:

    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}}

Multiple Fields Group By & Count with Sort using Field:

    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}},

Multiple Fields Group By & Count with Sort using Count:

    {"$group" : {_id:{source:"$source",status:"$status"}, count:{$sum:1}}},

how do I print an unsigned char as hex in c++ using ostream?

You can read more about this at and I am only posting this because it has become clear that the author of the above articles does not intend to.

The simplest and most correct technique to do print a char as hex is

unsigned char a = 0;
unsigned char b = 0xff;
auto flags = cout.flags(); //I only include resetting the ioflags because so
                           //many answers on this page call functions where
                           //flags are changed and leave no way to  
                           //return them to the state they were in before 
                           //the function call
cout << "a is " << hex << +a <<"; b is " << +b << endl;

The readers digest version of how this works is that the unary + operator forces a no op type conversion to an int with the correct signedness. So, an unsigned char converts to unsigned int, a signed char converts to int, and a char converts to either unsigned int or int depending on whether char is signed or unsigned on your platform (it comes as a shock to many that char is special and not specified as either signed or unsigned).

The only negative of this technique is that it may not be obvious what is happening to a someone that is unfamiliar with it. However, I think that it is better to use the technique that is correct and teach others about it rather than doing something that is incorrect but more immediately clear.

Simple example of threading in C++

When searching for an example of a C++ class that calls one of its own instance methods in a new thread, this question comes up, but we were not able to use any of these answers that way. Here's an example that does that:


class DataManager
    bool hasData;
    void getData();
    bool dataAvailable();


#include "DataManager.h"

void DataManager::getData()
    // perform background data munging
    hasData = true;
    // be sure to notify on the main thread

bool DataManager::dataAvailable()
    if (hasData)
        return true;
        std::thread t(&DataManager::getData, this);
        t.detach(); // as opposed to .join, which runs on the current thread

Note that this example doesn't get into mutex or locking.

Displaying standard DataTables in MVC

Here is the answer in Razor syntax

 <table border="1" cellpadding="5">
          @foreach (System.Data.DataColumn col in Model.Columns)
    @foreach(System.Data.DataRow row in Model.Rows)
          @foreach (var cell in row.ItemArray)

In Flask, What is request.args and how is it used?

According to the flask.Request.args documents.

A MultiDict with the parsed contents of the query string. (The part in the URL after the question mark).

So the args.get() is method get() for MultiDict, whose prototype is as follows:

get(key, default=None, type=None)

In newer version of flask (v1.0.x and v1.1.x), flask.Request.args is an ImmutableMultiDict(an immutable MultiDict), so the prototype and specific method above is still valid.

How to append output to the end of a text file

To append a file use >>

echo "hello world"  >> read.txt   
cat read.txt     
echo "hello siva" >> read.txt   
cat read.txt

then the output should be

hello world   # from 1st echo command
hello world   # from 2nd echo command
hello siva

To overwrite a file use >

echo "hello tom" > read.txt
cat read.txt  

then the out put is

hello tom

Cannot connect to repo with TortoiseSVN

I've found that replacing the first part of the URL with IP address numbers instead of words worked for me.

For example use:

instead of:

Checking for #N/A in Excel cell from VBA code

First check for an error (N/A value) and then try the comparisation against cvErr(). You are comparing two different things, a value and an error. This may work, but not always. Simply casting the expression to an error may result in similar problems because it is not a real error only the value of an error which depends on the expression.

If IsError(ActiveWorkbook.Sheets("Publish").Range("G4").offset(offsetCount, 0).Value) Then
  If (ActiveWorkbook.Sheets("Publish").Range("G4").offset(offsetCount, 0).Value <> CVErr(xlErrNA)) Then
    'do something
  End If
End If

Android API 21 Toolbar Padding

Simpley add this two line in toolbar. Then we get new removed left side space bcoz by default it 16dp.


"No cached version... available for offline mode."

With the new Android Studio 3.6 to toggle Gradle's offline mode go to View > Tool Windows > Gradle from the menu bar and toggle the value of Offline Mode that is near the top of the Gradle window.

enter image description here

enter image description here

Regex to match alphanumeric and spaces

The circumflex inside the square brackets means all characters except the subsequent range. You want a circumflex outside of square brackets.

How can I use NSError in my iPhone App?

Well it's a little bit out of question scope but in case you don't have an option for NSError you can always display the Low level error:

 NSLog(@"Error = %@ ",[NSString stringWithUTF8String:strerror(errno)]);

Convert a string to a datetime

Try converting date like this:

    Dim expenddt as Date = Date.ParseExact(edate, "dd/mm/yyyy", 

Hope this helps.

TypeError: 'undefined' is not a function (evaluating '$(document)')

I had this problem only on Chrome.

I tried adding

var $ =jQuery.noConflict();

just before calling

$(document).ready(function () {

It worked.

Thanks a lot

Permissions for /var/www/html

I have just been in a similar position with regards to setting the 777 permissions on the apache website hosting directory. After a little bit of tinkering it seems that changing the group ownership of the folder to the "apache" group allowed access to the folder based on the user group.

1) make sure that the group ownership of the folder is set to the group apache used / generates for use. (check /etc/groups, mine was www-data on Ubuntu)

2) set the folder permissions to 774 to stop "everyone" from having any change access, but allowing the owner and group permissions required.

3) add your user account to the group that has permission on the folder (mine was www-data).

How to use pagination on HTML tables?

Pure js. Can apply it to multiple tables at once. Aborts if only one page is required. I used anushree as my starting point.

Sorry to the asker, obviously this is not a simplePagignation.js solution. However, it's the top google result when you type "javascript table paging", and it's a reasonable solution to many who may be considering a library but unsure whether to go that route or not.

Use like this:

addPagerToTables('#someTable', 8);

Requires no css, though it may be wise to initially hide table tBody rows in css anyway to prevent the effect of rows showing then quicky being hidden (not happening with me right now, but it's something I've seen before).

The code:

function addPagerToTables(tables, rowsPerPage = 10) {

    tables = 
        typeof tables == "string"
      ? document.querySelectorAll(tables)
      : tables;

    for (let table of tables) 
        addPagerToTable(table, rowsPerPage);


function addPagerToTable(table, rowsPerPage = 10) {

    let tBodyRows = table.querySelectorAll('tBody tr');
    let numPages = Math.ceil(tBodyRows.length/rowsPerPage);

    let colCount = 
    .reduce((a,b) => a + parseInt(b.colSpan), 0);

    .innerHTML = `<td colspan=${colCount}><div class="nav"></div></td>`;

    if(numPages == 1)

    for(i = 0;i < numPages;i++) {

        let pageNum = i + 1;

            `<a href="#" rel="${i}">${pageNum}</a> `        


    changeToPage(table, 1, rowsPerPage);

    for (let navA of table.querySelectorAll('.nav a'))
            e => changeToPage(


function changeToPage(table, page, rowsPerPage) {

    let startItem = (page - 1) * rowsPerPage;
    let endItem = startItem + rowsPerPage;
    let navAs = table.querySelectorAll('.nav a');
    let tBodyRows = table.querySelectorAll('tBody tr');

    for (let nix = 0; nix < navAs.length; nix++) {

        if (nix == page - 1)

        for (let trix = 0; trix < tBodyRows.length; trix++) 
            tBodyRows[trix].style.display = 
                (trix >= startItem && trix < endItem)
                ? 'table-row'
                : 'none';  



How to read one single line of csv data in Python?

To read only the first row of the csv file use next() on the reader object.

with open('some.csv', newline='') as f:
  reader = csv.reader(f)
  row1 = next(reader)  # gets the first line
  # now do something here 
  # if first row is the header, then you can do one more next() to get the next row:
  # row2 = next(f)

or :

with open('some.csv', newline='') as f:
  reader = csv.reader(f)
  for row in reader:
    # do something here with `row`

How to make a TextBox accept only alphabetic characters?

Try following code in KeyPress event of textbox

if (char.IsLetter(e.KeyChar) == false  & 
        Convert.ToString(e.KeyChar) != Microsoft.VisualBasic.Constants.vbBack)
            e.Handled = true

Insert node at a certain position in a linked list C++

Insert an element at very beginning position. case-1 when the list is empty. case-2 When the list is not empty.


using namespace std;

struct Node{
int data;
Node* next; //link == head =stored the address of the next node

Node* head;  //pointer to Head node with empty list

void Insert(int y);
void print();

int main(){
    head = nullptr; //empty list
    int n,y;
    cout<<"how many number do you want to enter?"<<endl;
    for (int i=0;i<n;i++){
        cout<<"Enter the number "<<i+1<<endl;

void Insert(int y){
    Node* temp = new Node(); //create dynamic memory allocation
    temp->data = y;
    temp->next = head; // temp->next = null; when list is empty
    head = temp;

void print(){
    Node* temp = head;
    cout<<"List is: "<<endl;
    while(temp!= nullptr){
        cout<<temp->data<<" ";
        temp = temp->next;

Setting a property with an EventTrigger

Stopping the Storyboard can be done in the code behind, or the xaml, depending on where the need comes from.

If the EventTrigger is moved outside of the button, then we can go ahead and target it with another EventTrigger that will tell the storyboard to stop. When the storyboard is stopped in this manner it will not revert to the previous value.

Here I've moved the Button.Click EventTrigger to a surrounding StackPanel and added a new EventTrigger on the the CheckBox.Click to stop the Button's storyboard when the CheckBox is clicked. This lets us check and uncheck the CheckBox when it is clicked on and gives us the desired unchecking behavior from the button as well.

    <StackPanel x:Name="myStackPanel">

        <CheckBox x:Name="myCheckBox"
                  Content="My CheckBox" />

        <Button Content="Click to Uncheck"
                x:Name="myUncheckButton" />

        <Button Content="Click to check the box in code."
                Click="OnClick" />


            <EventTrigger RoutedEvent="Button.Click"
                    <BeginStoryboard x:Name="myBeginStoryboard">
                        <Storyboard x:Name="myStoryboard">
                            <BooleanAnimationUsingKeyFrames Storyboard.TargetName="myCheckBox"
                                <DiscreteBooleanKeyFrame KeyTime="00:00:00"
                                                         Value="False" />

            <EventTrigger RoutedEvent="CheckBox.Click"
                    <StopStoryboard BeginStoryboardName="myBeginStoryboard" />


To stop the storyboard in the code behind, we will have to do something slightly different. The third button provides the method where we will stop the storyboard and set the IsChecked property back to true through code.

We can't call myStoryboard.Stop() because we did not begin the Storyboard through the code setting the isControllable parameter. Instead, we can remove the Storyboard. To do this we need the FrameworkElement that the storyboard exists on, in this case our StackPanel. Once the storyboard is removed, we can once again set the IsChecked property with it persisting to the UI.

    private void OnClick(object sender, RoutedEventArgs e)
        myCheckBox.IsChecked = true;

What does the JSLint error 'body of a for in should be wrapped in an if statement' mean?

Just to add on to the topic of for in/for/$.each, I added a jsperf test case for using $.each vs for in:

Different browsers/versions handle it differently, but it seems $.each and straight out for in are the cheapest options performance-wise.

If you're using for in to iterate through an associative array/object, knowing what you're after and ignoring everything else, use $.each if you use jQuery, or just for in (and then a break; once you've reached what you know should be the last element)

If you're iterating through an array to perform something with each key pair in it, should use the hasOwnProperty method if you DON'T use jQuery, and use $.each if you DO use jQuery.

Always use for(i=0;i<o.length;i++) if you don't need an associative array though... lol chrome performed that 97% faster than a for in or $.each

R Markdown - changing font size and font type in html output

I think fontsize: command in YAML only works for LaTeX / pdf. Apart, in standard latex classes (article, book, and report) only three font sizes are accepted (10pt, 11pt, and 12pt).

Regarding appearance (different font types and colors), you can specify a theme:. See Appearance and Style.

I guess, what you are looking for is your own css. Make a file called style.css, save it in the same folder as your .Rmd and include it in the YAML header:

    css: style.css

In the css-file you define your font-type and size:

/* Whole document: */
  font-family: Helvetica;
  font-size: 16pt;
/* Headers */
  font-size: 24pt;

How to delete SQLite database from Android programmatically

context.deleteDatabase(DATABASE_NAME); will delete the database only if all the connections are closed. If you are maintaining singleton instance for handling your database helper - it is easy to close the opened Connection.

Incase the databasehelper is used in multiple place by instantiating directly, the deleteDatabase + killProcess will do the job even if some connections are open. This can be used if the application scenario doesn't have any issues in restarting the app.

send bold & italic text on telegram bot with html

If you are using PHP you can use this, and I'm sure it's almost similar in other languages as well

$WebsiteURL = "".$BotToken;
$text = "<b>This</b> <i>is some Text</i>";
$Update = file_get_contents($WebsiteURL."/sendMessage?chat_id=$chat_id&text=$text&parse_mode=html);

echo $Update;

Here is the list of all tags that you can use

<b>bold</b>, <strong>bold</strong>
<i>italic</i>, <em>italic</em>
<a href="">inline URL</a>
<code>inline fixed-width code</code>
<pre>pre-formatted fixed-width code block</pre>

Query grants for a table in postgres

The query below will give you a list of all users and their permissions on the table in a schema.

select a.schemaname, a.tablename, b.usename,
  HAS_TABLE_PRIVILEGE(usename, quote_ident(schemaname) || '.' || quote_ident(tablename), 'select') as has_select,
  HAS_TABLE_PRIVILEGE(usename, quote_ident(schemaname) || '.' || quote_ident(tablename), 'insert') as has_insert,
  HAS_TABLE_PRIVILEGE(usename, quote_ident(schemaname) || '.' || quote_ident(tablename), 'update') as has_update,
  HAS_TABLE_PRIVILEGE(usename, quote_ident(schemaname) || '.' || quote_ident(tablename), 'delete') as has_delete, 
  HAS_TABLE_PRIVILEGE(usename, quote_ident(schemaname) || '.' || quote_ident(tablename), 'references') as has_references 
from pg_tables a, pg_user b 
where a.schemaname = 'your_schema_name' and a.tablename='your_table_name';

More details on has_table_privilages can be found here.

Insert at first position of a list in Python

From the documentation:

list.insert(i, x)
Insert an item at a given position. The first argument is the index of the element before which to insert, so a.insert(0, x) inserts at the front of the list, and a.insert(len(a),x) is equivalent to a.append(x)

How to parse month full form string using DateFormat in Java?

val currentTime = Calendar.getInstance().time
SimpleDateFormat("MMMM", Locale.getDefault()).format(date.time)

Changing one character in a string

Like other people have said, generally Python strings are supposed to be immutable.

However, if you are using CPython, the implementation at, it is possible to use ctypes to modify the string structure in memory.

Here is an example where I use the technique to clear a string.

Mark data as sensitive in python

I mention this for the sake of completeness, and this should be your last resort as it is hackish.

git is not installed or not in the PATH

Installing git and running npm install from git-bash worked for me. Make sure you are in the correct directory.

How (and why) to use display: table-cell (CSS)

How (and why) to use display: table-cell (CSS)

I just wanted to mention, since I don't think any of the other answers did directly, that the answer to "why" is: there is no good reason, and you should probably never do this.

In my over a decade of experience in web development, I can't think of a single time I would have been better served to have a bunch of <div>s with display styles than to just have table elements.

The only hypothetical I could come up with is if you have tabular data stored in some sort of non-HTML-table format (eg. a CSV file). In a very specific version of this case it might be easier to just add <div> tags around everything and then add descendent-based styles, instead of adding actual table tags.

But that's an extremely contrived example, and in all real cases I know of simply using table tags would be better.

How do I debug jquery AJAX calls?

here is what I would do in order to debug and get the php errors into javascript console.log after an ajax call:

    $('#ChangePermission').click(function () {
        url: 'change_permission.php',
        type: 'POST',
        data: {
            'user': document.GetElementById("user").value,
            'perm': document.GetElementById("perm").value
        success: function (data) {
        error: function (data) {

on the php side I would start by asking for all error returned to string concat and echo as a json encoded response that will includes php error messages as well if any.


require_once(functions . php);
$result = "true";
$DBH = new PDO("mysql:host=$dbhost;dbname=$dbname", $dbuser, $dbpass);  
$STH = $DBH->prepare("INSERT INTO people ( username, permissions ) values (?, ?)");

if (isset($_POST["user"]) && isset($_POST["perm"])) {
    $STH->bindParam(1, $_POST["user"], PDO::PARAM_STR);
    $STH->bindParam(2, $_POST["perm"], PDO::PARAM_STR);
try {
} catch (PDOException $e) {
    $result .= $e->getMessage;
echo json_encode($result);

Log.INFO vs. Log.DEBUG

I usually try to use it like this:

  • DEBUG: Information interesting for Developers, when trying to debug a problem.
  • INFO: Information interesting for Support staff trying to figure out the context of a given error
  • WARN to FATAL: Problems and Errors depending on level of damage.

jQuery UI Dialog Box - does not open after being closed

on the last line, don't use $(this).remove() use $(this).hide() instead.

EDIT: To clarify,on the close click event you're removing the #terms div from the DOM which is why its not coming back. You just need to hide it instead.

How to sort an array in Bash

In the 3-hour train trip from Munich to Frankfurt (which I had trouble to reach because Oktoberfest starts tomorrow) I was thinking about my first post. Employing a global array is a much better idea for a general sort function. The following function handles arbitary strings (newlines, blanks etc.):

declare BSORT=()
function bubble_sort()
{   #
    # @param [ARGUMENTS]...
    # Sort all positional arguments and store them in global array BSORT.
    # Without arguments sort this array. Return the number of iterations made.
    # Bubble sorting lets the heaviest element sink to the bottom.
    (($# > 0)) && BSORT=("$@")
    local j=0 ubound=$((${#BSORT[*]} - 1))
    while ((ubound > 0))
        local i=0
        while ((i < ubound))
            if [ "${BSORT[$i]}" \> "${BSORT[$((i + 1))]}" ]
                local t="${BSORT[$i]}"
                BSORT[$i]="${BSORT[$((i + 1))]}"
                BSORT[$((i + 1))]="$t"
    echo $j

bubble_sort a c b 'z y' 3 5
echo ${BSORT[@]}

This prints:

3 5 a b c z y

The same output is created from

BSORT=(a c b 'z y' 3 5) 
echo ${BSORT[@]}

Note that probably Bash internally uses smart-pointers, so the swap-operation could be cheap (although I doubt it). However, bubble_sort demonstrates that more advanced functions like merge_sort are also in the reach of the shell language.

jQuery date/time picker

This is some code I use to have a user select one datetimepicker, set the datetime, and have the other datetimepicker add One Minute to that time.

I needed this for a custom medication control....

Anyway, thought it might help someone else since I could not find the answer any where online...

(at least not a complete answer)

Keep in mind that the 60000 added, adds one minute. (60 * 1000 milliseconds)

        onClose: function(dateText, inst) {
            var endDateTextBox = $('.todtPicker');
            if (endDateTextBox.val() != '') {
                var testStartDate = new Date(dateText);
                var testEndDate = new Date(endDateTextBox.val());
                if (testStartDate > testEndDate) {

                    var testStartDate = new Date(dateText).getTime() + 60000;
                    var testStartDate2 = new Date(testStartDate); 

                    endDateTextBox.datetimepicker('setDate', (new Date(testStartDate2)));
            else {
                var testStartDate = new Date(dateText).getTime() + 60000;
                var testStartDate2 = new Date(testStartDate);
                endDateTextBox.datetimepicker('setDate', (new Date(testStartDate2)));
            $('.frdtPicker').val(dateText); //endDateTextBox.val());

        onSelect: function(selectedDateTime) {
            var start = $(this).datetimepicker('getDate');
            $('.todtPicker').datetimepicker('option', 'minDate', new Date(start.getTime()));

Convert binary to ASCII and vice versa

For ASCII characters in the range [ -~] on Python 2:

>>> import binascii
>>> bin(int(binascii.hexlify('hello'), 16))

In reverse:

>>> n = int('0b110100001100101011011000110110001101111', 2)
>>> binascii.unhexlify('%x' % n)

In Python 3.2+:

>>> bin(int.from_bytes('hello'.encode(), 'big'))

In reverse:

>>> n = int('0b110100001100101011011000110110001101111', 2)
>>> n.to_bytes((n.bit_length() + 7) // 8, 'big').decode()

To support all Unicode characters in Python 3:

def text_to_bits(text, encoding='utf-8', errors='surrogatepass'):
    bits = bin(int.from_bytes(text.encode(encoding, errors), 'big'))[2:]
    return bits.zfill(8 * ((len(bits) + 7) // 8))

def text_from_bits(bits, encoding='utf-8', errors='surrogatepass'):
    n = int(bits, 2)
    return n.to_bytes((n.bit_length() + 7) // 8, 'big').decode(encoding, errors) or '\0'

Here's single-source Python 2/3 compatible version:

import binascii

def text_to_bits(text, encoding='utf-8', errors='surrogatepass'):
    bits = bin(int(binascii.hexlify(text.encode(encoding, errors)), 16))[2:]
    return bits.zfill(8 * ((len(bits) + 7) // 8))

def text_from_bits(bits, encoding='utf-8', errors='surrogatepass'):
    n = int(bits, 2)
    return int2bytes(n).decode(encoding, errors)

def int2bytes(i):
    hex_string = '%x' % i
    n = len(hex_string)
    return binascii.unhexlify(hex_string.zfill(n + (n & 1)))


>>> text_to_bits('hello')
>>> text_from_bits('110100001100101011011000110110001101111') == u'hello'

Java generics: multiple generic parameters?

Yes - it's possible (though not with your method signature) and yes, with your signature the types must be the same.

With the signature you have given, T must be associated to a single type (e.g. String or Integer) at the call-site. You can, however, declare method signatures which take multiple type parameters

public <S, T> void func(Set<S> s, Set<T> t)

Note in the above signature that I have declared the types S and T in the signature itself. These are therefore different to and independent of any generic types associated with the class or interface which contains the function.

public class MyClass<S, T> {
   public        void foo(Set<S> s, Set<T> t); //same type params as on class
   public <U, V> void bar(Set<U> s, Set<V> t); //type params independent of class

You might like to take a look at some of the method signatures of the collection classes in the java.util package. Generics is really rather a complicated subject, especially when wildcards (? extends and ? super) are considered. For example, it's often the case that a method which might take a Set<Number> as a parameter should also accept a Set<Integer>. In which case you'd see a signature like this:

public void baz(Set<? extends T> s);

There are plenty of questions already on SO for you to look at on the subject!

Not sure what the point of returning an int from the function is, although you could do that if you want!

Maven build Compilation error : Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.1:compile (default-compile) on project Maven

I had the same problem and I Changed this


here 1.7 is my JDK was solved.

Binding Combobox Using Dictionary as the Datasource

I used Sorin Comanescu's solution, but hit a problem when trying to get the selected value. My combobox was a toolstrip combobox. I used the "combobox" property, which exposes a normal combobox.

I had a

 Dictionary<Control, string> controls = new Dictionary<Control, string>();

Binding code (Sorin Comanescu's solution - worked like a charm):

 controls.Add(pictureBox1, "Image");
 controls.Add(dgvText, "Text");
 cbFocusedControl.ComboBox.DataSource = new BindingSource(controls, null);
 cbFocusedControl.ComboBox.ValueMember = "Key";
 cbFocusedControl.ComboBox.DisplayMember = "Value";

The problem was that when I tried to get the selected value, I didn't realize how to retrieve it. After several attempts I got this:

 var control = ((KeyValuePair<Control, string>) cbFocusedControl.ComboBox.SelectedItem).Key

Hope it helps someone else!

How to print a string multiple times?

The question is a bit unclear can't you just repeat the for loop?


for i in a:
    print i

for i in a:
    print i

Best practice to return errors in ASP.NET Web API

Use the built in "InternalServerError" method (available in ApiController):

return InternalServerError();
return InternalServerError(new YourException("your message"));

When should iteritems() be used instead of items()?

Just as @Wessie noted, dict.iteritems, dict.iterkeys and dict.itervalues (which return an iterator in Python2.x) as well as dict.viewitems, dict.viewkeys and dict.viewvalues (which return view objects in Python2.x) were all removed in Python3.x

And dict.items, dict.keys and dict.values used to return a copy of the dictionary's list in Python2.x now return view objects in Python3.x, but they are still not the same as iterator.

If you want to return an iterator in Python3.x, use iter(dictview) :

$ python3.3

>>> d = {'one':'1', 'two':'2'}
>>> type(d.items())
<class 'dict_items'>
>>> type(d.keys())
<class 'dict_keys'>
>>> ii = iter(d.items())
>>> type(ii)
<class 'dict_itemiterator'>
>>> ik = iter(d.keys())
>>> type(ik)
<class 'dict_keyiterator'>

How do I print the key-value pairs of a dictionary in python

You can access your keys and/or values by calling items() on your dictionary.

for key, value in d.iteritems():
    print(key, value)

Android SDK Setup under Windows 7 Pro 64 bit

When it says JDK not found, just press ‘back’ button and then press again ‘next’ button..

I got this from the bottom of this post:

How to display hidden characters by default (ZERO WIDTH SPACE ie. &#8203)

Not sure what you meant, but you can permanently turn showing whitespaces on and off in Settings -> Editor -> General -> Appearance -> Show whitespaces.

Also, you can set it for a current file only in View -> Active Editor -> Show WhiteSpaces.


Had some free time since it looks like a popular issue, I had written a plugin to inspect the code for such abnormalities. It is called Zero Width Characters locator and you're welcome to give it a try.

Can't ping a local VM from the host

I had the same issue. Fixed it by adding a static route on my host to my VM via the VMnet8 adapter:

route ADD VM_addr MASK VMnet8_addr

As previously mentioned, you need a bridged connection.

How can I install a package with go get?

Command go

Download and install packages and dependencies


go get [-d] [-f] [-t] [-u] [-v] [-fix] [-insecure] [build flags] [packages]

Get downloads the packages named by the import paths, along with their dependencies. It then installs the named packages, like 'go install'.

The -d flag instructs get to stop after downloading the packages; that is, it instructs get not to install the packages.

The -f flag, valid only when -u is set, forces get -u not to verify that each package has been checked out from the source control repository implied by its import path. This can be useful if the source is a local fork of the original.

The -fix flag instructs get to run the fix tool on the downloaded packages before resolving dependencies or building the code.

The -insecure flag permits fetching from repositories and resolving custom domains using insecure schemes such as HTTP. Use with caution.

The -t flag instructs get to also download the packages required to build the tests for the specified packages.

The -u flag instructs get to use the network to update the named packages and their dependencies. By default, get uses the network to check out missing packages but does not use it to look for updates to existing packages.

The -v flag enables verbose progress and debug output.

Get also accepts build flags to control the installation. See 'go help build'.

When checking out a new package, get creates the target directory GOPATH/src/. If the GOPATH contains multiple entries, get uses the first one. For more details see: 'go help gopath'.

When checking out or updating a package, get looks for a branch or tag that matches the locally installed version of Go. The most important rule is that if the local installation is running version "go1", get searches for a branch or tag named "go1". If no such version exists it retrieves the default branch of the package.

When go get checks out or updates a Git repository, it also updates any git submodules referenced by the repository.

Get never checks out or updates code stored in vendor directories.

For more about specifying packages, see 'go help packages'.

For more about how 'go get' finds source code to download, see 'go help importpath'.

This text describes the behavior of get when using GOPATH to manage source code and dependencies. If instead the go command is running in module-aware mode, the details of get's flags and effects change, as does 'go help get'. See 'go help modules' and 'go help module-get'.

See also: go build, go install, go clean.

For example, showing verbose output,

$ go get -v (download) (download) (download)

Curl and PHP - how can I pass a json through curl by PUT,POST,GET

I was Working with Elastic SQL plugin. Query is done with GET method using cURL as below:

curl -XGET http://localhost:9200/_sql/_explain -H 'Content-Type: application/json' \
-d 'SELECT city.keyword as city FROM routes group by city.keyword order by city'

I exposed a custom port at public server, doing a reverse proxy with Basic Auth set.

This code, works fine plus Basic Auth Header:

$host = '';
$uri = "/_sql/_explain";
$auth = "john:doe";
$data = "SELECT city.keyword as city FROM routes group by city.keyword order by city";

function restCurl($host, $uri, $data = null, $auth = null, $method = 'DELETE'){
    $ch = curl_init();

    curl_setopt($ch, CURLOPT_URL, $host.$uri);
    curl_setopt($ch, CURLOPT_CUSTOMREQUEST, $method);
    curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json'));
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

    if ($method == 'POST')
        curl_setopt($ch, CURLOPT_POST, 1);
    if ($auth)
        curl_setopt($ch, CURLOPT_USERPWD, $auth);
    if (strlen($data) > 0)
        curl_setopt($ch, CURLOPT_POSTFIELDS,$data);

    $resp  = curl_exec($ch);
        $resp = (json_encode(array(array("error" => curl_error($ch), "code" => curl_errno($ch)))));
    return $resp;

$resp = restCurl($host, $uri); //DELETE
$resp = restCurl($host, $uri, $data, $auth, 'GET'); //GET
$resp = restCurl($host, $uri, $data, $auth, 'POST'); //POST
$resp = restCurl($host, $uri, $data, $auth, 'PUT'); //PUT

Allow all remote connections, MySQL

mysql> CREATE USER 'monty'@'192.168.%.%' IDENTIFIED BY 'some_pass';
mysql> GRANT ALL PRIVILEGES ON *.* TO 'monty'@'192.168.%.%'

Check if SQL Connection is Open or Closed

You should be using SqlConnection.State


using System.Data;

if (myConnection != null && myConnection.State == ConnectionState.Closed)
   // do something
   // ...

How to use HTML to print header and footer on every printed page of a document?

Muhammad Musavi's comment is the best answer, so here it is surfaced as an actual Answer:

thead/tfoot are automatically repeated on the top and bottom of each page. However, tfoot isn't sticky to the bottom of the last page.

position: fixed in print will repeat on each page, and the footer will stick to the bottom of all pages including the last one - but, it won't create space for its contents.

Combine them:


<header>(repeated header)</header>

<table class=paging><thead><tr><td>&nbsp;</td></tr></thead><tbody><tr><td>

(content goes here)


<footer>(repeated footer)</footer>


@page {
    size: letter;
    margin: .5in;

@media print {
    table.paging thead td, table.paging tfoot td {
        height: .5in;

header, footer {
    width: 100%; height: .5in;

header {
    position: absolute;
    top: 0;

@media print {
    header, footer {
        position: fixed;
    footer {
        bottom: 0;

There are a lot of niceties you can add in here, but I've intentionally slashed this to the bare minimum to get a cleanly rendering header and footer, appearing once on-screen and at the top and bottom of every printed page.

Most useful NLog configurations

Treating exceptions differently

We often want to get more information when there is an exception. The following configuration has two targets, a file and the console, which filter on whether or not there is any exception info. (EDIT: Jarek has posted about a new method of doing this in vNext.)

The key is to have a wrapper target with xsi:type="FilteringWrapper" condition="length('${exception}')>0"

<nlog xmlns=""
      internalLogFile="nlog log.log"
    <variable name="VerboseLayout" 
              value="${longdate} ${level:upperCase=true} ${message}  
                    (${callsite:includSourcePath=true})"            />
    <variable name="ExceptionVerboseLayout"  
              value="${VerboseLayout} (${stacktrace:topFrames=10})  
                     ${exception:format=ToString}"                  />

    <targets async="true">
        <target name="file" xsi:type="File" fileName="log.log"

        <target name="fileAsException"  
            <target xsi:type="File"  
                    layout="${ExceptionVerboseLayout}" />

        <target xsi:type="ColoredConsole"

        <target xsi:type="FilteringWrapper"  
            <target xsi:type="ColoredConsole" 
                    layout="${ExceptionVerboseLayout}" />

        <logger name="*" minlevel="Trace" writeTo="console,consoleException" />
        <logger name="*" minlevel="Warn" writeTo="file,fileAsException" />


How to make PDF file downloadable in HTML link?

Instead of linking to the .PDF file, instead do something like

<a href="pdf_server.php?file=pdffilename">Download my eBook</a>

which outputs a custom header, opens the PDF (binary safe) and prints the data to the user's browser, then they can choose to save the PDF despite their browser settings. The pdf_server.php should look like this:

header("Content-Type: application/octet-stream");

$file = $_GET["file"] .".pdf";
header("Content-Disposition: attachment; filename=" . urlencode($file));   
header("Content-Type: application/octet-stream");
header("Content-Type: application/download");
header("Content-Description: File Transfer");            
header("Content-Length: " . filesize($file));
flush(); // this doesn't really matter.
$fp = fopen($file, "r");
while (!feof($fp))
    echo fread($fp, 65536);
    flush(); // this is essential for large downloads

PS: and obviously run some sanity checks on the "file" variable to prevent people from stealing your files such as don't accept file extensions, deny slashes, add .pdf to the value

Async always WaitingForActivation

this code seems to have address the issue for me. it comes for a streaming class, ergo some of the nomenclature.

''' <summary> Reference to the awaiting task. </summary>
''' <value> The awaiting task. </value>
Protected ReadOnly Property AwaitingTask As Threading.Tasks.Task

''' <summary> Reference to the Action task; this task status undergoes changes. </summary>
Protected ReadOnly Property ActionTask As Threading.Tasks.Task

''' <summary> Reference to the cancellation source. </summary>
Protected ReadOnly Property TaskCancellationSource As Threading.CancellationTokenSource

''' <summary> Starts the action task. </summary>
''' <param name="taskAction"> The action to stream the entities, which calls
'''                           <see cref="StreamEvents(Of T)(IEnumerable(Of T), IEnumerable(Of Date), Integer, String)"/>. </param>
''' <returns> The awaiting task. </returns>
Private Async Function AsyncAwaitTask(ByVal taskAction As Action) As Task
    Me._ActionTask = Task.Run(taskAction)
    Await Me.ActionTask '  Task.Run(streamEntitiesAction)
        Me.OnStreamTaskEnded(If(Me.ActionTask Is Nothing, TaskStatus.RanToCompletion, Me.ActionTask.Status))
    Catch ex As AggregateException
    End Try
End Function

''' <summary> Starts Streaming the events. </summary>
''' <exception cref="InvalidOperationException"> Thrown when the requested operation is invalid. </exception>
''' <param name="bucketKey">            The bucket key. </param>
''' <param name="timeout">              The timeout. </param>
''' <param name="streamEntitiesAction"> The action to stream the entities, which calls
'''                                     <see cref="StreamEvents(Of T)(IEnumerable(Of T), IEnumerable(Of Date), Integer, String)"/>. </param>
Public Overridable Sub StartStreamEvents(ByVal bucketKey As String, ByVal timeout As TimeSpan, ByVal streamEntitiesAction As Action)
    If Me.IsTaskActive Then
        Throw New InvalidOperationException($"Stream task is {Me.ActionTask.Status}")
        Me._TaskCancellationSource = New Threading.CancellationTokenSource
        Me.TaskCancellationSource.Token.Register(AddressOf Me.StreamTaskCanceled)
        ' the action class is created withing the Async/Await function
        Me._AwaitingTask = Me.AsyncAwaitTask(streamEntitiesAction)
    End If
End Sub

How to convert a JSON string to a dictionary?


  • Xcode Version 10.3 (10G8), Swift 5


import Foundation

// MARK: - CastingError

struct CastingError: Error {
    let fromType: Any.Type
    let toType: Any.Type
    init<FromType, ToType>(fromType: FromType.Type, toType: ToType.Type) {
        self.fromType = fromType
        self.toType = toType

extension CastingError: LocalizedError {
    var localizedDescription: String { return "Can not cast from \(fromType) to \(toType)" }

extension CastingError: CustomStringConvertible { var description: String { return localizedDescription } }

// MARK: - Data cast extensions

extension Data {
    func toDictionary(options: JSONSerialization.ReadingOptions = []) throws -> [String: Any] {
        return try to(type: [String: Any].self, options: options)

    func to<T>(type: T.Type, options: JSONSerialization.ReadingOptions = []) throws -> T {
        guard let result = try JSONSerialization.jsonObject(with: self, options: options) as? T else {
            throw CastingError(fromType: type, toType: T.self)
        return result

// MARK: - String cast extensions

extension String {
    func asJSON<T>(to type: T.Type, using encoding: String.Encoding = .utf8) throws -> T {
        guard let data = data(using: encoding) else { throw CastingError(fromType: type, toType: T.self) }
        return try T.self)

    func asJSONToDictionary(using encoding: String.Encoding = .utf8) throws -> [String: Any] {
        return try asJSON(to: [String: Any].self, using: encoding)

// MARK: - Dictionary cast extensions

extension Dictionary {
    func toData(options: JSONSerialization.WritingOptions = []) throws -> Data {
        return try self, options: options)


let value1 = try? data.toDictionary()
let value2 = try? [String: Any].self)
let value3 = try? [String: String].self)
let value4 = try? string.asJSONToDictionary()
let value5 = try? string.asJSON(to: [String: String].self)

Test sample

Do not forget to paste the solution code here

func testDescriber(text: String, value: Any) {
    print("-- \(text)\n\n  type: \(type(of: value))\n  value: \(value)")

let json1: [String: Any] = ["key1" : 1, "key2": true, "key3" : ["a": 1, "b": 2], "key4": [1,2,3]]
var jsonData = try? json1.toData()
testDescriber(text: "Sample test of func toDictionary()", value: json1)
if let data = jsonData {
    print("  Result: \(String(describing: try? data.toDictionary()))")

testDescriber(text: "Sample test of func to<T>() -> [String: Any]", value: json1)
if let data = jsonData {
    print("  Result: \(String(describing: try? [String: Any].self)))")

testDescriber(text: "Sample test of func to<T>() -> [String] with cast error", value: json1)
if let data = jsonData {
    do {
        print("  Result: \(String(describing: try [String].self)))")
    } catch {
        print("  ERROR: \(error)")

let array = [1,4,5,6]
testDescriber(text: "Sample test of func to<T>() -> [Int]", value: array)
if let data = try? array) {
    print("  Result: \(String(describing: try? [Int].self)))")

let json2 = ["key1": "a", "key2": "b"]
testDescriber(text: "Sample test of func to<T>() -> [String: String]", value: json2)
if let data = try? json2) {
    print("  Result: \(String(describing: try? [String: String].self)))")

let jsonString = "{\"key1\": \"a\", \"key2\": \"b\"}"
testDescriber(text: "Sample test of func to<T>() -> [String: String]", value: jsonString)
print("  Result: \(String(describing: try? jsonString.asJSON(to: [String: String].self)))")

testDescriber(text: "Sample test of func to<T>() -> [String: String]", value: jsonString)
print("  Result: \(String(describing: try? jsonString.asJSONToDictionary()))")

let wrongJsonString = "{\"key1\": \"a\", \"key2\":}"
testDescriber(text: "Sample test of func to<T>() -> [String: String] with JSONSerialization error", value: jsonString)
do {
    let json = try wrongJsonString.asJSON(to: [String: String].self)
    print("  Result: \(String(describing: json))")
} catch {
    print("  ERROR: \(error)")

Test log

-- Sample test of func toDictionary()

  type: Dictionary<String, Any>
  value: ["key4": [1, 2, 3], "key2": true, "key3": ["a": 1, "b": 2], "key1": 1]
  Result: Optional(["key4": <__NSArrayI 0x600002a35380>(
, "key2": 1, "key3": {
    a = 1;
    b = 2;
}, "key1": 1])

-- Sample test of func to<T>() -> [String: Any]

  type: Dictionary<String, Any>
  value: ["key4": [1, 2, 3], "key2": true, "key3": ["a": 1, "b": 2], "key1": 1]
  Result: Optional(["key4": <__NSArrayI 0x600002a254d0>(
, "key2": 1, "key1": 1, "key3": {
    a = 1;
    b = 2;

-- Sample test of func to<T>() -> [String] with cast error

  type: Dictionary<String, Any>
  value: ["key4": [1, 2, 3], "key2": true, "key3": ["a": 1, "b": 2], "key1": 1]
  ERROR: Can not cast from Array<String> to Array<String>

-- Sample test of func to<T>() -> [Int]

  type: Array<Int>
  value: [1, 4, 5, 6]
  Result: Optional([1, 4, 5, 6])

-- Sample test of func to<T>() -> [String: String]

  type: Dictionary<String, String>
  value: ["key1": "a", "key2": "b"]
  Result: Optional(["key1": "a", "key2": "b"])

-- Sample test of func to<T>() -> [String: String]

  type: String
  value: {"key1": "a", "key2": "b"}
  Result: Optional(["key1": "a", "key2": "b"])

-- Sample test of func to<T>() -> [String: String]

  type: String
  value: {"key1": "a", "key2": "b"}
  Result: Optional(["key1": a, "key2": b])

-- Sample test of func to<T>() -> [String: String] with JSONSerialization error

  type: String
  value: {"key1": "a", "key2": "b"}
  ERROR: Error Domain=NSCocoaErrorDomain Code=3840 "Invalid value around character 21." UserInfo={NSDebugDescription=Invalid value around character 21.}

Using ping in c#

Imports System.Net.NetworkInformation

Public Function PingHost(ByVal nameOrAddress As String) As Boolean
    Dim pingable As Boolean = False
    Dim pinger As Ping
    Dim lPingReply As PingReply

        pinger = New Ping()
        lPingReply = pinger.Send(nameOrAddress)
        If lPingReply.Status = IPStatus.Success Then

            pingable = True
            pingable = False
        End If

    Catch PingException As Exception
        pingable = False
    End Try
    Return pingable
End Function

Initializing data.frames()

I always just convert a matrix:

x <- = 100, ncol = 10))

convert string date to java.sql.Date

worked for me too:

SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy");
    Date parsed = null;
    try {
        parsed = sdf.parse("02/01/2014");
    } catch (ParseException e1) {
        // TODO Auto-generated catch block
    java.sql.Date data = new java.sql.Date(parsed.getTime());
    contato.setDataNascimento( data);

    // Contato DataNascimento era Calendar

    // grave nessa conexão!!! 
    ContatoDao dao = new ContatoDao("mysql");           

    // método elegante 
    System.out.println("Banco: ["+dao.getNome()+"] Gravado! Data: "+contato.getDataNascimento());

Get and set position with jQuery .offset()

var p = $("#elementId");
var offset = p.offset();

$("#secondElementId").offset({ top:, left: offset.left});

How to compare oldValues and newValues on React Hooks useEffect?

Using Ref will introduce a new kind of bug into the app.

Let's see this case using usePrevious that someone commented before:

  1. prop.minTime: 5 ==> ref.current = 5 | set ref.current
  2. prop.minTime: 5 ==> ref.current = 5 | new value is equal to ref.current
  3. prop.minTime: 8 ==> ref.current = 5 | new value is NOT equal to ref.current
  4. prop.minTime: 5 ==> ref.current = 5 | new value is equal to ref.current

As we can see here, we are not updating the internal ref because we are using useEffect

What are the "spec.ts" files generated by Angular CLI for?

if you generate new angular project using "ng new", you may skip a generating of spec.ts files. For this you should apply --skip-tests option.

ng new ng-app-name --skip-tests

Validating input using java.util.Scanner

If you are parsing string data from the console or similar, the best way is to use regular expressions. Read more on that here:

Otherwise, to parse an int from a string, try Integer.parseInt(string). If the string is not a number, you will get an exception. Otherise you can then perform your checks on that value to make sure it is not negative.

String input;
int number;
    number = Integer.parseInt(input);
    if(number > 0)
        System.out.println("You positive number is " + number);
} catch (NumberFormatException ex)
     System.out.println("That is not a positive number!");

To get a character-only string, you would probably be better of looping over each character checking for digits, using for instance Character.isLetter(char).

String input
for(int i = 0; i<input.length(); i++)
      System.out.println("This string does not contain only letters!");

Good luck!

How to get current working directory using vba?

Use these codes and enjoy it.

Public Function GetDirectoryName(ByVal source As String) As String()
Dim fso, oFolder, oSubfolder, oFile, queue As Collection
Set fso = CreateObject("Scripting.FileSystemObject")
Set queue = New Collection

Dim source_file() As String
Dim i As Integer        

queue.Add fso.GetFolder(source) 'obviously replace

Do While queue.Count > 0
    Set oFolder = queue(1)
    queue.Remove 1 'dequeue
    '...insert any folder processing code here...
    For Each oSubfolder In oFolder.SubFolders
        queue.Add oSubfolder 'enqueue
    Next oSubfolder
    For Each oFile In oFolder.Files
        '...insert any file processing code here...
        'Debug.Print oFile
        i = i + 1
        ReDim Preserve source_file(i)
        source_file(i) = oFile
    Next oFile
GetDirectoryName = source_file
End Function

And here you can call function:

Sub test()
Dim s
For Each s In GetDirectoryName("C:\New folder")
Debug.Print s
End Sub

Change the location of an object programmatically

If somehow balancePanel won't work, you could use this:

this.Location = new Point(127, 283);


anotherObject.Location = new Point(127, 283);

Truncate a SQLite table if it exists?

"sqllite" dosn't have "TRUNCATE " order like than mysql, then we have to get other way... this function (reset_table) frist delete all data in table and then reset AUTOINCREMENT key in table.... now you can use this function every where you want...

example :

private SQLiteDatabase mydb;
private final String dbPath = "data/data/your_project_name/databases/";
private final String dbName = "your_db_name";

public void reset_table(String table_name){

    mydb.execSQL("Delete from "+table_name);
    mydb.execSQL("DELETE FROM SQLITE_SEQUENCE WHERE name='"+table_name+"';");


public void open_db(){

    mydb = SQLiteDatabase.openDatabase(dbPath + dbName + ".db", null, SQLiteDatabase.OPEN_READWRITE);
public void close_db(){


Error - replacement has [x] rows, data has [y]

TL;DR ...and late to the party, but that short explanation might help future googlers..

In general that error message means that the replacement doesn't fit into the corresponding column of the dataframe.

A minimal example:

df <- data.frame(a = 1:2); df$a <- 1:3

throws the error

Error in $<*tmp*, a, value = 1:3) : replacement has 3 rows, data has 2

which is clear, because the vector a of df has 2 entries (rows) whilst the vector we try to replace it has 3 entries (rows).

How do I get a Date without time in Java?

Definitely not the most correct way, but if you just need a quick solution to get the date without the time and you do not wish to use a third party library this should do

    Date db = db.substring(0, 10) + db.substring(23,28);

I only needed the date for visual purposes and couldn't Joda so I substringed.

Set encoding and fileencoding to utf-8 in Vim

You can set the variable 'fileencodings' in your .vimrc.

This is a list of character encodings considered when starting to edit an existing file. When a file is read, Vim tries to use the first mentioned character encoding. If an error is detected, the next one in the list is tried. When an encoding is found that works, 'fileencoding' is set to it. If all fail, 'fileencoding' is set to an empty string, which means the value of 'encoding' is used.

See :help filencodings

If you often work with e.g. cp1252, you can add it there:

set fileencodings=ucs-bom,utf-8,cp1252,default,latin9

How to iterate over associative arrays in Bash

Welcome to input associative array 2.0!

    echo "Welcome to input associative array 2.0! (Spaces in keys and values now supported)"
    unset array
    declare -A array
    read -p 'Enter number for array size: ' num
    for (( i=0; i < num; i++ ))
            echo -n "(pair $(( $i+1 )))"
            read -p ' Enter key: ' k
            read -p '         Enter value: ' v
            echo " "
    echo " "
    echo "The keys are: " ${!array[@]}
    echo "The values are: " ${array[@]}
    echo " "
    echo "Key <-> Value"
    echo "-------------"
    for i in "${!array[@]}"; do echo $i "<->" ${array[$i]}; done
    echo " "
    echo "Thanks for using input associative array 2.0!"


Welcome to input associative array 2.0! (Spaces in keys and values now supported)
Enter number for array size: 4
(pair 1) Enter key: Key Number 1
         Enter value: Value#1

(pair 2) Enter key: Key Two
         Enter value: Value2

(pair 3) Enter key: Key3
         Enter value: Val3

(pair 4) Enter key: Key4
         Enter value: Value4

The keys are:  Key4 Key3 Key Number 1 Key Two
The values are:  Value4 Val3 Value#1 Value2

Key <-> Value
Key4 <-> Value4
Key3 <-> Val3
Key Number 1 <-> Value#1
Key Two <-> Value2

Thanks for using input associative array 2.0!

Input associative array 1.0

(keys and values that contain spaces are not supported)

    echo "Welcome to input associative array! (written by mO extraordinaire!)"
    unset array
    declare -A array
    read -p 'Enter number for array size: ' num
    for (( i=0; i < num; i++ ))
            read -p 'Enter key and value separated by a space: ' k v
    echo " "
    echo "The keys are: " ${!array[@]}
    echo "The values are: " ${array[@]}
    echo " "
    echo "Key <-> Value"
    echo "-------------"
    for i in ${!array[@]}; do echo $i "<->" ${array[$i]}; done
    echo " "
    echo "Thanks for using input associative array!"


Welcome to input associative array! (written by mO extraordinaire!)
Enter number for array size: 10
Enter key and value separated by a space: a1 10
Enter key and value separated by a space: b2 20
Enter key and value separated by a space: c3 30
Enter key and value separated by a space: d4 40
Enter key and value separated by a space: e5 50
Enter key and value separated by a space: f6 60
Enter key and value separated by a space: g7 70
Enter key and value separated by a space: h8 80
Enter key and value separated by a space: i9 90
Enter key and value separated by a space: j10 100

The keys are:  h8 a1 j10 g7 f6 e5 d4 c3 i9 b2
The values are:  80 10 100 70 60 50 40 30 90 20

Key <-> Value
h8 <-> 80
a1 <-> 10
j10 <-> 100
g7 <-> 70
f6 <-> 60
e5 <-> 50
d4 <-> 40
c3 <-> 30
i9 <-> 90
b2 <-> 20

Thanks for using input associative array!

How do I position a div at the bottom center of the screen

If you aren't comfortable with using negative margins, check this out.

div {
  position: fixed;
  left: 50%;
  bottom: 20px;
  transform: translate(-50%, -50%);
  margin: 0 auto;
  Your Text

Especially useful when you don't know the width of the div.

align="center" has no effect.

Since you have position:absolute, I would recommend positioning it 50% from the left and then subtracting half of its width from its left margin.

#manipulate {

Changing text of UIButton programmatically swift

Changing title when attributed is a bit different :

I just ran into a problem : If you have an UIButton with an Attributed Title, you have to use :

my_btn.setAttributedTitle(NSAttributedString(string: my_title), for: my_state)

as, per Apple SetTitle Doc :

If you set both a title and an attributed title for the button, the button prefers the use of the attributed title over this one.

I had an attributed title and I tried to setTitle on it, with no effect...

Pause Console in C++ program

I just want to add that there is a way to get what you want, but it will require to use some a third party library (or that you write the platform dependent code yourself).

As far as I'm concerned, the biggest drawback with cin is that you are required to hit Return, and not just any key.

Assuming you had a key-listener you could quite easily write a function that waits for the user to hit any key. However finding a platform independent key listener is no trivial task, and will most likely require you to load parts of a larger library.

I am thinking something along the lines of:

char wait_for_key() {
    int key;
    while ( ! (key == key_pressed(ANY)) ) {
    return convert_virtual_key_to_char(key);

The actual function would obviously be quite different from what I wrote, depending on the library you use.

I know the following libraries have keylisteners (Feel free to add more in an edit if you know of any.):

Maven parent pom vs modules pom

In my opinion, to answer this question, you need to think in terms of project life cycle and version control. In other words, does the parent pom have its own life cycle i.e. can it be released separately of the other modules or not?

If the answer is yes (and this is the case of most projects that have been mentioned in the question or in comments), then the parent pom needs his own module from a VCS and from a Maven point of view and you'll end up with something like this at the VCS level:

|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
`-- projectA
    |-- branches
    |-- tags
    `-- trunk
        |-- module1
        |   `-- pom.xml
        |-- moduleN
        |   `-- pom.xml
        `-- pom.xml

This makes the checkout a bit painful and a common way to deal with that is to use svn:externals. For example, add a trunks directory:

|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
|-- projectA
|   |-- branches
|   |-- tags
|   `-- trunk
|       |-- module1
|       |   `-- pom.xml
|       |-- moduleN
|       |   `-- pom.xml
|       `-- pom.xml
`-- trunks

With the following externals definition:

parent-pom http://host/svn/parent-pom/trunk
projectA http://host/svn/projectA/trunk

A checkout of trunks would then result in the following local structure (pattern #2):


Optionally, you can even add a pom.xml in the trunks directory:

|-- parent-pom
|   |-- branches
|   |-- tags
|   `-- trunk
|       `-- pom.xml
|-- projectA
|   |-- branches
|   |-- tags
|   `-- trunk
|       |-- module1
|       |   `-- pom.xml
|       |-- moduleN
|       |   `-- pom.xml
|       `-- pom.xml
`-- trunks
    `-- pom.xml

This pom.xml is a kind of "fake" pom: it is never released, it doesn't contain a real version since this file is never released, it only contains a list of modules. With this file, a checkout would result in this structure (pattern #3):


This "hack" allows to launch of a reactor build from the root after a checkout and make things even more handy. Actually, this is how I like to setup maven projects and a VCS repository for large builds: it just works, it scales well, it gives all the flexibility you may need.

If the answer is no (back to the initial question), then I think you can live with pattern #1 (do the simplest thing that could possibly work).

Now, about the bonus questions:

  • Where is the best place to define the various shared configuration as in source control, deployment directories, common plugins etc. (I'm assuming the parent but I've often been bitten by this and they've ended up in each project rather than a common one).

Honestly, I don't know how to not give a general answer here (like "use the level at which you think it makes sense to mutualize things"). And anyway, child poms can always override inherited settings.

  • How do the maven-release plugin, hudson and nexus deal with how you set up your multi-projects (possibly a giant question, it's more if anyone has been caught out when by how a multi-project build has been set up)?

The setup I use works well, nothing particular to mention.

Actually, I wonder how the maven-release-plugin deals with pattern #1 (especially with the <parent> section since you can't have SNAPSHOT dependencies at release time). This sounds like a chicken or egg problem but I just can't remember if it works and was too lazy to test it.

How to set dialog to show in full screen?

based on this link , the correct answer (which i've tested myself) is:

put this code in the constructor or the onCreate() method of the dialog:


in addition , set the style of the dialog to :

<style name="full_screen_dialog">
    <item name="android:windowFrame">@null</item>
    <item name="android:windowIsFloating">true</item>
    <item name="android:windowContentOverlay">@null</item>
    <item name="android:windowAnimationStyle">@android:style/Animation.Dialog</item>
    <item name="android:windowSoftInputMode">stateUnspecified|adjustPan</item>

this could be achieved via the constructor , for example :

public FullScreenDialog(Context context)

EDIT: an alternative to all of the above would be to set the style to and that's it.

Android AlertDialog Single Button

Couldn't that just be done by only using a positive button?

AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setMessage("Look at this dialog!")
       .setPositiveButton("OK", new DialogInterface.OnClickListener() {
           public void onClick(DialogInterface dialog, int id) {
                //do things
AlertDialog alert = builder.create();;

Programmatically add new column to DataGridView

Add new column to DataTable and use column Expression property to set your Status expression.

Here you can find good example: DataColumn.Expression Property

DataTable and DataColumn Expressions in ADO.NET - Calculated Columns


Code sample:

DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("colBestBefore", typeof(DateTime)));
dt.Columns.Add(new DataColumn("colStatus", typeof(string)));

dt.Columns["colStatus"].Expression = String.Format("IIF(colBestBefore < #{0}#, 'Ok','Not ok')", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));


demoGridView.DataSource = dt;


dt.Columns["colStatus"].Expression = String.Format("IIF(CONVERT(colBestBefore, 'System.DateTime') < #{0}#, 'Ok','Not ok')", DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss"));

Where is the user's Subversion config file stored on the major operating systems?

~/.subversion/config or /etc/subversion/config

for Mac/Linux



for Windows

Bootstrap4 adding scrollbar to div

Yes, It is possible, Just add a class like anyclass and give some CSS style. Live

.anyClass {
  overflow-y: scroll;

.anyClass {_x000D_
  overflow-y: scroll;_x000D_
<link href="" rel="stylesheet"/>_x000D_
<div class=" col-md-2">_x000D_
  <ul class="nav nav-pills nav-stacked anyClass">_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link active" href="#">Active</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    </li><li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    </li><li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    </li><li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    </li><li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link" href="#">Link</a>_x000D_
    <li class="nav-item">_x000D_
      <a class="nav-link disabled" href="#">Disabled</a>_x000D_

What does "commercial use" exactly mean?

If the usage of something is part of the process of you making money, then it's generally considered a commercial use. If the purpose of the site is to, through some means or another, directly or indirectly, make you money, then it's probably commercial use.

If, on the other hand, something is merely incidental (not part of the process of production/working, but instead simply tacked on to the side), there are potential grounds for it not to be considered commercial use.

How to find out the server IP address (using JavaScript) that the browser is connected to?

Fairly certain this cannot be done. However you could use your preferred server-side language to print the server's IP to the client, and then use it however you like. For example, in PHP:

<script type="text/javascript">
    var ip = "<?php echo $_SERVER['SERVER_ADDR']; ?>";

This depends on your server's security setup though - some may block this.

How to connect android emulator to the internet

Thanks for the pointers. They really helped. The "Firewall" word clicked an idea in my mind.

I have a Windows XP machine with WIFI connection and no proxy. I have Norton Internet Security running on my machine which has a Smart Firewall. This Smart Firewall manages programs' access to network, including emulator.exe. I went into Settings -> Program Control and then granted full access to emulator.exe. After this I started Android Emulator and bang... I could connect to internet.

Hope this helps new folks.


What is the difference between partitioning and bucketing a table in Hive ?

Before going into Bucketing, we need to understand what Partitioning is. Let us take the below table as an example. Note that I have given only 12 records in the below example for beginner level understanding. In real-time scenarios you might have millions of records.

enter image description here

Partitioning is used to obtain performance while querying the data. For example, in the above table, if we write the below sql, it need to scan all the records in the table which reduces the performance and increases the overhead.

select * from sales_table where product_id='P1'

To avoid full table scan and to read only the records related to product_id='P1' we can partition (split hive table's files) into multiple files based on the product_id column. By this the hive table's file will be split into two files one with product_id='P1' and other with product_id='P2'. Now when we execute the above query, it will scan only the product_id='P1' file.


The syntax for creating the partition is given below. Note that we should not use the product_id column definition along with the non-partitioned columns in the below syntax. This should be only in the partitioned by clause.

create table sales_table(sales_id int,trans_date date, amount int) 
partitioned by (product_id varchar(10))

Cons : We should be very careful while partitioning. That is, it should not be used for the columns where number of repeating values are very less (especially primary key columns) as it increases the number of partitioned files and increases the overhead for the Name node.

Bucketing is used to overcome the cons that I mentioned in the partitioning section. This should be used when there are very few repeating values in a column (example - primary key column). This is similar to the concept of index on primary key column in the RDBMS. In our table, we can take Sales_Id column for bucketing. It will be useful when we need to query the sales_id column.

Below is the syntax for bucketing.

create table sales_table(sales_id int,trans_date date, amount int) 
partitioned by (product_id varchar(10)) Clustered by(Sales_Id) into 3 buckets

Here we will further split the data into few more files on top of partitions.

enter image description here

Since we have specified 3 buckets, it is split into 3 files each for each product_id. It internally uses modulo operator to determine in which bucket each sales_id should be stored. For example, for the product_id='P1', the sales_id=1 will be stored in 000001_0 file (ie, 1%3=1), sales_id=2 will be stored in 000002_0 file (ie, 2%3=2),sales_id=3 will be stored in 000000_0 file (ie, 3%3=0) etc.

Android Material: Status bar color won't change

While colouring the status bar is not supported <5.0, on 4.4 you can use a work around to achieve a darker colour:

Make the status bar translucent

<item name="android:windowTranslucentStatus">true</item>

Then use AppCompat's Toolbar for your appbar, making sure that it fits system windows:


Make sure to set your toolbar as your activity's toolbar:

protected void onCreate(final Bundle savedInstanceState) {
    toolbar = (Toolbar) findViewById(;

The toolbar stretches underneath the status bar, and the semi translucency of the status bar makes it appear to be a darker secondary colour. If that's not the colour you want, this combination allows you to fit a view underneath your status bar sporting the background colour of your choice (though it's still tinted darker by the status bar).

Kind of an edge case workaround due to 4.4 only, but there ya go.

Git: "Corrupt loose object"

I followed many of the other steps here; Linus' description of how to look at the git tree/objects and find what's missing was especially helpful. git-git recover corrupted blob

But in the end, for me, I had loose/corrupt tree objects caused by a partial disk failure, and tree objects are not so easily recovered/not covered by that doc.

In the end, I moved the conflicting objects/<ha>/<hash> out of the way, and used git unpack-objects with a pack file from a reasonably up to date clone. It was able to restore the missing tree objects.

Still left me with a lot of dangling blobs, which can be a side effect of unpacking previously archived stuff, and addressed in other questions here

How can I center text (horizontally and vertically) inside a div block?

You can try the following methods:

  1. If you have a single word or one line sentence, then the following code can do the trick.

    Have a text inside a div tag and give it an id. Define the following properties for that id.

    id-name {
      height: 90px;
      line-height: 90px;
      text-align: center;
      border: 2px dashed red;

    Note: Make sure the line-height property is same as the height of the division.


    But, if the content is more than one single word or a line then this doesn’t work. Also, there will be times when you cannot specify the size of a division in px or % (when the division is really small and you want the content to be exactly in the middle).

  2. To solve this issue, we can try the following combination of properties.

    id-name {
      display: flex;
      justify-content: center;
      align-items: center;
      border: 2px dashed red;


    These 3 lines of code sets the content exactly in the middle of a division (irrespective of the size of the display). The "align-items: center" helps in vertical centering while "justify-content: center" will make it horizontally centered.

    Note: Flex does not work in all browsers. Make sure you add appropriate vendor prefixes for additional browser support.

How to change the background color of a UIButton while it's highlighted?

if you won't override just set two action touchDown touchUpInside

How do I create a right click context menu in Java Swing?

I will correct usage for that method that @BullyWillPlaza suggested. Reason is that when I try to add add textArea to only contextMenu it's not visible, and if i add it to both to contextMenu and some panel it ecounters: Different parent double association if i try to switch to Design editor.

TexetObjcet.addMouseListener(new MouseAdapter() {
        public void mouseClicked(MouseEvent e) {
            if (SwingUtilities.isRightMouseButton(e)){
      , 0, 0);

Make mouse listener like this for text object you need to have popup on. What this will do is when you right click on your text object it will then add that popup and display it. This way you don't encounter that error. Solution that @BullyWillPlaza made is very good, rich and fast to implement in your program so you should try it our see how you like it.

How to debug an apache virtual host configuration?

I recently had some issues with a VirtualHost. I used a2ensite to enable a host but before running a restart (which would kill the server on fail) I ran

apache2ctl -S

Which gives you some info about what's going on with your virtual hosts. It's not perfect, but it helps.

How to access component methods from “outside” in ReactJS?

As of React 16.3 React.createRef can be used, (use ref.current to access)

var ref = React.createRef()

var parent = (
    <Child ref={ref} />
    <button onClick={e=>console.log(ref.current)}

React.renderComponent(parent, document.body)

How do I convert an integer to string as part of a PostgreSQL query?

You could do this:

SELECT * FROM table WHERE cast(YOUR_INTEGER_VALUE as varchar) = 'string of numbers'

How to secure an ASP.NET Web API

Have you tried DevDefined.OAuth?

I have used it to secure my WebApi with 2-Legged OAuth. I have also successfully tested it with PHP clients.

It's quite easy to add support for OAuth using this library. Here's how you can implement the provider for ASP.NET MVC Web API:

1) Get the source code of DevDefined.OAuth: - the newest version allows for OAuthContextBuilder extensibility.

2) Build the library and reference it in your Web API project.

3) Create a custom context builder to support building a context from HttpRequestMessage:

using System;
using System.Collections.Generic;
using System.Collections.Specialized;
using System.Diagnostics.CodeAnalysis;
using System.Linq;
using System.Net.Http;
using System.Web;

using DevDefined.OAuth.Framework;

public class WebApiOAuthContextBuilder : OAuthContextBuilder
    public WebApiOAuthContextBuilder()
        : base(UriAdjuster)

    public IOAuthContext FromHttpRequest(HttpRequestMessage request)
        var context = new OAuthContext
                RawUri = this.CleanUri(request.RequestUri), 
                Cookies = this.CollectCookies(request), 
                Headers = ExtractHeaders(request), 
                RequestMethod = request.Method.ToString(), 
                QueryParameters = request.GetQueryNameValuePairs()

        if (request.Content != null)
            var contentResult = request.Content.ReadAsByteArrayAsync();
            context.RawContent = contentResult.Result;

                // the following line can result in a NullReferenceException
                var contentType = 
                context.RawContentType = contentType;

                if (contentType.ToLower()
                    var stringContentResult = request.Content
                    context.FormEncodedParameters = 
            catch (NullReferenceException)

        this.ParseAuthorizationHeader(context.Headers, context);

        return context;

    protected static NameValueCollection ExtractHeaders(
        HttpRequestMessage request)
        var result = new NameValueCollection();

        foreach (var header in request.Headers)
            var values = header.Value.ToArray();
            var value = string.Empty;

            if (values.Length > 0)
                value = values[0];

            result.Add(header.Key, value);

        return result;

    protected NameValueCollection CollectCookies(
        HttpRequestMessage request)
        IEnumerable<string> values;

        if (!request.Headers.TryGetValues("Set-Cookie", out values))
            return new NameValueCollection();

        var header = values.FirstOrDefault();

        return this.CollectCookiesFromHeaderString(header);

    /// <summary>
    /// Adjust the URI to match the RFC specification (no query string!!).
    /// </summary>
    /// <param name="uri">
    /// The original URI. 
    /// </param>
    /// <returns>
    /// The adjusted URI. 
    /// </returns>
    private static Uri UriAdjuster(Uri uri)
            new Uri(
                    uri.IsDefaultPort ?
                        string.Empty :
                        string.Format(":{0}", uri.Port), 

4) Use this tutorial for creating an OAuth provider: In the last step (Accessing Protected Resource Example) you can use this code in your AuthorizationFilterAttribute attribute:

public override void OnAuthorization(HttpActionContext actionContext)
    // the only change I made is use the custom context builder from step 3:
    OAuthContext context = 
        new WebApiOAuthContextBuilder().FromHttpRequest(actionContext.Request);


        // do nothing here
    catch (OAuthException authEx)
        // the OAuthException's Report property is of the type "OAuthProblemReport", it's ToString()
        // implementation is overloaded to return a problem report string as per
        // the error reporting OAuth extension:
        actionContext.Response = new HttpResponseMessage(HttpStatusCode.Unauthorized)
               RequestMessage = request, ReasonPhrase = authEx.Report.ToString()

I have implemented my own provider so I haven't tested the above code (except of course the WebApiOAuthContextBuilder which I'm using in my provider) but it should work fine.

Shorthand for if-else statement

?: Operator

If you want to shorten If/Else you can use ?: operator as:

condition ? action-on-true : action-on-false(else)

For instance:

let gender = isMale ? 'Male' : 'Female';

In this case else part is mandatory.

&& Operator

In another case, if you have only if condition you can use && operator as:

condition && action;

For instance:

!this.settings && (this.settings = new TableSettings());

FYI: You have to try to avoid using if-else or at least decrease using it and try to replace it with Polymorphism or Inheritance. Go for being Anti-If guy.

Order by in Inner Join

SQL doesn't return any ordering by default because it's faster this way. It doesn't have to go through your data first and then decide what to do.

You need to add an order by clause, and probably order by which ever ID you expect. (There's a duplicate of names, thus I'd assume you want One.ID)

select * From one
inner join two
ON one.one_name = two.one_name

How to use the CSV MIME-type?

With Internet Explorer you often have to specify the Pragma: public header as well for the download to function properly..

header('Pragma: public');

Just my 2 cents..

Check if Variable is Empty - Angular 2

It depends if you know the given variable Type. If you expect it to be an Object than you could check if myVar is an empty Object like this:

 public isEmpty(myVar): boolean {
     return (myVar && (Object.keys(myVar).length === 0));

Otherwise: if (!myVar) {}, should do the job

What is a plain English explanation of "Big O" notation?

Big O is a measure of how much time/space an algorithm uses relative to the size of its input.

If an algorithm is O(n) then the time/space will increase at the same rate as its input.

If an algorithm is O(n2) then the time/space increase at the rate of its input squared.

and so on.

Timeout jQuery effects

Update: As of jQuery 1.4 you can use the .delay( n ) method.


Note: $.show() and $.hide() by default are not queued, so if you want to use $.delay() with them, you need to configure them that way:

    .show({duration: 0, queue: true})
    .hide({duration: 0, queue: true});

You could possibly use the Queue syntax, this might work:


var e = $('.notice'); 
  }, 2000 ); 


or you could be really ingenious and make a jQuery function to do it.


  jQuery.fn.idle = function(time)
      var o = $(this); 
         }, time);

which would ( in theory , working on memory here ) permit you do to this:


Aren't Python strings immutable? Then why does a + " " + b work?

The string objects themselves are immutable.

The variable, a, which points to the string, is mutable.


a = "Foo"
# a now points to "Foo"
b = a
# b points to the same "Foo" that a points to
a = a + a
# a points to the new string "FooFoo", but b still points to the old "Foo"

print a
print b
# Outputs:

# FooFoo
# Foo

# Observe that b hasn't changed, even though a has.

How to open Visual Studio Code from the command line on OSX?

?? The Easy Solution.

  1. Download, install and open Visual Studio Code.
  2. Open the Command Palette (? + ? + P on Mac) OR View ? Command Palette

3. Type shell command to find Shell Command: Install 'code' command in PATH command

  1. Install it and you're done

Here's a complimentary GIF.

Install Code on Command line

After that, you can use code or code . in the terminal.


Peace! ??

If you'd like to go a little bit further and learn a couple of great tips/tricks for using the VSCode CLI, I made a YouTube video on my workflows.

append to url and refresh page

this should work (not tested!)

var url = window.location.href;    
if (url.indexOf('?') > -1){
   url += '&param=1'
   url += '?param=1'
window.location.href = url;

Convert datetime value into string


  DATE_FORMAT(NOW(), '%d %m %Y') AS your_date;

How can I make PHP display the error instead of giving me 500 Internal Server Error

Use "php -l <filename>" (that's an 'L') from the command line to output the syntax error that could be causing PHP to throw the status 500 error. It'll output something like:

PHP Parse error: syntax error, unexpected '}' in <filename> on line 18

How can I get current location from user in iOS

The answer of RedBlueThing worked quite well for me. Here is some sample code of how I did it.


#import <UIKit/UIKit.h>
#import <CoreLocation/CoreLocation.h>

@interface yourController : UIViewController <CLLocationManagerDelegate> {
    CLLocationManager *locationManager;



In the init method

locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.distanceFilter = kCLDistanceFilterNone;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
[locationManager startUpdatingLocation];

Callback function

- (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
    NSLog(@"OldLocation %f %f", oldLocation.coordinate.latitude, oldLocation.coordinate.longitude);
    NSLog(@"NewLocation %f %f", newLocation.coordinate.latitude, newLocation.coordinate.longitude);

iOS 6

In iOS 6 the delegate function was deprecated. The new delegate is

- (void)locationManager:(CLLocationManager *)manager didUpdateLocations:(NSArray *)locations

Therefore to get the new position use

[locations lastObject]

iOS 8

In iOS 8 the permission should be explicitly asked before starting to update location

locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.distanceFilter = kCLDistanceFilterNone;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;

if ([[[UIDevice currentDevice] systemVersion] floatValue] >= 8.0)
    [self.locationManager requestWhenInUseAuthorization];

[locationManager startUpdatingLocation];

You also have to add a string for the NSLocationAlwaysUsageDescription or NSLocationWhenInUseUsageDescription keys to the app's Info.plist. Otherwise calls to startUpdatingLocation will be ignored and your delegate will not receive any callback.

And at the end when you are done reading location call stopUpdating location at suitable place.

[locationManager stopUpdatingLocation];

Split a List into smaller lists of N size

One more

public static IList<IList<T>> SplitList<T>(this IList<T> list, int chunkSize)
    var chunks = new List<IList<T>>();
    List<T> chunk = null;
    for (var i = 0; i < list.Count; i++)
        if (i % chunkSize == 0)
            chunk = new List<T>(chunkSize);
    return chunks;

Pass parameters in setInterval function

You can use an anonymous function;

setInterval(function() { funca(10,3); },500);

Open URL in same window and in same tab

As MDN's refs says, just need give a name of the new window/tab.

open in the current tab page using _self

const autoOpenAlink = (url = ``) => {, "open testing page in the same tab page");

open in a new tab page using _blank

vue demo

    <div style="margin: 5px;">


    autoOpenAlink(e) {
        let url = this.url;, "iframe testing page");

In C#, what is the difference between public, private, protected, and having no access modifier?

Public - If you can see the class, then you can see the method

Private - If you are part of the class, then you can see the method, otherwise not.

Protected - Same as Private, plus all descendants can also see the method.

Static (class) - Remember the distinction between "Class" and "Object" ? Forget all that. They are the same with "static"... the class is the one-and-only instance of itself.

Static (method) - Whenever you use this method, it will have a frame of reference independent of the actual instance of the class it is part of.

Make the current commit the only (initial) commit in a Git repository?

Here you go:

# By Zibri (2019)
# Usage: gitclean username password giturl
gitclean () 
    if [ "$#" -ne 3 ]; then
        echo "Usage: gitclean username password giturl";
        return 1;
    temp=$(mktemp -d 2>/dev/null /dev/shm/git.XXX || mktemp -d 2>/dev/null /tmp/git.XXX);
    cd "$temp";
    url=$(echo "$3" |sed -e "s/[^/]*\/\/\([^@]*@\)\?\.*/\1/");
    git clone "https://$1:$2@$url" && { 
        cd *;
        for BR in "$(git branch|tr " " "\n"|grep -v '*')";
            echo working on branch $BR;
            git checkout $BR;
            git checkout --orphan $(basename "$temp"|tr -d .);
            git add -A;
            git commit -m "Initial Commit" && { 
                git branch -D $BR;
                git branch -m $BR;
                git push -f origin $BR;
                git gc --aggressive --prune=all
    cd $odir;
    rm -rf "$temp"

Also hosted here:

How do I remove the first characters of a specific column in a table?

SELECT RIGHT(MyColumn, LEN(MyColumn) - 4) AS MyTrimmedColumn

Edit: To explain, RIGHT takes 2 arguments - the string (or column) to operate on, and the number of characters to return (starting at the "right" side of the string). LEN returns the length of the column data, and we subtract four so that our RIGHT function leaves the leftmost 4 characters "behind".

Hope this makes sense.

Edit again - I just read Andrew's response, and he may very well have interperpereted correctly, and I might be mistaken. If this is the case (and you want to UPDATE the table rather than just return doctored results), you can do this:

SET MyColumn = RIGHT(MyColumn, LEN(MyColumn) - 4)

He's on the right track, but his solution will keep the 4 characters at the start of the string, rather than discarding said 4 characters.

T-SQL How to select only Second row from a table?

No need of row number functions if field ID is unique.

  FROM yourTable
) z

change the date format in laravel view page

You can check Date Mutators:

You need set in your User model column from_date in $dates array and then you can change format in $dateFormat

The another option is also put this method to your User model:

public function getFromDateAttribute($value) {
    return \Carbon\Carbon::parse($value)->format('d-m-Y');

and then in view if you run {{ $user->from_date }} you will be see format that you want.

How to fit in an image inside span tag?

Try using a div tag and block for span!

  <span style="padding-right:3px; padding-top: 3px; display:block;">
    <img class="manImg" src="images/ico_mandatory.gif"></img>

How to clear react-native cache?

If you are using WebStorm, press configuration selection drop down button left of the run button and select edit configurations:

edit configurations

Double click on Start React Native Bundler at bottom in Before launch section:

before launch

Enter --reset-cache to Arguments section:


Facebook api: (#4) Application request limit reached

now Application-Level Rate Limiting 200 calls per hour !

you can look this image.enter image description here

Java Error: "Your security settings have blocked a local application from running"

Java applets do create a security risk, hence are disabled in most modern browsers. You have to lower the security to run it. There is a guide by Oracle for that.

Unable to locate Spring NamespaceHandler for XML schema namespace []

In my case, this was caused by custom manifest entries added by the maven-jar-plugin.


Removing the following entries fixed the problem


Vendor code 17002 to connect to SQLDeveloper

In your case the "Vendor code 17002" is the equivalent of the ORA-12541 error: It's most likely that your listener is down, or has an improper port or service name. From the docs:

ORA-12541: TNS no listener

Cause: Listener for the source repository has not been started.

Action: Start the Listener on the machine where the source repository resides.

Using Page_Load and Page_PreRender in ASP.Net

The main point of the differences as pointed out @BizApps is that Load event happens right after the ViewState is populated while PreRender event happens later, right before Rendering phase, and after all individual children controls' action event handlers are already executing. Therefore, any modifications done by the controls' actions event handler should be updated in the control hierarchy during PreRender as it happens after.

How to do a "Save As" in vba code, saving my current Excel workbook with datestamp?

I was struggling, but the below worked for me finally!

Dim WB As Workbook

Set WB = Workbooks.Open("\\users\path\Desktop\test.xlsx")

WB.SaveAs fileName:="\\users\path\Desktop\test.xls", _
        FileFormat:=xlExcel8, Password:="", WriteResPassword:="", _
        ReadOnlyRecommended:=False, CreateBackup:=False

How to return multiple rows from the stored procedure? (Oracle PL/SQL)

create procedure <procedure_name>(p_cur out sys_refcursor) as begin open p_cur for select * from <table_name> end;

Regular expression to detect semi-colon terminated C++ for & while loops

Greg is absolutely correct. This kind of parsing cannot be done with regular expressions. I suppose it is possible to build some horrendous monstrosity that would work for many cases, but then you'll just run across something that does.

You really need to use more traditional parsing techniques. For example, its pretty simple to write a recursive decent parser to do what you need.

How to view log output using docker-compose run?

  1. use the command to start containers in detached mode: docker-compose up -d
  2. to view the containers use: docker ps
  3. to view logs for a container: docker logs <containerid>

Install apps silently, with granted INSTALL_PACKAGES permission

You should define

    android:name="android.permission.INSTALL_PACKAGES" />

in your manifest, then if whether you are in system partition (/system/app) or you have your application signed by the manufacturer, you are going to have INSTALL_PACKAGES permission.

My suggestion is to create a little android project with 1.5 compatibility level used to call installPackages via reflection and to export a jar with methods to install packages and to call the real methods. Then, by importing the jar in your project you will be ready to install packages.

Difference between $(document.body) and $('body')

They refer to the same element, the difference is that when you say document.body you are passing the element directly to jQuery. Alternatively, when you pass the string 'body', the jQuery selector engine has to interpret the string to figure out what element(s) it refers to.

In practice either will get the job done.

If you are interested, there is more information in the documentation for the jQuery function.

Multiplication on command line terminal

Yes, you can use bash's built-in Arithmetic Expansion $(( )) to do some simple maths

$ echo "$((5 * 5))"

Check the Shell Arithmetic section in the Bash Reference Manual for a complete list of operators.

For sake of completeness, as other pointed out, if you need arbitrary precision, bc or dc would be better.

Rename Pandas DataFrame Index

You can also use Index.set_names as follows:

In [25]: x = pd.DataFrame({'year':[1,1,1,1,2,2,2,2],
   ....:                   'country':['A','A','B','B','A','A','B','B'],
   ....:                   'prod':[1,2,1,2,1,2,1,2],
   ....:                   'val':[10,20,15,25,20,30,25,35]})

In [26]: x = x.set_index(['year','country','prod']).squeeze()

In [27]: x
year  country  prod
1     A        1       10
               2       20
      B        1       15
               2       25
2     A        1       20
               2       30
      B        1       25
               2       35
Name: val, dtype: int64
In [28]: x.index = x.index.set_names('foo', level=1)

In [29]: x
year  foo  prod
1     A    1       10
           2       20
      B    1       15
           2       25
2     A    1       20
           2       30
      B    1       25
           2       35
Name: val, dtype: int64

Web scraping with Java

There is also Jaunt Java Web Scraping & JSON Querying -

check if a string matches an IP address pattern in python?

Other regex answers in this page will accept an IP with a number over 255.

This regex will avoid this problem:

import re

def validate_ip(ip_str):
    reg = r"^(([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])\.){3}([0-9]|[1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5])$"
    if re.match(reg, ip_str):
        return True
        return False

jquery smooth scroll to an anchor?

Best solution I have seen so far: jQuery: Smooth Scrolling Internal Anchor Links


<a href="#comments" class="scroll">Scroll to comments</a>


jQuery(document).ready(function($) {
        $('html,body').animate({scrollTop:$(this.hash).offset().top}, 500);

How do I import a Swift file from another Swift file?

In Objective-C, if you wanted to use a class in another file you had to import it:

#import "SomeClass.h"

However, in Swift, you don't have to import at all. Simply use it as if it was already imported.


// This is a file named SomeClass.swift

class SomeClass : NSObject {


// This is a different file, named OtherClass.swift

class OtherClass : NSObject {
    let object = SomeClass()

As you can see, no import was needed. Hope this helps.

Is there a library function for Root mean square error (RMSE) in python?

from sklearn import metrics              
import numpy as np

How can I fix MySQL error #1064?


Error #1064 means that MySQL can't understand your command. To fix it:

  • Read the error message. It tells you exactly where in your command MySQL got confused.

  • Examine your command. If you use a programming language to create your command, use echo, console.log(), or its equivalent to show the entire command so you can see it.

  • Check the manual. By comparing against what MySQL expected at that point, the problem is often obvious.

  • Check for reserved words. If the error occurred on an object identifier, check that it isn't a reserved word (and, if it is, ensure that it's properly quoted).

  1. Aaaagh!! What does #1064 mean?

    Error messages may look like gobbledygook, but they're (often) incredibly informative and provide sufficient detail to pinpoint what went wrong. By understanding exactly what MySQL is telling you, you can arm yourself to fix any problem of this sort in the future.

    As in many programs, MySQL errors are coded according to the type of problem that occurred. Error #1064 is a syntax error.

    • What is this "syntax" of which you speak? Is it witchcraft?

      Whilst "syntax" is a word that many programmers only encounter in the context of computers, it is in fact borrowed from wider linguistics. It refers to sentence structure: i.e. the rules of grammar; or, in other words, the rules that define what constitutes a valid sentence within the language.

      For example, the following English sentence contains a syntax error (because the indefinite article "a" must always precede a noun):

      This sentence contains syntax error a.

    • What does that have to do with MySQL?

      Whenever one issues a command to a computer, one of the very first things that it must do is "parse" that command in order to make sense of it. A "syntax error" means that the parser is unable to understand what is being asked because it does not constitute a valid command within the language: in other words, the command violates the grammar of the programming language.

      It's important to note that the computer must understand the command before it can do anything with it. Because there is a syntax error, MySQL has no idea what one is after and therefore gives up before it even looks at the database and therefore the schema or table contents are not relevant.

  2. How do I fix it?

    Obviously, one needs to determine how it is that the command violates MySQL's grammar. This may sound pretty impenetrable, but MySQL is trying really hard to help us here. All we need to do is…

    • Read the message!

      MySQL not only tells us exactly where the parser encountered the syntax error, but also makes a suggestion for fixing it. For example, consider the following SQL command:

      UPDATE my_table WHERE id=101 SET name='foo'

      That command yields the following error message:

      ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE id=101 SET name='foo'' at line 1

      MySQL is telling us that everything seemed fine up to the word WHERE, but then a problem was encountered. In other words, it wasn't expecting to encounter WHERE at that point.

      Messages that say ...near '' at line... simply mean that the end of command was encountered unexpectedly: that is, something else should appear before the command ends.

    • Examine the actual text of your command!

      Programmers often create SQL commands using a programming language. For example a php program might have a (wrong) line like this:

      $result = $mysqli->query("UPDATE " . $tablename ."SET name='foo' WHERE id=101");

      If you write this this in two lines

      $query = "UPDATE " . $tablename ."SET name='foo' WHERE id=101"
      $result = $mysqli->query($query);

      then you can add echo $query; or var_dump($query) to see that the query actually says

      UPDATE userSET name='foo' WHERE id=101

      Often you'll see your error immediately and be able to fix it.

    • Obey orders!

      MySQL is also recommending that we "check the manual that corresponds to our MySQL version for the right syntax to use". Let's do that.

      I'm using MySQL v5.6, so I'll turn to that version's manual entry for an UPDATE command. The very first thing on the page is the command's grammar (this is true for every command):

      UPDATE [LOW_PRIORITY] [IGNORE] table_reference
          SET col_name1={expr1|DEFAULT} [, col_name2={expr2|DEFAULT}] ...
          [WHERE where_condition]
          [ORDER BY ...]
          [LIMIT row_count]

      The manual explains how to interpret this syntax under Typographical and Syntax Conventions, but for our purposes it's enough to recognise that: clauses contained within square brackets [ and ] are optional; vertical bars | indicate alternatives; and ellipses ... denote either an omission for brevity, or that the preceding clause may be repeated.

      We already know that the parser believed everything in our command was okay prior to the WHERE keyword, or in other words up to and including the table reference. Looking at the grammar, we see that table_reference must be followed by the SET keyword: whereas in our command it was actually followed by the WHERE keyword. This explains why the parser reports that a problem was encountered at that point.

    A note of reservation

    Of course, this was a simple example. However, by following the two steps outlined above (i.e. observing exactly where in the command the parser found the grammar to be violated and comparing against the manual's description of what was expected at that point), virtually every syntax error can be readily identified.

    I say "virtually all", because there's a small class of problems that aren't quite so easy to spot—and that is where the parser believes that the language element encountered means one thing whereas you intend it to mean another. Take the following example:

    UPDATE my_table SET where='foo'

    Again, the parser does not expect to encounter WHERE at this point and so will raise a similar syntax error—but you hadn't intended for that where to be an SQL keyword: you had intended for it to identify a column for updating! However, as documented under Schema Object Names:

    If an identifier contains special characters or is a reserved word, you must quote it whenever you refer to it. (Exception: A reserved word that follows a period in a qualified name must be an identifier, so it need not be quoted.) Reserved words are listed at Section 9.3, “Keywords and Reserved Words”.

    [ deletia ]

    The identifier quote character is the backtick (“`”):

    mysql> SELECT * FROM `select` WHERE `select`.id > 100;

    If the ANSI_QUOTES SQL mode is enabled, it is also permissible to quote identifiers within double quotation marks:

    mysql> CREATE TABLE "test" (col INT);
    ERROR 1064: You have an error in your SQL syntax...
    mysql> SET sql_mode='ANSI_QUOTES';
    mysql> CREATE TABLE "test" (col INT);
    Query OK, 0 rows affected (0.00 sec)

How to check for an active Internet connection on iOS or macOS?

The Reachability class is OK to find out if the Internet connection is available to a device or not...

But in case of accessing an intranet resource:

Pinging the intranet server with the reachability class always returns true.

So a quick solution in this scenario would be to create a web method called pingme along with other webmethods on the service. The pingme should return something.

So I wrote the following method on common functions

    NSURL *url=[NSURL URLWithString:@"http://www.serveraddress/service.asmx/Ping"];

    NSMutableURLRequest *urlReq=[NSMutableURLRequest requestWithURL:url];

    [urlReq setTimeoutInterval:10];

    NSURLResponse *response;

    NSError *error = nil;

    NSData *receivedData = [NSURLConnection sendSynchronousRequest:urlReq

    if (receivedData !=nil)
        return YES;
        NSLog(@"Data is null");
        return NO;

The above method was so useful for me, so whenever I try to send some data to the server I always check the reachability of my intranet resource using this low timeout URLRequest.

<embed> vs. <object>

OBJECT vs. EMBED - why not always use embed?

Bottom line: OBJECT is Good, EMBED is Old. Beside's IE's PARAM tags, any content between OBJECT tags will get rendered if the browser doesn't support OBJECT's referred plugin, and apparently, the content gets http requested regardless if it gets rendered or not.

object is the current standard tag to embed something on a page. embed was included by Netscape (along img) before anything like object were on the w3c mind.

This is how you include a PDF with object:

<object data="data/test.pdf" type="application/pdf" width="300" height="200">
  alt : <a href="data/test.pdf">test.pdf</a>

If you really need the inline PDF to show in almost every browser, as older browsers understand embed but not object, you'll need to do this:

<object data="abc.pdf" type="application/pdf">
    <embed src="abc.pdf" type="application/pdf" />

This version does not validate.

Finding the 'type' of an input element

To check input type

<!DOCTYPE html>

    <input type=number id="txtinp">
    <button onclick=checktype()>Try it</button>

        function checktype() 


Executing JavaScript without a browser?

I know you asked about Linux and Mac; I am going to provide the answer for Windows, in case other people who are interested in Windows find your question .

Windows includes a Javascript engine that can be used from the command line.

All versions of Windows, since Windows 98, have included something called "The Windows Script Host". It's a windows-standard way to support script "engines". Since the first release, WSH supports JScript, Microsoft's version of Javascript. Among other things, this means that, from a windows command line, you can just invoke the name of any *.js file, and it will run in the JScript engine. (via either wscript.exe or cscript.exe)

You can see this question: What is the ProgId or CLSID for IE9's Javascript engine (code-named "Chakra") to learn how to invoke the higher-performance IE9 Javascript engine from cscript.exe.

How to avoid the need to specify the WSDL location in a CXF or JAX-WS generated webservice client?

I was able to generate

static {
    WSDL_LOCATION = null;

by configuring pom file to have a null for wsdlurl:

                                <wsdlurl />

Call a "local" function within module.exports from another function in module.exports?

Another option, and closer to the original style of the OP, is to put the object you want to export into a variable and reference that variable to make calls to other methods in the object. You can then export that variable and you're good to go.

var self = {
  foo: function (req, res, next) {
    return ('foo');
  bar: function (req, res, next) {
module.exports = self;

How to use moment.js library in angular 2 typescript app?

For Angular 7+ (Also Supports 8, 9, 10, 11):

1: Install moment by command npm install moment --save

2: Do not add anything in the app.module.ts

3: Just add import statement in top of your component or any other file like this: import * as moment from 'moment';

4: Now you can use moment anywhere in your code. Just like:

myDate = moment(someDate).format('MM/DD/YYYY HH:mm');

How can I access an internal class from an external assembly?


using System.Reflection;

Vendor vendor = new Vendor();
object tag = vendor.Tag;

Type tagt = tag.GetType();
FieldInfo field = tagt.GetField("test");

string value = field.GetValue(tag);

Use the power wisely. Don't forget error checking. :)