- Programming Questions & Answers Hub For Beginners

Some Of The Best Answers From Latest Asked Questions

Free ASP.Net and/or CSS Themes

Microsoft hired one fo the kids from A List Apart to whip some out. The .Net projects are free of charge for download.

Algorithm to find Largest prime factor of a number

    //this method skips unnecessary trial divisions and makes 
    //trial division more feasible for finding large primes

    public static void main(String[] args) 
        long n= 1000000000039L; //this is a large prime number 
        long i = 2L;
        int test = 0;

        while (n > 1)
            while (n % i == 0)
                n /= i;     


            if(i*i > n && n > 1) 
                System.out.println(n); //prints n if it's prime
                test = 1;

        if (test == 0)  
            System.out.println(i-1); //prints n if it's the largest prime factor

What is the difference between procedural programming and functional programming?

I believe that procedural/functional/objective programming are about how to approach a problem.

The first style would plan everything in to steps, and solves the problem by implementing one step (a procedure) at a time. On the other hand, functional programming would emphasize the divide-and-conquer approach, where the problem is divided into sub-problem, then each sub-problem is solved (creating a function to solve that sub problem) and the results are combined to create the answer for the whole problem. Lastly, Objective programming would mimic the real world by create a mini-world inside the computer with many objects, each of which has a (somewhat) unique characteristics, and interacts with others. From those interactions the result would emerge.

Each style of programming has its own advantages and weaknesses. Hence, doing something such as "pure programming" (i.e. purely procedural - no one does this, by the way, which is kind of weird - or purely functional or purely objective) is very difficult, if not impossible, except some elementary problems specially designed to demonstrate the advantage of a programming style (hence, we call those who like pureness "weenie" :D).

Then, from those styles, we have programming languages that is designed to optimized for some each style. For example, Assembly is all about procedural. Okay, most early languages are procedural, not only Asm, like C, Pascal, (and Fortran, I heard). Then, we have all famous Java in objective school (Actually, Java and C# is also in a class called "money-oriented," but that is subject for another discussion). Also objective is Smalltalk. In functional school, we would have "nearly functional" (some considered them to be impure) Lisp family and ML family and many "purely functional" Haskell, Erlang, etc. By the way, there are many general languages such as Perl, Python, Ruby.

When do you use the "this" keyword?

When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.

To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.

What is the best way to prevent session hijacking?

Try Secure Cookie protocol described in this paper by Liu, Kovacs, Huang, and Gouda:

As stated in document:

A secure cookie protocol that runs between a client and a server needs to provide the following four services: authentication, confidentiality, integrity and anti-replay.

As for ease of deployment:

In terms of efficiency, our protocol does not involve any database lookup or public key cryptography. In terms of deployability, our protocol can be easily deployed on an existing web server, and it does not require any change to the Internet cookie specication.

In short: it is secure, lightweight, works for me just great.

Language Books/Tutorials for popular languages

The defacto standard for learning Grails is the excellent Getting Started with Grails by Jason Rudolph. You can debate whether it is an online tutorial or a book since it can be purchased but is available as a free download. There are more "real" books being published and I recommend Beginning Groovy and Grails.

How do I perform a Perl substitution on a string while keeping the original?

I hate foo and bar .. who dreamed up these non descriptive terms in programming anyway?

my $oldstring = "replace donotreplace replace donotreplace replace donotreplace";

my $newstring = $oldstring;
$newstring =~ s/replace/newword/g; # inplace replacement

print $newstring;
%: newword donotreplace newword donotreplace newword donotreplace

How to decode viewstate


The parser should work with most non-encrypted ViewStates. It doesn’t handle the serialization format used by .NET version 1 because that version is sorely outdated and therefore too unlikely to be encountered in any real situation.

Parsing .NET ViewState

What's the best mock framework for Java?

For something a little different, you could use JRuby and Mocha which are combined in JtestR to write tests for your Java code in expressive and succinct Ruby. There are some useful mocking examples with JtestR here. One advantage of this approach is that mocking concrete classes is very straightforward.

How to download a file over HTTP?

urlretrieve and requests.get are simple, however the reality not. I have fetched data for couple sites, including text and images, the above two probably solve most of the tasks. but for a more universal solution I suggest the use of urlopen. As it is included in Python 3 standard library, your code could run on any machine that run Python 3 without pre-installing site-package

import urllib.request
url_request = urllib.request.Request(url, headers=headers)
url_connect = urllib.request.urlopen(url_request)

#remember to open file in bytes mode
with open(filename, 'wb') as f:
    while True:
        buffer =
        if not buffer: break

        #an integer value of size of written data
        data_wrote = f.write(buffer)

#you could probably use with-open-as manner

This answer provides a solution to HTTP 403 Forbidden when downloading file over http using Python. I have tried only requests and urllib modules, the other module may provide something better, but this is the one I used to solve most of the problems.

Best practices for catching and re-throwing .NET exceptions

The rule of thumb is to avoid Catching and Throwing the basic Exception object. This forces you to be a little smarter about exceptions; in other words you should have an explicit catch for a SqlException so that your handling code doesn't do something wrong with a NullReferenceException.

In the real world though, catching and logging the base exception is also a good practice, but don't forget to walk the whole thing to get any InnerExceptions it might have.

Format numbers to strings in Python

If you have a value that includes a decimal, but the decimal value is negligible (ie: 100.0) and try to int that, you will get an error. It seems silly, but calling float first fixes this.


How can I develop for iPhone using a Windows development machine?

A devkit that allows one to develop iPhone apps in Objective-C, C++ or just plain C with Visual Studio:

Check it out at iOS build env

You can build iPhone apps directly within Visual Studio (2008, 2010, Express).

Pretty neat, it even builds IPA files for your app after a successful compilation. The code works as is on jailbroken devices, for the rest of the planet I believe the final compilation & submission to the App Store has to be done on a Mac. But still, it enables you to develop using a well-known IDE.

Cleanest Way to Invoke Cross-Thread Events

I've always wondered how costly it is to always assume that invoke is required...

private void OnCoolEvent(CoolObjectEventArgs e)
  BeginInvoke((o,e) => /*do work here*/,this, e);

Why can't I declare static methods in an interface?

It seems the static method in the interface might be supported in Java 8, well, my solution is just define them in the inner class.

interface Foo {
    // ...
    class fn {
        public static void func1(...) {
            // ...

The same technique can also be used in annotations:

public @interface Foo {
    String value();

    class fn {
        public static String getValue(Object obj) {
            Foo foo = obj.getClass().getAnnotation(Foo.class);
            return foo == null ? null : foo.value();

The inner class should always be accessed in the form of Interface.fn... instead of Class.fn..., then, you can get rid of ambiguous problem.

Reading from text file until EOF repeats last line

There's an alternative approach to this:

#include <iterator>
#include <algorithm>

// ...

    copy(istream_iterator<int>(iFile), istream_iterator<int>(),
         ostream_iterator<int>(cerr, "\n"));

.NET - Get protocol, host, and port

Even though @Rick has the accepted answer for this question, there's actually a shorter way to do this, using the poorly named Uri.GetLeftPart() method.

Uri url = new Uri("");
string output = url.GetLeftPart(UriPartial.Authority);

There is one catch to GetLeftPart(), however. If the port is the default port for the scheme, it will strip it out. Since port 80 is the default port for http, the output of GetLeftPart() in my example above will be

If the port number had been something other than 80, it would be included in the result.

What is the difference between #include <filename> and #include "filename"?

For #include "" a compiler normally searches the folder of the file which contains that include and then the other folders. For #include <> the compiler does not search the current file's folder.

In C++, what is a virtual base class?

Explaining multiple-inheritance with virtual bases requires a knowledge of the C++ object model. And explaining the topic clearly is best done in an article and not in a comment box.

The best, readable explanation I found that solved all my doubts on this subject was this article:

You really won't need to read anything else on the topic (unless you are a compiler writer) after reading that...

In SQL Server, how do I generate a CREATE TABLE statement for a given table?

I realise that it's been a very long time but thought I'd add anyway. If you just want the table, and not the create table statement you could use

select into x from db.schema.y where 1=0

to copy the table to a new DB

Dynamically load a JavaScript file

Dynamic module import landed in Firefox 67+.

(async () => {
   await import('./synth/BubbleSynth.js')

With error handling:

(async () => {
    await import('./synth/BubbleSynth.js').catch((error) => console.log('Loading failed' + error))

It also works for any kind of non-modules libraries, on this case the lib is available on the window object, the old way, but only on demand, which is nice.

Example using suncalc.js, the server must have CORS enabled to works this way!

(async () => {
 await import('')
   let times = SunCalc.getTimes(new Date(), 51.5,-0.1);
   console.log("Golden Hour today in London: " + times.goldenHour.getHours() + ':' + times.goldenHour.getMinutes() + ". Take your pics!")

Which .NET Dependency Injection frameworks are worth looking into?

I think a good place to start is with Ninject, it is new and has taken into account alot of fine tuning and is really fast. Nate, the developer, really has a great site and great support.

Simplest way to profile a PHP script

If subtracting microtimes gives you negative results, try using the function with the argument true (microtime(true)). With true, the function returns a float instead of a string (as it does if it is called without arguments).

Most efficient way to concatenate strings?

It really depends on your usage pattern. A detailed benchmark between string.Join, string,Concat and string.Format can be found here: String.Format Isn't Suitable for Intensive Logging

(This is actually the same answer I gave to this question)

List of standard lengths for database fields

Just looking though my email archives, there are a number of pretty long "first" names (of course what is meant by first is variable by culture). One example is Krishnamurthy - which is 13 letters long. A good guess might be 20 to 25 letters based on this. Email should be much longer since you might have [email protected]. Also, gmail and some other mail programs allow you to use [email protected] where "sometag" is anything you want to put there so that you can use it to sort incoming emails. I frequently run into web forms that don't allow me to put in my full email address without considering any tags. So, if you need a fixed email field maybe something like [email protected] in characters for a total of 90 characters (if I did my math right!).

Recommended SQL database design for tags or tagging

Normally I would agree with Yaakov Ellis but in this special case there is another viable solution:

Use two tables:

Table: Item
Columns: ItemID, Title, Content
Indexes: ItemID

Table: Tag
Columns: ItemID, Title
Indexes: ItemId, Title

This has some major advantages:

First it makes development much simpler: in the three-table solution for insert and update of item you have to lookup the Tag table to see if there are already entries. Then you have to join them with new ones. This is no trivial task.

Then it makes queries simpler (and perhaps faster). There are three major database queries which you will do: Output all Tags for one Item, draw a Tag-Cloud and select all items for one Tag Title.

All Tags for one Item:


SELECT Tag.Title 
  FROM Tag 
  JOIN ItemTag ON Tag.TagID = ItemTag.TagID
 WHERE ItemTag.ItemID = :id


SELECT Tag.Title
WHERE Tag.ItemID = :id



SELECT Tag.Title, count(*)
  FROM Tag
  JOIN ItemTag ON Tag.TagID = ItemTag.TagID
 GROUP BY Tag.Title


SELECT Tag.Title, count(*)
  FROM Tag
 GROUP BY Tag.Title

Items for one Tag:


  FROM Item
  JOIN ItemTag ON Item.ItemID = ItemTag.ItemID
  JOIN Tag ON ItemTag.TagID = Tag.TagID
 WHERE Tag.Title = :title


  FROM Item
  JOIN Tag ON Item.ItemID = Tag.ItemID
 WHERE Tag.Title = :title

But there are some drawbacks, too: It could take more space in the database (which could lead to more disk operations which is slower) and it's not normalized which could lead to inconsistencies.

The size argument is not that strong because the very nature of tags is that they are normally pretty small so the size increase is not a large one. One could argue that the query for the tag title is much faster in a small table which contains each tag only once and this certainly is true. But taking in regard the savings for not having to join and the fact that you can build a good index on them could easily compensate for this. This of course depends heavily on the size of the database you are using.

The inconsistency argument is a little moot too. Tags are free text fields and there is no expected operation like 'rename all tags "foo" to "bar"'.

So tldr: I would go for the two-table solution. (In fact I'm going to. I found this article to see if there are valid arguments against it.)

How do you clear a stringstream variable?

This should be the most reliable way regardless of the compiler:


Image UriSource and Data Binding

you may use

ImageSourceConverter class

to get what you want

    img1.Source = (ImageSource)new ImageSourceConverter().ConvertFromString("/Assets/check.png");

List of macOS text editors and code editors

I've tried Komodo out a bit, and I really like it so far. Aptana, an Eclipse variant, is also rather useful for a wide variety of things. There's always good ole' VI, too!

How do I use 3DES encryption/decryption in Java?

Here is a solution using the javax.crypto library and the apache commons codec library for encoding and decoding in Base64:

import javax.crypto.Cipher;
import javax.crypto.SecretKey;
import javax.crypto.SecretKeyFactory;
import javax.crypto.spec.DESedeKeySpec;
import org.apache.commons.codec.binary.Base64;

public class TrippleDes {

    private static final String UNICODE_FORMAT = "UTF8";
    public static final String DESEDE_ENCRYPTION_SCHEME = "DESede";
    private KeySpec ks;
    private SecretKeyFactory skf;
    private Cipher cipher;
    byte[] arrayBytes;
    private String myEncryptionKey;
    private String myEncryptionScheme;
    SecretKey key;

    public TrippleDes() throws Exception {
        myEncryptionKey = "ThisIsSpartaThisIsSparta";
        myEncryptionScheme = DESEDE_ENCRYPTION_SCHEME;
        arrayBytes = myEncryptionKey.getBytes(UNICODE_FORMAT);
        ks = new DESedeKeySpec(arrayBytes);
        skf = SecretKeyFactory.getInstance(myEncryptionScheme);
        cipher = Cipher.getInstance(myEncryptionScheme);
        key = skf.generateSecret(ks);

    public String encrypt(String unencryptedString) {
        String encryptedString = null;
        try {
            cipher.init(Cipher.ENCRYPT_MODE, key);
            byte[] plainText = unencryptedString.getBytes(UNICODE_FORMAT);
            byte[] encryptedText = cipher.doFinal(plainText);
            encryptedString = new String(Base64.encodeBase64(encryptedText));
        } catch (Exception e) {
        return encryptedString;

    public String decrypt(String encryptedString) {
        String decryptedText=null;
        try {
            cipher.init(Cipher.DECRYPT_MODE, key);
            byte[] encryptedText = Base64.decodeBase64(encryptedString);
            byte[] plainText = cipher.doFinal(encryptedText);
            decryptedText= new String(plainText);
        } catch (Exception e) {
        return decryptedText;

    public static void main(String args []) throws Exception
        TrippleDes td= new TrippleDes();

        String target="imparator";
        String encrypted=td.encrypt(target);
        String decrypted=td.decrypt(encrypted);

        System.out.println("String To Encrypt: "+ target);
        System.out.println("Encrypted String:" + encrypted);
        System.out.println("Decrypted String:" + decrypted);



Running the above program results with the following output:

String To Encrypt: imparator
Encrypted String:FdBNaYWfjpWN9eYghMpbRA==
Decrypted String:imparator

MyISAM versus InnoDB

A bit late to the game...but here's a quite comprehensive post I wrote a few months back, detailing the major differences between MYISAM and InnoDB. Grab a cuppa (and maybe a biscuit), and enjoy.

The major difference between MyISAM and InnoDB is in referential integrity and transactions. There are also other difference such as locking, rollbacks, and full-text searches.

Referential Integrity

Referential integrity ensures that relationships between tables remains consistent. More specifically, this means when a table (e.g. Listings) has a foreign key (e.g. Product ID) pointing to a different table (e.g. Products), when updates or deletes occur to the pointed-to table, these changes are cascaded to the linking table. In our example, if a product is renamed, the linking table’s foreign keys will also update; if a product is deleted from the ‘Products’ table, any listings which point to the deleted entry will also be deleted. Furthermore, any new listing must have that foreign key pointing to a valid, existing entry.

InnoDB is a relational DBMS (RDBMS) and thus has referential integrity, while MyISAM does not.

Transactions & Atomicity

Data in a table is managed using Data Manipulation Language (DML) statements, such as SELECT, INSERT, UPDATE and DELETE. A transaction group two or more DML statements together into a single unit of work, so either the entire unit is applied, or none of it is.

MyISAM do not support transactions whereas InnoDB does.

If an operation is interrupted while using a MyISAM table, the operation is aborted immediately, and the rows (or even data within each row) that are affected remains affected, even if the operation did not go to completion.

If an operation is interrupted while using an InnoDB table, because it using transactions, which has atomicity, any transaction which did not go to completion will not take effect, since no commit is made.

Table-locking vs Row-locking

When a query runs against a MyISAM table, the entire table in which it is querying will be locked. This means subsequent queries will only be executed after the current one is finished. If you are reading a large table, and/or there are frequent read and write operations, this can mean a huge backlog of queries.

When a query runs against an InnoDB table, only the row(s) which are involved are locked, the rest of the table remains available for CRUD operations. This means queries can run simultaneously on the same table, provided they do not use the same row.

This feature in InnoDB is known as concurrency. As great as concurrency is, there is a major drawback that applies to a select range of tables, in that there is an overhead in switching between kernel threads, and you should set a limit on the kernel threads to prevent the server coming to a halt.

Transactions & Rollbacks

When you run an operation in MyISAM, the changes are set; in InnoDB, those changes can be rolled back. The most common commands used to control transactions are COMMIT, ROLLBACK and SAVEPOINT. 1. COMMIT - you can write multiple DML operations, but the changes will only be saved when a COMMIT is made 2. ROLLBACK - you can discard any operations that have not yet been committed yet 3. SAVEPOINT - sets a point in the list of operations to which a ROLLBACK operation can rollback to


MyISAM offers no data integrity - Hardware failures, unclean shutdowns and canceled operations can cause the data to become corrupt. This would require full repair or rebuilds of the indexes and tables.

InnoDB, on the other hand, uses a transactional log, a double-write buffer and automatic checksumming and validation to prevent corruption. Before InnoDB makes any changes, it records the data before the transactions into a system tablespace file called ibdata1. If there is a crash, InnoDB would autorecover through the replay of those logs.


InnoDB does not support FULLTEXT indexing until MySQL version 5.6.4. As of the writing of this post, many shared hosting providers’ MySQL version is still below 5.6.4, which means FULLTEXT indexing is not supported for InnoDB tables.

However, this is not a valid reason to use MyISAM. It’s best to change to a hosting provider that supports up-to-date versions of MySQL. Not that a MyISAM table that uses FULLTEXT indexing cannot be converted to an InnoDB table.


In conclusion, InnoDB should be your default storage engine of choice. Choose MyISAM or other data types when they serve a specific need.

Is there a way to make Firefox ignore invalid ssl-certificates?

Go to Tools > Options > Advanced "Tab"(?) > Encryption Tab

Click the "Validation" button, and uncheck the checkbox for checking validity

Be advised though that this is pretty unsecure as it leaves you wide open to accept any invalid certificate. I'd only do this if using the browser on an Intranet where the validity of the cert isn't a concern to you, or you aren't concerned in general.

What's the best way to parse command line arguments?

consoleargs deserves to be mentioned here. It is very easy to use. Check it out:

from consoleargs import command

def main(url, name=None):
  :param url: Remote URL 
  :param name: File name
  print """Downloading url '%r' into file '%r'""" % (url, name)

if __name__ == '__main__':

Now in console:

% python --help

URL:    Remote URL 

    --name -n   File name

% python
Downloading url '''' into file 'None'

% python --name=index.html
Downloading url '''' into file ''index.html''

How do you embed binary data in XML?

You could encode the binary data using base64 and put it into a Base64 element; the below article is a pretty good one on the subject.

Handling Binary Data in XML Documents

How to request a random row in SQL?

select, from table AS r
INNER JOIN(select CEIL(RAND() * (select MAX(id) from table)) as id) as r1

This will require a lesser computation time

Transpose/Unzip Function (inverse of zip)?

Naive approach

def transpose_finite_iterable(iterable):
    return zip(*iterable)  # `itertools.izip` for Python 2 users

works fine for finite iterable (e.g. sequences like list/tuple/str) of (potentially infinite) iterables which can be illustrated like

| |a_00| |a_10| ... |a_n0| |
| |a_01| |a_11| ... |a_n1| |
| |... | |... | ... |... | |
| |a_0i| |a_1i| ... |a_ni| |
| |... | |... | ... |... | |


  • n in N,
  • a_ij corresponds to j-th element of i-th iterable,

and after applying transpose_finite_iterable we get

| |a_00| |a_01| ... |a_0i| ... |
| |a_10| |a_11| ... |a_1i| ... |
| |... | |... | ... |... | ... |
| |a_n0| |a_n1| ... |a_ni| ... |

Python example of such case where a_ij == j, n == 2

>>> from itertools import count
>>> iterable = [count(), count()]
>>> result = transpose_finite_iterable(iterable)
>>> next(result)
(0, 0)
>>> next(result)
(1, 1)

But we can't use transpose_finite_iterable again to return to structure of original iterable because result is an infinite iterable of finite iterables (tuples in our case):

>>> transpose_finite_iterable(result)
... hangs ...
Traceback (most recent call last):
  File "...", line 1, in ...
  File "...", line 2, in transpose_finite_iterable

So how can we deal with this case?

... and here comes the deque

After we take a look at docs of itertools.tee function, there is Python recipe that with some modification can help in our case

def transpose_finite_iterables(iterable):
    iterator = iter(iterable)
        first_elements = next(iterator)
    except StopIteration:
        return ()
    queues = [deque([element])
              for element in first_elements]

    def coordinate(queue):
        while True:
            if not queue:
                    elements = next(iterator)
                except StopIteration:
                for sub_queue, element in zip(queues, elements):
            yield queue.popleft()

    return tuple(map(coordinate, queues))

let's check

>>> from itertools import count
>>> iterable = [count(), count()]
>>> result = transpose_finite_iterables(transpose_finite_iterable(iterable))
>>> result
(<generator object transpose_finite_iterables.<locals>.coordinate at ...>, <generator object transpose_finite_iterables.<locals>.coordinate at ...>)
>>> next(result[0])
>>> next(result[0])


Now we can define general function for working with iterables of iterables ones of which are finite and another ones are potentially infinite using functools.singledispatch decorator like

from collections import (abc,
from functools import singledispatch

def transpose(object_):
    Transposes given object.
    raise TypeError('Unsupported object type: {type}.'

def transpose_finite_iterables(object_):
    Transposes given iterable of finite iterables.
    iterator = iter(object_)
        first_elements = next(iterator)
    except StopIteration:
        return ()
    queues = [deque([element])
              for element in first_elements]

    def coordinate(queue):
        while True:
            if not queue:
                    elements = next(iterator)
                except StopIteration:
                for sub_queue, element in zip(queues, elements):
            yield queue.popleft()

    return tuple(map(coordinate, queues))

def transpose_finite_iterable(object_):
    Transposes given finite iterable of iterables.
    yield from zip(*object_)

    transpose.register(abc.Collection, transpose_finite_iterable)
except AttributeError:
    # Python3.5-
    transpose.register(abc.Mapping, transpose_finite_iterable)
    transpose.register(abc.Sequence, transpose_finite_iterable)
    transpose.register(abc.Set, transpose_finite_iterable)

which can be considered as its own inverse (mathematicians call this kind of functions "involutions") in class of binary operators over finite non-empty iterables.

As a bonus of singledispatching we can handle numpy arrays like

import numpy as np
transpose.register(np.ndarray, np.transpose)

and then use it like

>>> array = np.arange(4).reshape((2,2))
>>> array
array([[0, 1],
       [2, 3]])
>>> transpose(array)
array([[0, 2],
       [1, 3]])


Since transpose returns iterators and if someone wants to have a tuple of lists like in OP -- this can be made additionally with map built-in function like

>>> original = [('a', 1), ('b', 2), ('c', 3), ('d', 4)]
>>> tuple(map(list, transpose(original)))
(['a', 'b', 'c', 'd'], [1, 2, 3, 4])


I've added generalized solution to lz package from 0.5.0 version which can be used like

>>> from lz.transposition import transpose
>>> list(map(tuple, transpose(zip(range(10), range(10, 20)))))
[(0, 1, 2, 3, 4, 5, 6, 7, 8, 9), (10, 11, 12, 13, 14, 15, 16, 17, 18, 19)]


There is no solution (at least obvious) for handling potentially infinite iterable of potentially infinite iterables, but this case is less common though.

Build a basic Python iterator

All answers on this page are really great for a complex object. But for those containing builtin iterable types as attributes, like str, list, set or dict, or any implementation of collections.Iterable, you can omit certain things in your class.

class Test(object):
    def __init__(self, string):
        self.string = string

    def __iter__(self):
        # since your string is already iterable
        return (ch for ch in self.string)
        # or simply
        return self.string.__iter__()
        # also
        return iter(self.string)

It can be used like:

for x in Test("abcde"):

# prints
# a
# b
# c
# d
# e

What is the correct way to create a single-instance WPF application?

My favourite solution is from MVP Daniel Vaughan: Enforcing Single Instance Wpf Applications

It use MemoryMappedFile to send command line arguments to the first instance:

/// <summary>
/// This class allows restricting the number of executables in execution, to one.
/// </summary>
public sealed class SingletonApplicationEnforcer
    readonly Action<IEnumerable<string>> processArgsFunc;
    readonly string applicationId;
    Thread thread;
    string argDelimiter = "_;;_";

    /// <summary>
    /// Gets or sets the string that is used to join 
    /// the string array of arguments in memory.
    /// </summary>
    /// <value>The arg delimeter.</value>
    public string ArgDelimeter
            return argDelimiter;
            argDelimiter = value;

    /// <summary>
    /// Initializes a new instance of the <see cref="SingletonApplicationEnforcer"/> class.
    /// </summary>
    /// <param name="processArgsFunc">A handler for processing command line args 
    /// when they are received from another application instance.</param>
    /// <param name="applicationId">The application id used 
    /// for naming the <seealso cref="EventWaitHandle"/>.</param>
    public SingletonApplicationEnforcer(Action<IEnumerable<string>> processArgsFunc, 
        string applicationId = "DisciplesRock")
        if (processArgsFunc == null)
            throw new ArgumentNullException("processArgsFunc");
        this.processArgsFunc = processArgsFunc;
        this.applicationId = applicationId;

    /// <summary>
    /// Determines if this application instance is not the singleton instance.
    /// If this application is not the singleton, then it should exit.
    /// </summary>
    /// <returns><c>true</c> if the application should shutdown, 
    /// otherwise <c>false</c>.</returns>
    public bool ShouldApplicationExit()
        bool createdNew;
        string argsWaitHandleName = "ArgsWaitHandle_" + applicationId;
        string memoryFileName = "ArgFile_" + applicationId;

        EventWaitHandle argsWaitHandle = new EventWaitHandle(
            false, EventResetMode.AutoReset, argsWaitHandleName, out createdNew);


        if (createdNew)
            /* This is the main, or singleton application. 
                * A thread is created to service the MemoryMappedFile. 
                * We repeatedly examine this file each time the argsWaitHandle 
                * is Set by a non-singleton application instance. */
            thread = new Thread(() =>
                        using (MemoryMappedFile file = MemoryMappedFile.CreateOrOpen(memoryFileName, 10000))
                            while (true)
                                using (MemoryMappedViewStream stream = file.CreateViewStream())
                                    var reader = new BinaryReader(stream);
                                    string args;
                                        args = reader.ReadString();
                                    catch (Exception ex)
                                        Debug.WriteLine("Unable to retrieve string. " + ex);
                                    string[] argsSplit = args.Split(new string[] { argDelimiter }, 

                    catch (Exception ex)
                        Debug.WriteLine("Unable to monitor memory file. " + ex);

            thread.IsBackground = true;
            /* Non singleton application instance. 
                * Should exit, after passing command line args to singleton process, 
                * via the MemoryMappedFile. */
            using (MemoryMappedFile mmf = MemoryMappedFile.OpenExisting(memoryFileName))
                using (MemoryMappedViewStream stream = mmf.CreateViewStream())
                    var writer = new BinaryWriter(stream);
                    string[] args = Environment.GetCommandLineArgs();
                    string joined = string.Join(argDelimiter, args);

        return !createdNew;

Is there a simple, elegant way to define singletons?

As the accepted answer says, the most idiomatic way is to just use a module.

With that in mind, here's a proof of concept:

def singleton(cls):
    obj = cls()
    # Always return the same object
    cls.__new__ = staticmethod(lambda cls: obj)
    # Disable __init__
        del cls.__init__
    except AttributeError:
    return cls

See the Python data model for more details on __new__.


class Duck(object):

if Duck() is Duck():
    print "It works!"
    print "It doesn't work!"


  1. You have to use new-style classes (derive from object) for this.

  2. The singleton is initialized when it is defined, rather than the first time it's used.

  3. This is just a toy example. I've never actually used this in production code, and don't plan to.

How do you configure an OpenFileDialog to select folders?

First Solution

I developed this as a cleaned up version of .NET Win 7-style folder select dialog by Bill Seddon of (I have no affiliation). (I learned of his code from another answer on this page). I wrote my own because his solution requires an additional Reflection class that isn't needed for this focused purpose, uses exception-based flow control, doesn't cache the results of its reflection calls. Note that the nested static VistaDialog class is so that its static reflection variables don't try to get populated if the Show method is never called. It falls back to the pre-Vista dialog if not in a high enough Windows version. Should work in Windows 7, 8, 9, 10 and higher (theoretically).

using System;
using System.Reflection;
using System.Windows.Forms;

namespace ErikE.Shuriken {
    /// <summary>
    /// Present the Windows Vista-style open file dialog to select a folder. Fall back for older Windows Versions
    /// </summary>
    public class FolderSelectDialog {
        private string _initialDirectory;
        private string _title;
        private string _fileName = "";

        public string InitialDirectory {
            get { return string.IsNullOrEmpty(_initialDirectory) ? Environment.CurrentDirectory : _initialDirectory; }
            set { _initialDirectory = value; }
        public string Title {
            get { return _title ?? "Select a folder"; }
            set { _title = value; }
        public string FileName { get { return _fileName; } }

        public bool Show() { return Show(IntPtr.Zero); }

        /// <param name="hWndOwner">Handle of the control or window to be the parent of the file dialog</param>
        /// <returns>true if the user clicks OK</returns>
        public bool Show(IntPtr hWndOwner) {
            var result = Environment.OSVersion.Version.Major >= 6
                ? VistaDialog.Show(hWndOwner, InitialDirectory, Title)
                : ShowXpDialog(hWndOwner, InitialDirectory, Title);
            _fileName = result.FileName;
            return result.Result;

        private struct ShowDialogResult {
            public bool Result { get; set; }
            public string FileName { get; set; }

        private static ShowDialogResult ShowXpDialog(IntPtr ownerHandle, string initialDirectory, string title) {
            var folderBrowserDialog = new FolderBrowserDialog {
                Description = title,
                SelectedPath = initialDirectory,
                ShowNewFolderButton = false
            var dialogResult = new ShowDialogResult();
            if (folderBrowserDialog.ShowDialog(new WindowWrapper(ownerHandle)) == DialogResult.OK) {
                dialogResult.Result = true;
                dialogResult.FileName = folderBrowserDialog.SelectedPath;
            return dialogResult;

        private static class VistaDialog {
            private const string c_foldersFilter = "Folders|\n";

            private const BindingFlags c_flags = BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic;
            private readonly static Assembly s_windowsFormsAssembly = typeof(FileDialog).Assembly;
            private readonly static Type s_iFileDialogType = s_windowsFormsAssembly.GetType("System.Windows.Forms.FileDialogNative+IFileDialog");
            private readonly static MethodInfo s_createVistaDialogMethodInfo = typeof(OpenFileDialog).GetMethod("CreateVistaDialog", c_flags);
            private readonly static MethodInfo s_onBeforeVistaDialogMethodInfo = typeof(OpenFileDialog).GetMethod("OnBeforeVistaDialog", c_flags);
            private readonly static MethodInfo s_getOptionsMethodInfo = typeof(FileDialog).GetMethod("GetOptions", c_flags);
            private readonly static MethodInfo s_setOptionsMethodInfo = s_iFileDialogType.GetMethod("SetOptions", c_flags);
            private readonly static uint s_fosPickFoldersBitFlag = (uint) s_windowsFormsAssembly
            private readonly static ConstructorInfo s_vistaDialogEventsConstructorInfo = s_windowsFormsAssembly
                .GetConstructor(c_flags, null, new[] { typeof(FileDialog) }, null);
            private readonly static MethodInfo s_adviseMethodInfo = s_iFileDialogType.GetMethod("Advise");
            private readonly static MethodInfo s_unAdviseMethodInfo = s_iFileDialogType.GetMethod("Unadvise");
            private readonly static MethodInfo s_showMethodInfo = s_iFileDialogType.GetMethod("Show");

            public static ShowDialogResult Show(IntPtr ownerHandle, string initialDirectory, string title) {
                var openFileDialog = new OpenFileDialog {
                    AddExtension = false,
                    CheckFileExists = false,
                    DereferenceLinks = true,
                    Filter = c_foldersFilter,
                    InitialDirectory = initialDirectory,
                    Multiselect = false,
                    Title = title

                var iFileDialog = s_createVistaDialogMethodInfo.Invoke(openFileDialog, new object[] { });
                s_onBeforeVistaDialogMethodInfo.Invoke(openFileDialog, new[] { iFileDialog });
                s_setOptionsMethodInfo.Invoke(iFileDialog, new object[] { (uint) s_getOptionsMethodInfo.Invoke(openFileDialog, new object[] { }) | s_fosPickFoldersBitFlag });
                var adviseParametersWithOutputConnectionToken = new[] { s_vistaDialogEventsConstructorInfo.Invoke(new object[] { openFileDialog }), 0U };
                s_adviseMethodInfo.Invoke(iFileDialog, adviseParametersWithOutputConnectionToken);

                try {
                    int retVal = (int) s_showMethodInfo.Invoke(iFileDialog, new object[] { ownerHandle });
                    return new ShowDialogResult {
                        Result = retVal == 0,
                        FileName = openFileDialog.FileName
                finally {
                    s_unAdviseMethodInfo.Invoke(iFileDialog, new[] { adviseParametersWithOutputConnectionToken[1] });

        // Wrap an IWin32Window around an IntPtr
        private class WindowWrapper : IWin32Window {
            private readonly IntPtr _handle;
            public WindowWrapper(IntPtr handle) { _handle = handle; }
            public IntPtr Handle { get { return _handle; } }

It is used like so in a Windows Form:

var dialog = new FolderSelectDialog {
    InitialDirectory = musicFolderTextBox.Text,
    Title = "Select a folder to import music from"
if (dialog.Show(Handle)) {
    musicFolderTextBox.Text = dialog.FileName;

You can of course play around with its options and what properties it exposes. For example, it allows multiselect in the Vista-style dialog.

Second Solution

Simon Mourier gave an answer that shows how to do the exact same job using interop against the Windows API directly, though his version would have to be supplemented to use the older style dialog if in an older version of Windows. Unfortunately, I hadn't found his post yet when I worked up my solution. Name your poison!

How to insert a line break in a SQL Server VARCHAR/NVARCHAR string

I'd say

concat('This is line 1.', 0xd0a, 'This is line 2.')


concat(N'This is line 1.', 0xd000a, N'This is line 2.')

Is there an "exists" function for jQuery?

By default - No.

There's the length property that is commonly used for the same result in the following way:

if ($(selector).length)

Here, 'selector' is to be replaced by the actual selector you are interested to find if it exists or not. If it does exist, the length property will output an integer more than 0 and hence the if statement will become true and hence execute the if block. If it doesn't, it will output the integer '0' and hence the if block won't get executed.

Get file version in PowerShell

Nowadays you can get the FileVersionInfo from Get-Item or Get-ChildItem, but it will show the original FileVersion from the shipped product, and not the updated version. For instance:

(Get-Item C:\Windows\System32\Lsasrv.dll).VersionInfo.FileVersion

Interestingly, you can get the updated (patched) ProductVersion by using this:

(Get-Command C:\Windows\System32\Lsasrv.dll).Version

The distinction I'm making between "original" and "patched" is basically due to the way the FileVersion is calculated (see the docs here). Basically ever since Vista, the Windows API GetFileVersionInfo is querying part of the version information from the language neutral file (exe/dll) and the non-fixed part from a language-specific mui file (which isn't updated every time the files change).

So with a file like lsasrv (which got replaced due to security problems in SSL/TLS/RDS in November 2014) the versions reported by these two commands (at least for a while after that date) were different, and the second one is the more "correct" version.

However, although it's correct in LSASrv, it's possible for the ProductVersion and FileVersion to be different (it's common, in fact). So the only way to get the updated Fileversion straight from the assembly file is to build it up yourself from the parts, something like this:

Get-Item C:\Windows\System32\Lsasrv.dll | ft FileName, File*Part

Or by pulling the data from this:


You can easily add this to all FileInfo objects by updating the TypeData in PowerShell:

Update-TypeData -TypeName System.IO.FileInfo -MemberName FileVersion -MemberType ScriptProperty -Value {
   [System.Diagnostics.FileVersionInfo]::GetVersionInfo($this.FullName) | % {
      [Version](($_.FileMajorPart, $_.FileMinorPart, $_.FileBuildPart, $_.FilePrivatePart)-join".") 

Now every time you do Get-ChildItem or Get-Item you'll have a FileVersion property that shows the updated FileVersion ...

Difference between the Apache HTTP Server and Apache Tomcat?

Well, Apache is HTTP webserver, where as Tomcat is also webserver for Servlets and JSP. Moreover Apache is preferred over Apache Tomcat in real time

How do I tell Maven to use the latest version of a dependency?

The truth is even in 3.x it still works, surprisingly the projects builds and deploys. But the LATEST/RELEASE keyword causing problems in m2e and eclipse all over the place, ALSO projects depends on the dependency which deployed through the LATEST/RELEASE fail to recognize the version.

It will also causing problem if you are try to define the version as property, and reference it else where.

So the conclusion is use the versions-maven-plugin if you can.

What is a reasonable length limit on person "Name" fields?

The average first name is about 6 letters. That leaves 43 for a last name. :) Seems like you could probably shorten it if you like.

The main question is how many rows do you think you will have? I don't think varchar(50) is going to kill you until you get several million rows.

SQL Server: Get data for only the past year

The most readable, IMO:

   DATEADD(yy, -1, CONVERT(datetime, CONVERT(varchar, GETDATE(), 101)))


  1. Gets now's datetime GETDATE() = #8/27/2008 10:23am#
  2. Converts to a string with format 101 CONVERT(varchar, #8/27/2008 10:23am#, 101) = '8/27/2007'
  3. Converts to a datetime CONVERT(datetime, '8/27/2007') = #8/27/2008 12:00AM#
  4. Subtracts 1 year DATEADD(yy, -1, #8/27/2008 12:00AM#) = #8/27/2007 12:00AM#

There's variants with DATEDIFF and DATEADD to get you midnight of today, but they tend to be rather obtuse (though slightly better on performance - not that you'd notice compared to the reads required to fetch the data).

Can Windows' built-in ZIP compression be scripted?

Yes, this can be scripted with VBScript. For example the following code can create a zip from a directory:

Dim fso, winShell, MyTarget, MySource, file
Set fso = CreateObject("Scripting.FileSystemObject")
Set winShell = createObject("shell.application")

MyTarget = Wscript.Arguments.Item(0)
MySource = Wscript.Arguments.Item(1)

Wscript.Echo "Adding " & MySource & " to " & MyTarget

'create a new clean zip archive
Set file = fso.CreateTextFile(MyTarget, True)
file.write("PK" & chr(5) & chr(6) & string(18,chr(0)))

winShell.NameSpace(MyTarget).CopyHere winShell.NameSpace(MySource).Items

do until winShell.namespace(MyTarget).items.count = winShell.namespace(MySource).items.count
    wscript.sleep 1000 

Set winShell = Nothing
Set fso = Nothing

You may also find helpful as it includes a full Unzip/Zip implementation in VBScript.

If you do a size check every 500 ms rather than a item count it works better for large files. Win 7 writes the file instantly although it's not finished compressing:

set fso=createobject("scripting.filesystemobject")
Set h=fso.getFile(DestZip)
    wscript.sleep 500
    max = h.size
loop while h.size > max 

Works great for huge amounts of log files.

Boolean Field in Oracle

In our databases we use an enum that ensures we pass it either TRUE or FALSE. If you do it either of the first two ways it is too easy to either start adding new meaning to the integer without going through a proper design, or ending up with that char field having Y, y, N, n, T, t, F, f values and having to remember which section of code uses which table and which version of true it is using.

JavaScript and Threads

The new v8 engine which should come out today supports it (i think)

How do I fire an event when a iframe has finished loading in jQuery?

$("#iFrameId").ready(function (){
    // do something once the iframe is loaded

have you tried .ready instead?

How to submit a form when the return key is pressed?

IMO, this is the cleanest answer:

<form action="" method="get">_x000D_
  Name: <input type="text" name="name"/><br/>_x000D_
  Pwd: <input type="password" name="password"/><br/>_x000D_
  <div class="yourCustomDiv"/>_x000D_
  <input type="submit" style="display:none"/>_x000D_

Better yet, if you are using javascript to submit the form using the custom div, you should also use javascript to create it, and to set the display:none style on the button. This way users with javascript disabled will still see the submit button and can click on it.

It has been noted that display:none will cause IE to ignore the input. I created a new JSFiddle example that starts as a standard form, and uses progressive enhancement to hide the submit and create the new div. I did use the CSS styling from StriplingWarrior.

How to catch SQLServer timeout exceptions

When a client sends ABORT, no transactions are rolled back. To avoid this behavior we have to use SET_XACT_ABORT ON

Cannot access a disposed object - How to fix?

because the solution folder was inside OneDrive folder.

If you moving the solution folders out of the one drive folder made the errors go away.


How to create an exit message

The abort function does this. For example:

abort("Message goes here")

Note: the abort message will be written to STDERR as opposed to puts which will write to STDOUT.

How can I cast int to enum?

I am using this piece of code to cast int to my enum:

if (typeof(YourEnum).IsEnumDefined(valueToCast)) return (YourEnum)valueToCast;
else { //handle it here, if its not defined }

I find it the best solution.

Best GUI designer for eclipse?

Visual Editor is a good choice.

It generates very clean code, with no "layout" files beside of your sourcen using a simple but convenient pattern. It's very easy to patch the generated code and directly see the result. There are some stability problems (some times, the preview window does not refresh anymore...), but nothing that a "clean Project" can't fix...

Converting bool to text in C++

We're talking about C++ right? Why on earth are we still using macros!?

C++ inline functions give you the same speed as a macro, with the added benefit of type-safety and parameter evaluation (which avoids the issue that Rodney and dwj mentioned.

inline const char * const BoolToString(bool b)
  return b ? "true" : "false";

Aside from that I have a few other gripes, particularly with the accepted answer :)

// this is used in C, not C++. if you want to use printf, instead include <cstdio>
//#include <stdio.h>
// instead you should use the iostream libs
#include <iostream>

// not only is this a C include, it's totally unnecessary!
//#include <stdarg.h>

// Macros - not type-safe, has side-effects. Use inline functions instead
//#define BOOL_STR(b) (b?"true":"false")
inline const char * const BoolToString(bool b)
  return b ? "true" : "false";

int main (int argc, char const *argv[]) {
    bool alpha = true;

    // printf? that's C, not C++
    //printf( BOOL_STR(alpha) );
    // use the iostream functionality
    std::cout << BoolToString(alpha);
    return 0;

Cheers :)

@DrPizza: Include a whole boost lib for the sake of a function this simple? You've got to be kidding?

What are the differences between delegates and events?

Delegate is a type-safe function pointer. Event is an implementation of publisher-subscriber design pattern using delegate.

Getting ssh to execute a command in the background on target machine

First follow this procedure:

Log in on A as user a and generate a pair of authentication keys. Do not enter a passphrase:

a@A:~> ssh-keygen -t rsa
Generating public/private rsa key pair.
Enter file in which to save the key (/home/a/.ssh/id_rsa): 
Created directory '/home/a/.ssh'.
Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/a/.ssh/id_rsa.
Your public key has been saved in /home/a/.ssh/
The key fingerprint is:
3e:4f:05:79:3a:9f:96:7c:3b:ad:e9:58:37:bc:37:e4 a@A

Now use ssh to create a directory ~/.ssh as user b on B. (The directory may already exist, which is fine):

a@A:~> ssh b@B mkdir -p .ssh
b@B's password: 

Finally append a's new public key to b@B:.ssh/authorized_keys and enter b's password one last time:

a@A:~> cat .ssh/ | ssh b@B 'cat >> .ssh/authorized_keys'
b@B's password: 

From now on you can log into B as b from A as a without password:

a@A:~> ssh b@B

then this will work without entering a password

ssh b@B "cd /some/directory; program-to-execute &"

Good Free Alternative To MS Access

The issue is finding an alternative to MS Access that includes a visual, drag and drop development environment with a "reasonable" database where the whole kit and caboodle can be deployed free of charge.

My first suggestion would be to look at this very complete list of MS Access alternatives (many of which are free), followed by a gander at this list of open source database development tools on

My second suggestion would be to check out WaveMaker, which is sort of an open source PowerBuilder for the cloud (disclaimer: I work there so should not be considered to be an unbiased source of information ;-)

WaveMaker combines a drag and drop IDE with an open source Java back end. It is licensed under the Apache license and boasts a 15,000-strong developer community.

Best JavaScript compressor

Revisiting this question a few years later, UglifyJS, seems to be the best option as of now.

As stated below, it runs on the NodeJS platform, but can be easily modified to run on any JavaScript engine.

--- Old answer below---

Google released Closure Compiler which seems to be generating the smallest files so far as seen here and here

Previous to that the various options were as follow

Basically Packer does a better job at initial compression , but if you are going to gzip the files before sending on the wire (which you should be doing) YUI Compressor gets the smallest final size.

The tests were done on jQuery code btw.

  • Original jQuery library 62,885 bytes , 19,758 bytes after gzip
  • jQuery minified with JSMin 36,391 bytes , 11,541 bytes after gzip
  • jQuery minified with Packer 21,557 bytes , 11,119 bytes after gzip
  • jQuery minified with the YUI Compressor 31,822 bytes , 10,818 bytes after gzip

@daniel james mentions in the comment compressorrater which shows Packer leading the chart in best compression, so I guess ymmv

Passing $_POST values with cURL

    function executeCurl($arrOptions) {

        $mixCH = curl_init();

        foreach ($arrOptions as $strCurlOpt => $mixCurlOptValue) {
            curl_setopt($mixCH, $strCurlOpt, $mixCurlOptValue);

        $mixResponse = curl_exec($mixCH);
        return $mixResponse;

    // If any HTTP authentication is needed.
    $username = 'http-auth-username';
    $password = 'http-auth-password';

    $requestType = 'POST'; // This can be PUT or POST

    // This is a sample array. You can use $arrPostData = $_POST
    $arrPostData = array(
        'key1'  => 'value-1-for-k1y-1',
        'key2'  => 'value-2-for-key-2',
        'key3'  => array(
                'key31'   => 'value-for-key-3-1',
                'key32'   => array(
                    'key321' => 'value-for-key321'
        'key4'  => array(
            'key'   => 'value'

    // You can set your post data
    $postData = http_build_query($arrPostData); // Raw PHP array

    $postData = json_encode($arrPostData); // Only USE this when request JSON data.

    $mixResponse = executeCurl(array(
        CURLOPT_URL => '',
        CURLOPT_HTTPGET => true,
        CURLOPT_VERBOSE => true,
        CURLOPT_AUTOREFERER => true,
        CURLOPT_CUSTOMREQUEST => $requestType,
        CURLOPT_POSTFIELDS  => $postData,
        CURLOPT_HTTPHEADER  => array(
            "X-HTTP-Method-Override: " . $requestType,
            'Content-Type: application/json', // Only USE this when requesting JSON data

        // If HTTP authentication is required, use the below lines.
        CURLOPT_USERPWD  => $username. ':' . $password

    // $mixResponse contains your server response.

PHP equivalent of .NET/Java's toString()

I use variableToString. It handles every PHP type and is flexible (you can extend it if you want).

Regular cast vs. static_cast vs. dynamic_cast

You should look at the article C++ Programming/Type Casting.

It contains a good description of all of the different cast types. The following taken from the above link:


const_cast(expression) The const_cast<>() is used to add/remove const(ness) (or volatile-ness) of a variable.


static_cast(expression) The static_cast<>() is used to cast between the integer types. 'e.g.' char->long, int->short etc.

Static cast is also used to cast pointers to related types, for example casting void* to the appropriate type.


Dynamic cast is used to convert pointers and references at run-time, generally for the purpose of casting a pointer or reference up or down an inheritance chain (inheritance hierarchy).


The target type must be a pointer or reference type, and the expression must evaluate to a pointer or reference. Dynamic cast works only when the type of object to which the expression refers is compatible with the target type and the base class has at least one virtual member function. If not, and the type of expression being cast is a pointer, NULL is returned, if a dynamic cast on a reference fails, a bad_cast exception is thrown. When it doesn't fail, dynamic cast returns a pointer or reference of the target type to the object to which expression referred.


Reinterpret cast simply casts one type bitwise to another. Any pointer or integral type can be casted to any other with reinterpret cast, easily allowing for misuse. For instance, with reinterpret cast one might, unsafely, cast an integer pointer to a string pointer.

SQL Group By with an Order By

MySQL prior to version 5 did not allow aggregate functions in ORDER BY clauses.

You can get around this limit with the deprecated syntax:

SELECT COUNT(id), `Tag` from `images-tags`

1, since it's the first column you want to group on.

Calculate distance between two latitude-longitude points? (Haversine formula)

Here's the accepted answer implementation ported to Java in case anyone needs it.

package com.project529.garage.util;

 * Mean radius.
private static double EARTH_RADIUS = 6371;

 * Returns the distance between two sets of latitudes and longitudes in meters.
 * <p/>
 * Based from the following JavaScript SO answer:
 * which is based on (error rate: ~0.55%).
public double getDistanceBetween(double lat1, double lon1, double lat2, double lon2) {
    double dLat = toRadians(lat2 - lat1);
    double dLon = toRadians(lon2 - lon1);

    double a = Math.sin(dLat / 2) * Math.sin(dLat / 2) +
            Math.cos(toRadians(lat1)) * Math.cos(toRadians(lat2)) *
                    Math.sin(dLon / 2) * Math.sin(dLon / 2);
    double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
    double d = EARTH_RADIUS * c;

    return d;

public double toRadians(double degrees) {
    return degrees * (Math.PI / 180);

What's the difference between a temp table and table variable in SQL Server?

It surprises me that no one mentioned the key difference between these two is that the temp table supports parallel insert while the table variable doesn't. You should be able to see the difference from the execution plan. And here is the video from SQL Workshops on Channel 9.

This also explains why you should use a table variable for smaller tables, otherwise use a temp table, as SQLMenace answered before.

How can I discover the "path" of an embedded resource?

This will get you a string array of all the resources:


Getting parts of a URL (Regex)

A single regex to parse and breakup a full URL including query parameters and anchors e.g.


RexEx positions:

url: RegExp['$&'],







you could then further parse the host ('.' delimited) quite easily.

What I would do is use something like this:

proto $1
host $2
port $3
the-rest $4

the further parse 'the rest' to be as specific as possible. Doing it in one regex is, well, a bit crazy.

How can I add (simple) tracing in C#?

I followed around five different answers as well as all the blog posts in the previous answers and still had problems. I was trying to add a listener to some existing code that was tracing using the TraceSource.TraceEvent(TraceEventType, Int32, String) method where the TraceSource object was initialised with a string making it a 'named source'.

For me the issue was not creating a valid combination of source and switch elements to target this source. Here is an example that will log to a file called tracelog.txt. For the following code:

TraceSource source = new TraceSource("sourceName");
source.TraceEvent(TraceEventType.Verbose, 1, "Trace message");

I successfully managed to log with the following diagnostics configuration:

        <source name="sourceName" switchName="switchName">
                    initializeData="tracelog.txt" />

        <add name="switchName" value="Verbose" />

What issues should be considered when overriding equals and hashCode in Java?

There are a couple of ways to do your check for class equality before checking member equality, and I think both are useful in the right circumstances.

  1. Use the instanceof operator.
  2. Use this.getClass().equals(that.getClass()).

I use #1 in a final equals implementation, or when implementing an interface that prescribes an algorithm for equals (like the java.util collection interfaces—the right way to check with with (obj instanceof Set) or whatever interface you're implementing). It's generally a bad choice when equals can be overridden because that breaks the symmetry property.

Option #2 allows the class to be safely extended without overriding equals or breaking symmetry.

If your class is also Comparable, the equals and compareTo methods should be consistent too. Here's a template for the equals method in a Comparable class:

final class MyClass implements Comparable<MyClass>


  public boolean equals(Object obj)
    /* If compareTo and equals aren't final, we should check with getClass instead. */
    if (!(obj instanceof MyClass)) 
      return false;
    return compareTo((MyClass) obj) == 0;


Detecting an undefined object property

Reading through this, I'm amazed I didn't see this. I have found multiple algorithms that would work for this.

Never Defined

If the value of an object was never defined, this will prevent from returning true if it is defined as null or undefined. This is helpful if you want true to be returned for values set as undefined

if(obj.prop === void 0) console.log("The value has never been defined");

Defined as undefined Or never Defined

If you want it to result as true for values defined with the value of undefined, or never defined, you can simply use === undefined

if(obj.prop === undefined) console.log("The value is defined as undefined, or never defined");

Defined as a falsy value, undefined,null, or never defined.

Commonly, people have asked me for an algorithm to figure out if a value is either falsy, undefined, or null. The following works.

if(obj.prop == false || obj.prop === null || obj.prop === undefined) {
    console.log("The value is falsy, null, or undefined");

How to convert std::string to LPCWSTR in C++ (Unicode)

string  myMessage="helloworld";
int len;
int slength = (int)myMessage.length() + 1;
len = MultiByteToWideChar(CP_ACP, 0, myMessage.c_str(), slength, 0, 0); 
wchar_t* buf = new wchar_t[len];
MultiByteToWideChar(CP_ACP, 0, myMessage.c_str(), slength, buf, len);
std::wstring r(buf);
 std::wstring stemp = r.C_str();
LPCWSTR result = stemp.c_str();

Comparing Arrays of Objects in JavaScript

EDIT: You cannot overload operators in current, common browser-based implementations of JavaScript interpreters.

To answer the original question, one way you could do this, and mind you, this is a bit of a hack, simply serialize the two arrays to JSON and then compare the two JSON strings. That would simply tell you if the arrays are different, obviously you could do this to each of the objects within the arrays as well to see which ones were different.

Another option is to use a library which has some nice facilities for comparing objects - I use and recommend MochiKit.

EDIT: The answer kamens gave deserves consideration as well, since a single function to compare two given objects would be much smaller than any library to do what I suggest (although my suggestion would certainly work well enough).

Here is a naïve implemenation that may do just enough for you - be aware that there are potential problems with this implementation:

function objectsAreSame(x, y) {
   var objectsAreSame = true;
   for(var propertyName in x) {
      if(x[propertyName] !== y[propertyName]) {
         objectsAreSame = false;
   return objectsAreSame;

The assumption is that both objects have the same exact list of properties.

Oh, and it is probably obvious that, for better or worse, I belong to the only-one-return-point camp. :)

How do I REALLY reset the Visual Studio window layout?

I had similar problem except that it happened without installing any plugin. I begin to get this dialog about source control every time I open the project + tons of windows popping up and floating which I had to close one by one.

enter image description here

Windows -> Rest Windows Layout, fixed it for me without any problems. It does bring the default setting which I don't mind at all :)

Getting all types that implement an interface

You could use some LINQ to get the list:

var types = from type in this.GetType().Assembly.GetTypes()
            where type is ISomeInterface
            select type;

But really, is that more readable?

What is MVC and what are the advantages of it?

One of the major advantages of MVC which has not mentioned here is that MVC provides RESTful urls which enables SEO. When you name your Controllers and Actions wisely, it makes it easier for search engines to find your site if they only take a look at your site Urls. For example you have a car sale website and a page which displays available Lamborghini Veneno cars, instead of having referring to the page you can choose url for SEO purpose.

Here is a good answer to MVC Advantages and here is an article How to create a SEO friendly Url.

How to set encoding in .getJSON jQuery

You need to analyze the JSON calls using Wireshark, so you will see if you include the charset in the formation of the JSON page or not, for example:

  • If the page is simple if text / html
0000  48 54 54 50 2f 31 2e 31  20 32 30 30 20 4f 4b 0d   HTTP/1.1  200 OK.
0010  0a 43 6f 6e 74 65 6e 74  2d 54 79 70 65 3a 20 74   .Content -Type: t
0020  65 78 74 2f 68 74 6d 6c  0d 0a 43 61 63 68 65 2d   ext/html ..Cache-
0030  43 6f 6e 74 72 6f 6c 3a  20 6e 6f 2d 63 61 63 68   Control:  no-cach
  • If the page is of the type including custom JSON with MIME "charset = ISO-8859-1"
0000  48 54 54 50 2f 31 2e 31  20 32 30 30 20 4f 4b 0d   HTTP/1.1  200 OK.
0010  0a 43 61 63 68 65 2d 43  6f 6e 74 72 6f 6c 3a 20   .Cache-C ontrol: 
0020  6e 6f 2d 63 61 63 68 65  0d 0a 43 6f 6e 74 65 6e   no-cache ..Conten
0030  74 2d 54 79 70 65 3a 20  74 65 78 74 2f 68 74 6d   t-Type:  text/htm
0040  6c 3b 20 63 68 61 72 73  65 74 3d 49 53 4f 2d 38   l; chars et=ISO-8
0050  38 35 39 2d 31 0d 0a 43  6f 6e 6e 65 63 74 69 6f   859-1..C onnectio

Why is that? because we can not put on the page of JSON a goal like this:

In my case I use the manufacturer Connect Me 9210 Digi:

  • I had to use a flag to indicate that one would use non-standard MIME: p-> theCgiPtr-> = fDataType eRpDataTypeOther;
  • It added the new MIME in the variable: strcpy (p-> theCgiPtr-> fOtherMimeType, "text / html; charset = ISO-8859-1 ");

It worked for me without having to convert the data passed by JSON for UTF-8 and then redo the conversion on the page ...

How can I pass arguments to a batch file?

Another useful tip is to use %* to mean "all". For example:

echo off
set arg1=%1
set arg2=%2
fake-command /u %arg1% /p %arg2% %*

When you run:

test-command admin password foo bar

the above batch file will run:

fake-command /u admin /p password admin password foo bar

I may have the syntax slightly wrong, but this is the general idea.

How can I play sound in Java?

I wrote the following code that works fine. But I think it only works with .wav format.

public static synchronized void playSound(final String url) {
  new Thread(new Runnable() {
  // The wrapper thread is unnecessary, unless it blocks on the
  // Clip finishing; see comments.
    public void run() {
      try {
        Clip clip = AudioSystem.getClip();
        AudioInputStream inputStream = AudioSystem.getAudioInputStream(
          Main.class.getResourceAsStream("/path/to/sounds/" + url));;
      } catch (Exception e) {

Filtering collections in C#

If your list is very big and you are filtering repeatedly - you can sort the original list on the filter attribute, binary search to find the start and end points.

Initial time O(n*log(n)) then O(log(n)).

Standard filtering will take O(n) each time.

What is the best way to connect and use a sqlite database from C#

Here I am trying to help you do the job step by step: (this may be the answer to other questions)

  1. Go to this address , down the page you can see something like "List of Release Packages". Based on your system and .net framework version choose the right one for you. for example if your want to use .NET Framework 4.6 on a 64-bit Windows, choose this version and download it.
  2. Then install the file somewhere on your hard drive, just like any other software.
  3. Open Visual studio and your project. Then in solution explorer, right-click on "References" and choose "add Reference...".
  4. Click the browse button and choose where you install the previous file and go to .../bin/System.Data.SQLite.dll and click add and then OK buttons.

that is pretty much it. now you can use SQLite in your project. to use it in your project on the code level you may use this below example code:

  1. make a connection string:

    string connectionString = @"URI=file:{the location of your sqlite database}";

  2. establish a sqlite connection:

    SQLiteConnection theConnection = new SQLiteConnection(connectionString );

  3. open the connection:


  4. create a sqlite command:

    SQLiteCommand cmd = new SQLiteCommand(theConnection);

  5. Make a command text, or better said your SQLite statement:

    cmd.CommandText = "INSERT INTO table_name(col1, col2) VALUES(val1, val2)";

  6. Execute the command


that is it.

How can I measure the similarity between two images?

How to measure similarity between two images entirely depends on what you would like to measure, for example: contrast, brightness, modality, noise... and then choose the best suitable similarity measure there is for you. You can choose from MAD (mean absolute difference), MSD (mean squared difference) which are good for measuring brightness...there is also available CR (correlation coefficient) which is good in representing correlation between two images. You could also choose from histogram based similarity measures like SDH (standard deviation of difference image histogram) or multimodality similarity measures like MI (mutual information) or NMI (normalized mutual information).

Because this similarity measures cost much in time, it is advised to scale images down before applying these measures on them.

Insert into ... values ( SELECT ... FROM ... )

Instead of VALUES part of INSERT query, just use SELECT query as below.

INSERT INTO table1 ( column1 , 2, 3... )
SELECT col1, 2, 3... FROM table2

Best programming based games

Kara is about programming a bug(!) coming up in various versions, e.g. Finite State Machine, Java, Turing Machine, Multithreading


Copy/duplicate database without using mysqldump

an SQL that shows SQL commands, need to run to duplicate a database from one database to another. for each table there is create a table statement and an insert statement. it assumes both databases are on the same server:

select @fromdb:="crm";
select @todb:="crmen";

SET group_concat_max_len=100000000;

SELECT  GROUP_CONCAT( concat("CREATE TABLE `",@todb,"`.`",table_name,"` LIKE `",@fromdb,"`.`",table_name,"`;\n",
"INSERT INTO `",@todb,"`.`",table_name,"` SELECT * FROM `",@fromdb,"`.`",table_name,"`;") 


as sqlstatement
 FROM information_schema.tables where table_schema=@fromdb and TABLE_TYPE='BASE TABLE';

Delete all but the most recent X files in bash

The problems with the existing answers:

  • inability to handle filenames with embedded spaces or newlines.
    • in the case of solutions that invoke rm directly on an unquoted command substitution (rm `...`), there's an added risk of unintended globbing.
  • inability to distinguish between files and directories (i.e., if directories happened to be among the 5 most recently modified filesystem items, you'd effectively retain fewer than 5 files, and applying rm to directories will fail).

wnoise's answer addresses these issues, but the solution is GNU-specific (and quite complex).

Here's a pragmatic, POSIX-compliant solution that comes with only one caveat: it cannot handle filenames with embedded newlines - but I don't consider that a real-world concern for most people.

For the record, here's the explanation for why it's generally not a good idea to parse ls output:

ls -tp | grep -v '/$' | tail -n +6 | xargs -I {} rm -- {}

Note: This command operates in the current directory; to target a directory explicitly, use a subshell ((...)):
(cd /path/to && ls -tp | grep -v '/$' | tail -n +6 | xargs -I {} rm -- {})
The same applies analogously to the commands below.

The above is inefficient, because xargs has to invoke rm once for each filename.
Your platform's xargs may allow you to solve this problem:

If you have GNU xargs, use -d '\n', which makes xargs consider each input line a separate argument, yet passes as many arguments as will fit on a command line at once:

ls -tp | grep -v '/$' | tail -n +6 | xargs -d '\n' -r rm --

-r (--no-run-if-empty) ensures that rm is not invoked if there's no input.

If you have BSD xargs (including on macOS), you can use -0 to handle NUL-separated input, after first translating newlines to NUL (0x0) chars., which also passes (typically) all filenames at once (will also work with GNU xargs):

ls -tp | grep -v '/$' | tail -n +6 | tr '\n' '\0' | xargs -0 rm --


  • ls -tp prints the names of filesystem items sorted by how recently they were modified , in descending order (most recently modified items first) (-t), with directories printed with a trailing / to mark them as such (-p).

    • Note: It is the fact that ls -tp always outputs file / directory names only, not full paths, that necessitates the subshell approach mentioned above for targeting a directory other than the current one ((cd /path/to && ls -tp ...)).
  • grep -v '/$' then weeds out directories from the resulting listing, by omitting (-v) lines that have a trailing / (/$).

    • Caveat: Since a symlink that points to a directory is technically not itself a directory, such symlinks will not be excluded.
  • tail -n +6 skips the first 5 entries in the listing, in effect returning all but the 5 most recently modified files, if any.
    Note that in order to exclude N files, N+1 must be passed to tail -n +.

  • xargs -I {} rm -- {} (and its variations) then invokes on rm on all these files; if there are no matches at all, xargs won't do anything.

    • xargs -I {} rm -- {} defines placeholder {} that represents each input line as a whole, so rm is then invoked once for each input line, but with filenames with embedded spaces handled correctly.
    • -- in all cases ensures that any filenames that happen to start with - aren't mistaken for options by rm.

A variation on the original problem, in case the matching files need to be processed individually or collected in a shell array:

# One by one, in a shell loop (POSIX-compliant):
ls -tp | grep -v '/$' | tail -n +6 | while IFS= read -r f; do echo "$f"; done

# One by one, but using a Bash process substitution (<(...), 
# so that the variables inside the `while` loop remain in scope:
while IFS= read -r f; do echo "$f"; done < <(ls -tp | grep -v '/$' | tail -n +6)

# Collecting the matches in a Bash *array*:
IFS=$'\n' read -d '' -ra files  < <(ls -tp | grep -v '/$' | tail -n +6)
printf '%s\n' "${files[@]}" # print array elements

What is the best free memory leak detector for a C/C++ program and its plug-in DLLs?

I personally use Visual Leak Detector, though it can cause large delays when large blocks are leaked (it displays the contents of the entire leaked block).

Python module for converting PDF to text

Additionally there is PDFTextStream which is a commercial Java library that can also be used from Python.

Get OS-level system information

Add OSHI dependency via maven:


Get a battery capacity left in percentage:

SystemInfo si = new SystemInfo();
HardwareAbstractionLayer hal = si.getHardware();
for (PowerSource pSource : hal.getPowerSources()) {
    System.out.println(String.format("%n %s @ %.1f%%", pSource.getName(), pSource.getRemainingCapacity() * 100d));

How do I get the full url of the page I am on in C#

If you need the port number also, you can use



string url = Request.Url.Authority + HttpContext.Current.Request.RawUrl.ToString();

if (Request.ServerVariables["HTTPS"] == "on")
    url = "https://" + url;
    url = "http://" + url;

Is Java "pass-by-reference" or "pass-by-value"?

The crux of the matter is that the word reference in the expression "pass by reference" means something completely different from the usual meaning of the word reference in Java.

Usually in Java reference means a a reference to an object. But the technical terms pass by reference/value from programming language theory is talking about a reference to the memory cell holding the variable, which is something completely different.

What are the differences between a HashMap and a Hashtable in Java?

Beside all the other important aspects already mentioned here, Collections API (e.g. Map interface) is being modified all the time to conform to the "latest and greatest" additions to Java spec.

For example, compare Java 5 Map iterating:

for (Elem elem : map.keys()) {

versus the old Hashtable approach:

for (Enumeration en = htable.keys(); en.hasMoreElements(); ) {
  Elem elem = (Elem) en.nextElement();

In Java 1.8 we are also promised to be able to construct and access HashMaps like in good old scripting languages:

Map<String,Integer> map = { "orange" : 12, "apples" : 15 };

Update: No, they won't land in 1.8... :(

Are Project Coin's collection enhancements going to be in JDK8?

What is the command to truncate a SQL Server log file?

In management studio:

  • Don't do this on a live environment, but to ensure you shrink your dev db as much as you can:
    • Right-click the database, choose Properties, then Options.
    • Make sure "Recovery model" is set to "Simple", not "Full"
    • Click OK
  • Right-click the database again, choose Tasks -> Shrink -> Files
  • Change file type to "Log"
  • Click OK.

Alternatively, the SQL to do it:

 DBCC SHRINKFILE (mydatabase_Log, 1)


How to tell which disk Windows Used to Boot

a simpler way search downloads in the start menu and click on downloads in the search results to see where it will take you the drive will be highlighted in the explorer.

Force unmount of NFS-mounted directory

Try running

lsof | grep /mnt/data

That should list any process that is accessing /mnt/data that would prevent it from being unmounted.

How do you beta test an iphone app?

There's a relatively new service called HockeyApp, which seems to rival TestFlight, however they claim to give you access to unlimited users, but it does cost some $$ unlike TestFlight which has now been integrated directly into iTunes Connect.

How to find a Java Memory Leak

I use following approach to finding memory leaks in Java. I've used jProfiler with great success, but I believe that any specialized tool with graphing capabilities (diffs are easier to analyze in graphical form) will work.

  1. Start the application and wait until it get to "stable" state, when all the initialization is complete and the application is idle.
  2. Run the operation suspected of producing a memory leak several times to allow any cache, DB-related initialization to take place.
  3. Run GC and take memory snapshot.
  4. Run the operation again. Depending on the complexity of operation and sizes of data that is processed operation may need to be run several to many times.
  5. Run GC and take memory snapshot.
  6. Run a diff for 2 snapshots and analyze it.

Basically analysis should start from greatest positive diff by, say, object types and find what causes those extra objects to stick in memory.

For web applications that process requests in several threads analysis gets more complicated, but nevertheless general approach still applies.

I did quite a number of projects specifically aimed at reducing memory footprint of the applications and this general approach with some application specific tweaks and trick always worked well.