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Some Of The Best Answers From Latest Asked Questions

How do I install and use the ASP.NET AJAX Control Toolkit in my .NET 3.5 web applications?

Install the ASP.NET AJAX Control Toolkit

  1. Download the ZIP file AjaxControlToolkit-Framework3.5SP1-DllOnly.zip from the ASP.NET AJAX Control Toolkit Releases page of the CodePlex web site.

  2. Copy the contents of this zip file directly into the bin directory of your web site.

Update web.config

  1. Put this in your web.config under the <controls> section:

    <?xml version="1.0"?>
    <configuration>
        ...
        <system.web>
            ...
            <pages>
                ...
                <controls>
                    ...
                    <add tagPrefix="ajaxtoolkit"
                        namespace="AjaxControlToolkit"
                        assembly="AjaxControlToolKit"/>
                </controls>
            </pages>
            ...
        </system.web>
        ...
    </configuration>
    

Setup Visual Studio

  1. Right-click on the Toolbox and select "Add Tab", and add a tab called "AJAX Control Toolkit"

  2. Inside that tab, right-click on the Toolbox and select "Choose Items..."

  3. When the "Choose Toolbox Items" dialog appears, click the "Browse..." button. Navigate to your project's "bin" folder. Inside that folder, select "AjaxControlToolkit.dll" and click OK. Click OK again to close the Choose Items Dialog.

You can now use the controls in your web sites!

How do I remove an item from a stl vector with a certain value?

If you want to remove an item, the following will be a bit more efficient.

std::vector<int> v;


auto it = std::find(v.begin(), v.end(), 5);
if(it != v.end())
    v.erase(it);

or you may avoid overhead of moving the items if the order does not matter to you:

std::vector<int> v;

auto it = std::find(v.begin(), v.end(), 5);

if (it != v.end()) {
  using std::swap;

  // swap the one to be removed with the last element
  // and remove the item at the end of the container
  // to prevent moving all items after '5' by one
  swap(*it, v.back());
  v.pop_back();
}

Why doesn't JavaScript support multithreading?

It's the implementations that doesn't support multi-threading. Currently Google Gears is providing a way to use some form of concurrency by executing external processes but that's about it.

The new browser Google is supposed to release today (Google Chrome) executes some code in parallel by separating it in process.

The core language, of course can have the same support as, say Java, but support for something like Erlang's concurrency is nowhere near the horizon.

Embed a PowerPoint presentation into HTML

The 'actual answer' is that you cannot do it directly. You have to convert your PowerPoint presentation to something that the browser can process. You can save each page of the PowerPoint presentation as a JPEG image and then display as a series of images. You can save the PowerPoint presentation as HTML. Both of these solutions will render only static pages, without any of the animations of PowerPoint. You can use a tool to convert your PowerPoint presentation to Flash (.swf) and embed it that way. This will preserve any animations and presumably allow you to do an automatic slideshow without the need for writing special code to change the images.

Is a GUID unique 100% of the time?

GUID algorithms are usually implemented according to the v4 GUID specification, which is essentially a pseudo-random string. Sadly, these fall into the category of "likely non-unique", from Wikipedia (I don't know why so many people ignore this bit): "... other GUID versions have different uniqueness properties and probabilities, ranging from guaranteed uniqueness to likely non-uniqueness."

The pseudo-random properties of V8's JavaScript Math.random() are TERRIBLE at uniqueness, with collisions often coming after only a few thousand iterations, but V8 isn't the only culprit. I've seen real-world GUID collisions using both PHP and Ruby implementations of v4 GUIDs.

Because it's becoming more and more common to scale ID generation across multiple clients, and clusters of servers, entropy takes a big hit -- the chances of the same random seed being used to generate an ID escalate (time is often used as a random seed in pseudo-random generators), and GUID collisions escalate from "likely non-unique" to "very likely to cause lots of trouble".

To solve this problem, I set out to create an ID algorithm that could scale safely, and make better guarantees against collision. It does so by using the timestamp, an in-memory client counter, client fingerprint, and random characters. The combination of factors creates an additive complexity that is particularly resistant to collision, even if you scale it across a number of hosts:

http://usecuid.org/

How to loop through files matching wildcard in batch file

You can use this line to print the contents of your desktop:

FOR %%I in (C:\windows\desktop\*.*) DO echo %%I 

Once you have the %%I variable it's easy to perform a command on it (just replace the word echo with your program)

In addition, substitution of FOR variable references has been enhanced You can now use the following optional syntax:

%~I         - expands %I removing any surrounding quotes (")
%~fI        - expands %I to a fully qualified path name
%~dI        - expands %I to a drive letter only
%~pI        - expands %I to a path only (directory with \)
%~nI        - expands %I to a file name only
%~xI        - expands %I to a file extension only
%~sI        - expanded path contains short names only
%~aI        - expands %I to file attributes of file
%~tI        - expands %I to date/time of file
%~zI        - expands %I to size of file
%~$PATH:I   - searches the directories listed in the PATH
               environment variable and expands %I to the
               fully qualified name of the first one found.
               If the environment variable name is not
               defined or the file is not found by the
               search, then this modifier expands to the
               empty string

https://ss64.com/nt/syntax-args.html

In the above examples %I and PATH can be replaced by other valid values. The %~ syntax is terminated by a valid FOR variable name. Picking upper case variable names like %I makes it more readable and avoids confusion with the modifiers, which are not case sensitive.

You can get the full documentation by typing FOR /?

Best way to do multi-row insert in Oracle?

Whenever I need to do this I build a simple PL/SQL block with a local procedure like this:

declare
   procedure ins
   is
      (p_exch_wh_key INTEGER, 
       p_exch_nat_key INTEGER, 
       p_exch_date DATE, exch_rate NUMBER, 
       p_from_curcy_cd VARCHAR2, 
       p_to_curcy_cd VARCHAR2, 
       p_exch_eff_date DATE, 
       p_exch_eff_end_date DATE, 
       p_exch_last_updated_date DATE);
   begin
      insert into tmp_dim_exch_rt 
      (exch_wh_key, 
       exch_nat_key, 
       exch_date, exch_rate, 
       from_curcy_cd, 
       to_curcy_cd, 
       exch_eff_date, 
       exch_eff_end_date, 
       exch_last_updated_date) 
      values
      (p_exch_wh_key, 
       p_exch_nat_key, 
       p_exch_date, exch_rate, 
       p_from_curcy_cd, 
       p_to_curcy_cd, 
       p_exch_eff_date, 
       p_exch_eff_end_date, 
       p_exch_last_updated_date);
   end;
begin
   ins (1, 1, '28-AUG-2008', 109.49, 'USD', 'JPY', '28-AUG-2008', '28-AUG-2008', '28-AUG-2008'),
   ins (2, 1, '28-AUG-2008', .54, 'USD', 'GBP', '28-AUG-2008', '28-AUG-2008', '28-AUG-2008'),
   ins (3, 1, '28-AUG-2008', 1.05, 'USD', 'CAD', '28-AUG-2008', '28-AUG-2008', '28-AUG-2008'),
   ins (4, 1, '28-AUG-2008', .68, 'USD', 'EUR', '28-AUG-2008', '28-AUG-2008', '28-AUG-2008'),
   ins (5, 1, '28-AUG-2008', 1.16, 'USD', 'AUD', '28-AUG-2008', '28-AUG-2008', '28-AUG-2008'),
   ins (6, 1, '28-AUG-2008', 7.81, 'USD', 'HKD', '28-AUG-2008', '28-AUG-2008', '28-AUG-2008');
end;
/

What is the best way to convert an array to a hash in Ruby

NOTE: For a concise and efficient solution, please see Marc-André Lafortune's answer below.

This answer was originally offered as an alternative to approaches using flatten, which were the most highly upvoted at the time of writing. I should have clarified that I didn't intend to present this example as a best practice or an efficient approach. Original answer follows.


Warning! Solutions using flatten will not preserve Array keys or values!

Building on @John Topley's popular answer, let's try:

a3 = [ ['apple', 1], ['banana', 2], [['orange','seedless'], 3] ]
h3 = Hash[*a3.flatten]

This throws an error:

ArgumentError: odd number of arguments for Hash
        from (irb):10:in `[]'
        from (irb):10

The constructor was expecting an Array of even length (e.g. ['k1','v1,'k2','v2']). What's worse is that a different Array which flattened to an even length would just silently give us a Hash with incorrect values.

If you want to use Array keys or values, you can use map:

h3 = Hash[a3.map {|key, value| [key, value]}]
puts "h3: #{h3.inspect}"

This preserves the Array key:

h3: {["orange", "seedless"]=>3, "apple"=>1, "banana"=>2}

What is the yield keyword used for in C#?

At first sight, yield return is a .NET sugar to return an IEnumerable.

Without yield, all the items of the collection are created at once:

class SomeData
{
    public SomeData() { }

    static public IEnumerable<SomeData> CreateSomeDatas()
    {
        return new List<SomeData> {
            new SomeData(), 
            new SomeData(), 
            new SomeData()
        };
    }
}

Same code using yield, it returns item by item:

class SomeData
{
    public SomeData() { }

    static public IEnumerable<SomeData> CreateSomeDatas()
    {
        yield return new SomeData();
        yield return new SomeData();
        yield return new SomeData();
    }
}

The advantage of using yield is that if the function consuming your data simply needs the first item of the collection, the rest of the items won't be created.

The yield operator allows the creation of items as it is demanded. That's a good reason to use it.

How large is a DWORD with 32- and 64-bit code?

No ... on all Windows platforms DWORD is 32 bits. LONGLONG or LONG64 is used for 64 bit types.

How do I install a color theme for IntelliJ IDEA 7.0.x

Interesting I never spent too much time adjusting the colours in IntelliJ although tried once.

See link below with an already defined colour scheme you can import.

Where can I download IntelliJ IDEA 10 Color Schemes?

http://devnet.jetbrains.net/docs/DOC-1154

Download the jar file, file import the jar where you will see a what to import ;)

enter image description here

Search and replace a line in a file in Python

Expanding on @Kiran's answer, which I agree is more succinct and Pythonic, this adds codecs to support the reading and writing of UTF-8:

import codecs 

from tempfile import mkstemp
from shutil import move
from os import remove


def replace(source_file_path, pattern, substring):
    fh, target_file_path = mkstemp()

    with codecs.open(target_file_path, 'w', 'utf-8') as target_file:
        with codecs.open(source_file_path, 'r', 'utf-8') as source_file:
            for line in source_file:
                target_file.write(line.replace(pattern, substring))
    remove(source_file_path)
    move(target_file_path, source_file_path)

How do I merge two dictionaries in a single expression (taking union of dictionaries)?

I benchmarked the suggested with perfplot and found that the good old

temp = x.copy()
temp.update(y)

is the fastest solution together with the new

x | y

enter image description here


Code to reproduce the plot:

from collections import ChainMap
from itertools import chain
import perfplot


def setup(n):
    x = dict(zip(range(n), range(n)))
    y = dict(zip(range(n, 2 * n), range(n, 2 * n)))
    return x, y


def copy_update(data):
    x, y = data
    temp = x.copy()
    temp.update(y)
    return temp


def add_items(data):
    x, y = data
    return dict(list(x.items()) + list(y.items()))


def curly_star(data):
    x, y = data
    return {**x, **y}


def chain_map(data):
    x, y = data
    return dict(ChainMap({}, y, x))


def itertools_chain(data):
    x, y = data
    return dict(chain(x.items(), y.items()))


def python39_concat(data):
    x, y = data
    return x | y


perfplot.show(
    setup=setup,
    kernels=[
        copy_update,
        add_items,
        curly_star,
        chain_map,
        itertools_chain,
        python39_concat,
    ],
    labels=[
        "copy_update",
        "dict(list(x.items()) + list(y.items()))",
        "{**x, **y}",
        "chain_map",
        "itertools.chain",
        "x | y",
    ],
    n_range=[2 ** k for k in range(15)],
    xlabel="len(x), len(y)",
    equality_check=None,
)

What static analysis tools are available for C#?

Aside from the excellent list by madgnome, I would add a duplicate code detector that is based off the command line (but is free):

http://sourceforge.net/projects/duplo/

Using Django time/date widgets in custom form

The below will also work as a last resort if the above failed

class PaymentsForm(forms.ModelForm):
    class Meta:
        model = Payments

    def __init__(self, *args, **kwargs):
        super(PaymentsForm, self).__init__(*args, **kwargs)
        self.fields['date'].widget = SelectDateWidget()

Same as

class PaymentsForm(forms.ModelForm):
    date = forms.DateField(widget=SelectDateWidget())

    class Meta:
        model = Payments

put this in your forms.py from django.forms.extras.widgets import SelectDateWidget

What is the difference between "INNER JOIN" and "OUTER JOIN"?

The precise algorithm for INNER JOIN, LEFT/RIGHT OUTER JOIN are as following:

  1. Take each row from the first table: a
  2. Consider all rows from second table beside it: (a, b[i])
  3. Evaluate the ON ... clause against each pair: ON( a, b[i] ) = true/false?
    • When the condition evaluates to true, return that combined row (a, b[i]).
    • When reach end of second table without any match, and this is an Outer Join then return a (virtual) pair using Null for all columns of other table: (a, Null) for LEFT outer join or (Null, b) for RIGHT outer join. This is to ensure all rows of first table exists in final results.

Note: the condition specified in ON clause could be anything, it is not required to use Primary Keys (and you don't need to always refer to Columns from both tables)! For example:

Inner Join vs. Left Outer Join


enter image description here

Note: Left Join = Left Outer Join, Right Join = Right Outer Join.

What is the purpose of class methods?

I used to work with PHP and recently I was asking myself, whats going on with this classmethod? Python manual is very technical and very short in words so it wont help with understanding that feature. I was googling and googling and I found answer -> http://code.anjanesh.net/2007/12/python-classmethods.html.

If you are lazy to click it. My explanation is shorter and below. :)

in PHP (maybe not all of you know PHP, but this language is so straight forward that everybody should understand what I'm talking about) we have static variables like this:


class A
{

    static protected $inner_var = null;

    static public function echoInnerVar()
    {
        echo self::$inner_var."\n";
    }

    static public function setInnerVar($v)
    {
        self::$inner_var = $v;
    }

}

class B extends A
{
}

A::setInnerVar(10);
B::setInnerVar(20);

A::echoInnerVar();
B::echoInnerVar();

The output will be in both cases 20.

However in python we can add @classmethod decorator and thus it is possible to have output 10 and 20 respectively. Example:


class A(object):
    inner_var = 0

    @classmethod
    def setInnerVar(cls, value):
        cls.inner_var = value

    @classmethod
    def echoInnerVar(cls):
        print cls.inner_var


class B(A):
    pass


A.setInnerVar(10)
B.setInnerVar(20)

A.echoInnerVar()
B.echoInnerVar()

Smart, ain't?

How can I get the DateTime for the start of the week?

Let's combine the culture-safe answer and the extension method answer:

public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        System.Globalization.CultureInfo ci = System.Threading.Thread.CurrentThread.CurrentCulture;
        DayOfWeek fdow = ci.DateTimeFormat.FirstDayOfWeek;
        return DateTime.Today.AddDays(-(DateTime.Today.DayOfWeek- fdow));
    }
}

How do I find the authoritative name-server for a domain name?

You could find out the nameservers for a domain with the "host" command:

[davidp@supernova:~]$ host -t ns stackoverflow.com
stackoverflow.com name server ns51.domaincontrol.com.
stackoverflow.com name server ns52.domaincontrol.com.

What is the regex pattern for datetime (2008-09-01 12:35:45 )?

http://regexlib.com/REDetails.aspx?regexp_id=610

^(?=\d)(?:(?:31(?!.(?:0?[2469]|11))|(?:30|29)(?!.0?2)|29(?=.0?2.(?:(?:(?:1[6-9]|[2-9]\d)?(?:0[48]|[2468][048]|[13579][26])|(?:(?:16|[2468][048]|[3579][26])00)))(?:\x20|$))|(?:2[0-8]|1\d|0?[1-9]))([-./])(?:1[012]|0?[1-9])\1(?:1[6-9]|[2-9]\d)?\d\d(?:(?=\x20\d)\x20|$))?(((0?[1-9]|1[012])(:[0-5]\d){0,2}(\x20[AP]M))|([01]\d|2[0-3])(:[0-5]\d){1,2})?$


This RE validates both dates and/or times patterns. Days in Feb. are also validated for Leap years. Dates: in dd/mm/yyyy or d/m/yy format between 1/1/1600 - 31/12/9999. Leading zeroes are optional. Date separators can be either matching dashes(-), slashes(/) or periods(.) Times: in the hh:MM:ss AM/PM 12 hour format (12:00 AM - 11:59:59 PM) or hh:MM:ss military time format (00:00:00 - 23:59:59). The 12 hour time format: 1) may have a leading zero for the hour. 2) Minutes and seconds are optional for the 12 hour format 3) AM or PM is required and case sensitive. Military time 1) must have a leading zero for all hours less than 10. 2) Minutes are manditory. 3) seconds are optional. Datetimes: combination of the above formats. A date first then a time separated by a space. ex) dd/mm/yyyy hh:MM:ss


Edit: Make sure you copy the RegEx from the regexlib.com website as StackOverflow sometimes removes/destroys special chars.

How to replace plain URLs with links?

Replacing URLs with links (Answer to the General Problem)

The regular expression in the question misses a lot of edge cases. When detecting URLs, it's always better to use a specialized library that handles international domain names, new TLDs like .museum, parentheses and other punctuation within and at the end of the URL, and many other edge cases. See the Jeff Atwood's blog post The Problem With URLs for an explanation of some of the other issues.

The best summary of URL matching libraries is in Dan Dascalescu's Answer +100
(as of Feb 2014)


"Make a regular expression replace more than one match" (Answer to the specific problem)

Add a "g" to the end of the regular expression to enable global matching:

/ig;

But that only fixes the problem in the question where the regular expression was only replacing the first match. Do not use that code.

What is reflection and why is it useful?

As name itself suggest it reflects what it holds for example class method,etc apart from providing feature to invoke method creating instance dynamically at runtime.

It is used by many frameworks and application under the wood to invoke services without actually knowing the code.

How do I determine the size of my array in C?

The macro ARRAYELEMENTCOUNT(x) that everyone is making use of evaluates incorrectly. This, realistically, is just a sensitive matter, because you can't have expressions that result in an 'array' type.

/* Compile as: CL /P "macro.c" */
# define ARRAYELEMENTCOUNT(x) (sizeof (x) / sizeof (x[0]))

ARRAYELEMENTCOUNT(p + 1);

Actually evaluates as:

(sizeof (p + 1) / sizeof (p + 1[0]));

Whereas

/* Compile as: CL /P "macro.c" */
# define ARRAYELEMENTCOUNT(x) (sizeof (x) / sizeof (x)[0])

ARRAYELEMENTCOUNT(p + 1);

It correctly evaluates to:

(sizeof (p + 1) / sizeof (p + 1)[0]);

This really doesn't have a lot to do with the size of arrays explicitly. I've just noticed a lot of errors from not truly observing how the C preprocessor works. You always wrap the macro parameter, not an expression in might be involved in.


This is correct; my example was a bad one. But that's actually exactly what should happen. As I previously mentioned p + 1 will end up as a pointer type and invalidate the entire macro (just like if you attempted to use the macro in a function with a pointer parameter).

At the end of the day, in this particular instance, the fault doesn't really matter (so I'm just wasting everyone's time; huzzah!), because you don't have expressions with a type of 'array'. But really the point about preprocessor evaluation subtles I think is an important one.

iPhone App Minus App Store?

With the help of this post, I have made a script that will install via the app Installous for rapid deployment:

# compress application.
/bin/mkdir -p $CONFIGURATION_BUILD_DIR/Payload
/bin/cp -R $CONFIGURATION_BUILD_DIR/MyApp.app $CONFIGURATION_BUILD_DIR/Payload
/bin/cp iTunesCrap/logo_itunes.png $CONFIGURATION_BUILD_DIR/iTunesArtwork
/bin/cp iTunesCrap/iTunesMetadata.plist $CONFIGURATION_BUILD_DIR/iTunesMetadata.plist

cd $CONFIGURATION_BUILD_DIR

# zip up the HelloWorld directory

/usr/bin/zip -r MyApp.ipa Payload iTunesArtwork iTunesMetadata.plist

What Is missing in the post referenced above, is the iTunesMetadata. Without this, Installous will not install apps correctly. Here is an example of an iTunesMetadata:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>appleId</key>
    <string></string>
    <key>artistId</key>
    <integer>0</integer>
    <key>artistName</key>
    <string>MYCOMPANY</string>
    <key>buy-only</key>
    <true/>
    <key>buyParams</key>
    <string></string>
    <key>copyright</key>
    <string></string>
    <key>drmVersionNumber</key>
    <integer>0</integer>
    <key>fileExtension</key>
    <string>.app</string>
    <key>genre</key>
    <string></string>
    <key>genreId</key>
    <integer>0</integer>
    <key>itemId</key>
    <integer>0</integer>
    <key>itemName</key>
    <string>MYAPP</string>
    <key>kind</key>
    <string>software</string>
    <key>playlistArtistName</key>
    <string>MYCOMPANY</string>
    <key>playlistName</key>
    <string>MYAPP</string>
    <key>price</key>
    <integer>0</integer>
    <key>priceDisplay</key>
    <string>nil</string>
    <key>rating</key>
    <dict>
        <key>content</key>
        <string></string>
        <key>label</key>
        <string>4+</string>
        <key>rank</key>
        <integer>100</integer>
        <key>system</key>
        <string>itunes-games</string>
    </dict>
    <key>releaseDate</key>
    <string>Sunday, December 12, 2010</string>
    <key>s</key>
    <integer>143441</integer>
    <key>softwareIcon57x57URL</key>
    <string></string>
    <key>softwareIconNeedsShine</key>
    <false/>
    <key>softwareSupportedDeviceIds</key>
    <array>
        <integer>1</integer>
    </array>
    <key>softwareVersionBundleId</key>
    <string>com.mycompany.myapp</string>
    <key>softwareVersionExternalIdentifier</key>
    <integer>0</integer>
    <key>softwareVersionExternalIdentifiers</key>
    <array>
        <integer>1466803</integer>
        <integer>1529132</integer>
        <integer>1602608</integer>
        <integer>1651681</integer>
        <integer>1750461</integer>
        <integer>1930253</integer>
        <integer>1961532</integer>
        <integer>1973932</integer>
        <integer>2026202</integer>
        <integer>2526384</integer>
        <integer>2641622</integer>
        <integer>2703653</integer>
    </array>
    <key>vendorId</key>
    <integer>0</integer>
    <key>versionRestrictions</key>
    <integer>0</integer>
</dict>
</plist>

Obviously, replace all instances of MyApp with the name of your app and MyCompany with the name of your company.

Basically, this will install on any jailbroken device with Installous installed. After it is set up, this results in very fast deployment, as it can be installed from anywhere, just upload it to your companies website, and download the file directly to the device, and copy / move it to ~/Documents/Installous/Downloads.

How do I make a fully statically linked .exe with Visual Studio Express 2005?

I've had this same dependency problem and I also know that you can include the VS 8.0 DLLs (release only! not debug!---and your program has to be release, too) in a folder of the appropriate name, in the parent folder with your .exe:

How to: Deploy using XCopy (MSDN)

Also note that things are guaranteed to go awry if you need to have C++ and C code in the same statically linked .exe because you will get linker conflicts that can only be resolved by ignoring the correct libXXX.lib and then linking dynamically (DLLs).

Lastly, with a different toolset (VC++ 6.0) things "just work", since Windows 2000 and above have the correct DLLs installed.

How to deal with "java.lang.OutOfMemoryError: Java heap space" error?

Easy way to solve OutOfMemoryError in java is to increase the maximum heap size by using JVM options -Xmx512M, this will immediately solve your OutOfMemoryError. This is my preferred solution when I get OutOfMemoryError in Eclipse, Maven or ANT while building project because based upon size of project you can easily ran out of Memory.

Here is an example of increasing maximum heap size of JVM, Also its better to keep -Xmx to -Xms ration either 1:1 or 1:1.5 if you are setting heap size in your java application.

export JVM_ARGS="-Xms1024m -Xmx1024m"

Reference Link

What is the syntax for an inner join in LINQ to SQL?

basically LINQ join operator provides no benefit for SQL. I.e. the following query

var r = from dealer in db.Dealers
   from contact in db.DealerContact
   where dealer.DealerID == contact.DealerID
   select dealerContact;

will result in INNER JOIN in SQL

join is useful for IEnumerable<> because it is more efficient:

from contact in db.DealerContact  

clause would be re-executed for every dealer But for IQueryable<> it is not the case. Also join is less flexible.

SQL Query for Logins

Is this what you're after?

select * from master.syslogins

How do you remove a specific revision in the git history?

Here is a way to remove non-interactively a specific <commit-id>, knowing only the <commit-id> you would like to remove:

git rebase --onto <commit-id>^ <commit-id> HEAD

Make browser window blink in task Bar

I've made a jQuery plugin for the purpose of blinking notification messages in the browser title bar. You can specify different options like blinking interval, duration, if the blinking should stop when the window/tab gets focused, etc. The plugin works in Firefox, Chrome, Safari, IE6, IE7 and IE8.

Here is an example on how to use it:

$.titleAlert("New mail!", {
    requireBlur:true,
    stopOnFocus:true,
    interval:600
});

If you're not using jQuery, you might still want to look at the source code (there are a few quirky bugs and edge cases that you need to work around when doing title blinking if you want to fully support all major browsers).

CSS - Make divs align horizontally

you can use the clip property:

#container {
  position: absolute;
  clip: rect(0px,200px,100px,0px);
  overflow: hidden;
  background: red;
}

note the position: absolute and overflow: hidden needed in order to get clip to work.

Java: notify() vs. notifyAll() all over again

Taken from blog on Effective Java:

The notifyAll method should generally be used in preference to notify. 

If notify is used, great care must be taken to ensure liveness.

So, what i understand is (from aforementioned blog, comment by "Yann TM" on accepted answer and Java docs):

  • notify() : JVM awakens one of the waiting threads on this object. Thread selection is made arbitrarily without fairness. So same thread can be awakened again and again. So system's state changes but no real progress is made. Thus creating a livelock.
  • notifyAll() : JVM awakens all threads and then all threads race for the lock on this object. Now, CPU scheduler selects a thread which acquires lock on this object. This selection process would be much better than selection by JVM. Thus, ensuring liveness.

How do you use script variables in psql?

You need to use one of the procedural languages such as PL/pgSQL not the SQL proc language. In PL/pgSQL you can use vars right in SQL statements. For single quotes you can use the quote literal function.

How can I represent an 'Enum' in Python?

def M_add_class_attribs(attribs):
    def foo(name, bases, dict_):
        for v, k in attribs:
            dict_[k] = v
        return type(name, bases, dict_)
    return foo

def enum(*names):
    class Foo(object):
        __metaclass__ = M_add_class_attribs(enumerate(names))
        def __setattr__(self, name, value):  # this makes it read-only
            raise NotImplementedError
    return Foo()

Use it like this:

Animal = enum('DOG', 'CAT')
Animal.DOG # returns 0
Animal.CAT # returns 1
Animal.DOG = 2 # raises NotImplementedError

if you just want unique symbols and don't care about the values, replace this line:

__metaclass__ = M_add_class_attribs(enumerate(names))

with this:

__metaclass__ = M_add_class_attribs((object(), name) for name in names)

What does ** (double star/asterisk) and * (star/asterisk) do for parameters?

Let us first understand what are positional arguments and keyword arguments. Below is an example of function definition with Positional arguments.

def test(a,b,c):
     print(a)
     print(b)
     print(c)

test(1,2,3)
#output:
1
2
3

So this is a function definition with positional arguments. You can call it with keyword/named arguments as well:

def test(a,b,c):
     print(a)
     print(b)
     print(c)

test(a=1,b=2,c=3)
#output:
1
2
3

Now let us study an example of function definition with keyword arguments:

def test(a=0,b=0,c=0):
     print(a)
     print(b)
     print(c)
     print('-------------------------')

test(a=1,b=2,c=3)
#output :
1
2
3
-------------------------

You can call this function with positional arguments as well:

def test(a=0,b=0,c=0):
    print(a)
    print(b)
    print(c)
    print('-------------------------')

test(1,2,3)
# output :
1
2
3
---------------------------------

So we now know function definitions with positional as well as keyword arguments.

Now let us study the '*' operator and '**' operator.

Please note these operators can be used in 2 areas:

a) function call

b) function definition

The use of '*' operator and '**' operator in function call.

Let us get straight to an example and then discuss it.

def sum(a,b):  #receive args from function calls as sum(1,2) or sum(a=1,b=2)
    print(a+b)

my_tuple = (1,2)
my_list = [1,2]
my_dict = {'a':1,'b':2}

# Let us unpack data structure of list or tuple or dict into arguments with help of '*' operator
sum(*my_tuple)   # becomes same as sum(1,2) after unpacking my_tuple with '*'
sum(*my_list)    # becomes same as sum(1,2) after unpacking my_list with  '*'
sum(**my_dict)   # becomes same as sum(a=1,b=2) after unpacking by '**' 

# output is 3 in all three calls to sum function.

So remember

when the '*' or '**' operator is used in a function call -

'*' operator unpacks data structure such as a list or tuple into arguments needed by function definition.

'**' operator unpacks a dictionary into arguments needed by function definition.

Now let us study the '*' operator use in function definition. Example:

def sum(*args): #pack the received positional args into data structure of tuple. after applying '*' - def sum((1,2,3,4))
    sum = 0
    for a in args:
        sum+=a
    print(sum)

sum(1,2,3,4)  #positional args sent to function sum
#output:
10

In function definition the '*' operator packs the received arguments into a tuple.

Now let us see an example of '**' used in function definition:

def sum(**args): #pack keyword args into datastructure of dict after applying '**' - def sum({a:1,b:2,c:3,d:4})
    sum=0
    for k,v in args.items():
        sum+=v
    print(sum)

sum(a=1,b=2,c=3,d=4) #positional args sent to function sum

In function definition The '**' operator packs the received arguments into a dictionary.

So remember:

In a function call the '*' unpacks data structure of tuple or list into positional or keyword arguments to be received by function definition.

In a function call the '**' unpacks data structure of dictionary into positional or keyword arguments to be received by function definition.

In a function definition the '*' packs positional arguments into a tuple.

In a function definition the '**' packs keyword arguments into a dictionary.

How do you set up use HttpOnly cookies in PHP

<?php
//None HttpOnly cookie:
setcookie("abc", "test", NULL, NULL, NULL, NULL, FALSE); 

//HttpOnly cookie:
setcookie("abc", "test", NULL, NULL, NULL, NULL, TRUE); 

?>

Source

Should a function have only one return statement?

The more return statements you have in a function, the higher complexity in that one method. If you find yourself wondering if you have too many return statements, you might want to ask yourself if you have too many lines of code in that function.

But, not, there is nothing wrong with one/many return statements. In some languages, it is a better practice (C++) than in others (C).

Struct like objects in Java

You can make a simple class with public fields and no methods in Java, but it is still a class and is still handled syntactically and in terms of memory allocation just like a class. There is no way to genuinely reproduce structs in Java.

What is a 'Closure'?

Closure is a feature in JavaScript where a function has access to its own scope variables, access to the outer function variables and access to the global variables.

Closure has access to its outer function scope even after the outer function has returned. This means a closure can remember and access variables and arguments of its outer function even after the function has finished.

The inner function can access the variables defined in its own scope, the outer function’s scope, and the global scope. And the outer function can access the variable defined in its own scope and the global scope.

Example of Closure:

var globalValue = 5;

function functOuter() {
  var outerFunctionValue = 10;

  //Inner function has access to the outer function value
  //and the global variables
  function functInner() {
    var innerFunctionValue = 5;
    alert(globalValue + outerFunctionValue + innerFunctionValue);
  }
  functInner();
}
functOuter();  

Output will be 20 which sum of its inner function own variable, outer function variable and global variable value.

Why functional languages?

Have you been following the evolution of programming languages lately? Every new release of all mainstream programming languages seems to borrow more and more features from functional programming.

  • Closures, anonymous functions, passing and returning functions as values used to be exotic features known only to Lisp and ML hackers. But gradually, C#, Delphi, Python, Perl, Javascript, have added support for closures. Its not possible for any up-and-coming language to be taken seriously without closures.

  • Several languages, notably Python, C#, and Ruby have native support for list comprehensions and list generators.

  • ML pioneered generic programming in 1973, but support for generics ("parametric polymorphism") has only become an industry standard in the last 5 years or so. If I remember correctly, Fortran supported generics in 2003, followed by Java 2004, C# in 2005, Delphi in 2008. (I know C++ has supported templates since 1979, but 90% of discussions on C++'s STL start with "here there be demons".)

What makes these features appealing to programmers? It should be plainly obvious: it helps programmers write shorter code. All languages in the future are going to support -- at a minimum -- closures if they want to stay competitive. In this respect, functional programming is already in the mainstream.

Most applications are simple enough to be solved in normal OO ways

Who says can't use functional programming for simple things too? Not every functional program needs to be a compiler, theorem prover, or massively parallel telecommunications switch. I regularly use F# for ad hoc throwaway scripts in addition to my more complicated projects.

How to pass a single object[] to a params object[]

The params parameter modifier gives callers a shortcut syntax for passing multiple arguments to a method. There are two ways to call a method with a params parameter:

1) Calling with an array of the parameter type, in which case the params keyword has no effect and the array is passed directly to the method:

object[] array = new[] { "1", "2" };

// Foo receives the 'array' argument directly.
Foo( array );

2) Or, calling with an extended list of arguments, in which case the compiler will automatically wrap the list of arguments in a temporary array and pass that to the method:

// Foo receives a temporary array containing the list of arguments.
Foo( "1", "2" );

// This is equivalent to:
object[] temp = new[] { "1", "2" );
Foo( temp );


In order to pass in an object array to a method with a "params object[]" parameter, you can either:

1) Create a wrapper array manually and pass that directly to the method, as mentioned by lassevk:

Foo( new object[] { array } );  // Equivalent to calling convention 1.

2) Or, cast the argument to object, as mentioned by Adam, in which case the compiler will create the wrapper array for you:

Foo( (object)array );  // Equivalent to calling convention 2.


However, if the goal of the method is to process multiple object arrays, it may be easier to declare it with an explicit "params object[][]" parameter. This would allow you to pass multiple arrays as arguments:

void Foo( params object[][] arrays ) {
  foreach( object[] array in arrays ) {
    // process array
  }
}

...
Foo( new[] { "1", "2" }, new[] { "3", "4" } );

// Equivalent to:
object[][] arrays = new[] {
  new[] { "1", "2" },
  new[] { "3", "4" }
};
Foo( arrays );

Edit: Raymond Chen describes this behavior and how it relates to the C# specification in a new post.

What is 'Currying'?

If you understand partial you're halfway there. The idea of partial is to preapply arguments to a function and give back a new function that wants only the remaining arguments. When this new function is called it includes the preloaded arguments along with whatever arguments were supplied to it.

In Clojure + is a function but to make things starkly clear:

(defn add [a b] (+ a b))

You may be aware that the inc function simply adds 1 to whatever number it's passed.

(inc 7) # => 8

Let's build it ourselves using partial:

(def inc (partial add 1))

Here we return another function that has 1 loaded into the first argument of add. As add takes two arguments the new inc function wants only the b argument -- not 2 arguments as before since 1 has already been partially applied. Thus partial is a tool from which to create new functions with default values presupplied. That is why in a functional language functions often order arguments from general to specific. This makes it easier to reuse such functions from which to construct other functions.

Now imagine if the language were smart enough to understand introspectively that add wanted two arguments. When we passed it one argument, rather than balking, what if the function partially applied the argument we passed it on our behalf understanding that we probably meant to provide the other argument later? We could then define inc without explicitly using partial.

(def inc (add 1)) #partial is implied

This is the way some languages behave. It is exceptionally useful when one wishes to compose functions into larger transformations. This would lead one to transducers.

How to sort a list of strings?

Please use sorted() function in Python3

items = ["love", "like", "play", "cool", "my"]
sorted(items2)

How to get controls in WPF to fill available space?

There are also some properties you can set to force a control to fill its available space when it would otherwise not do so. For example, you can say:

HorizontalContentAlignment="Stretch"

... to force the contents of a control to stretch horizontally. Or you can say:

HorizontalAlignment="Stretch"

... to force the control itself to stretch horizontally to fill its parent.

How do I execute a file in Cygwin?

To execute a file in the current directory, the syntax to use is: ./foo

As mentioned by allain, ./a.exe is the correct way to execute a.exe in the working directory using Cygwin.

Note: You may wish to use the -o parameter to cc to specify your own output filename. An example of this would be: cc helloworld.c -o helloworld.exe.

C-like structures in Python

You can use a tuple for a lot of things where you would use a struct in C (something like x,y coordinates or RGB colors for example).

For everything else you can use dictionary, or a utility class like this one:

>>> class Bunch:
...     def __init__(self, **kwds):
...         self.__dict__.update(kwds)
...
>>> mystruct = Bunch(field1=value1, field2=value2)

I think the "definitive" discussion is here, in the published version of the Python Cookbook.

Rolling back bad changes with svn in Eclipse

In Eclipse Ganymede (Subclipse)

Select project/file that contains bad change, and from pop-up menu choose:

Team -> Show History

Revisions related to that project/file will be shown in History tab.

Find revision where "bad changes" were committed and from pop-up menu choose:

Revert Changes from Revision X

This will merge changes in file(s) modified within bad revision, with revision prior to bad revision.

There are two scenarios from here:

  1. If you committed no changes for that file (bad revision is last revision for that file), it will simply remove changes made in bad revision. Those changes are merged to your working copy so you have to commit them.

  2. If you committed some changes for that file (bad revision is not last revision for that file), you will have to manually resolve conflict. Let say that you have file readme.txt with, and bad revision number is 33. Also, you've made another commit for that file in revision 34. After you choose Revert Changes from Revision 33 you will have following in your working copy:

readme.txt.merge-left.r33 - bad revision

readme.txt.merge-right.r32 - before bad revision

readme.txt.working - working copy version (same as in r34 if you don't have any uncommitted changes)

Original readme.txt will be marked conflicted, and will contain merged version (where changes from bad revision are removed) with some markers (<<<<<<< .working etc). If you just want to remove changes from bad revision and keep changes made after that, then all you have to do is remove markers. Otherwise, you can copy contents from one of 3 files mentioned above to original file. Whatever you choose, when you are done, mark conflict resolved by

Team - Mark Resolved

Temporary files will be removed and your file will be marked changed. As in 1, you have to commit changes.

Note that this does not remove revision from revision history in svn repository. You simply made new revision where changes from bad revision are removed.

Base64 Encoding Image

My synopsis of rfc2397 is:

Once you've got your base64 encoded image data put it inside the <Image></Image> tags prefixed with "data:{mimetype};base64," this is similar to the prefixing done in the parenthesis of url() definition in CSS or in the quoted value of the src attribute of the img tag in [X]HTML. You can test the data url in firefox by putting the data:image/... line into the URL field and pressing enter, it should show your image.

For actually encoding I think we need to go over all your options, not just PHP, because there's so many ways to base64 encode something.

  1. Use the base64 command line tool. It's part of the GNU coreutils (v6+) and pretty much default in any Cygwin, Linux, GnuWin32 install, but not the BSDs I tried. Issue: $ base64 imagefile.ico > imagefile.base64.txt
  2. Use a tool that features the option to convert to base64, like Notepad++ which has the feature under plugins->MIME tools->base64 Encode
  3. Email yourself the file and view the raw email contents, copy and paste.
  4. Use a web form.

A note on mime-types: I would prefer you use one of image/png image/jpeg or image/gif as I can't find the popular image/x-icon. Should that be image/vnd.microsoft.icon? Also the other formats are much shorter.

compare 265 bytes vs 1150 bytes:

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAAVFBMVEWcZjTcViTMuqT8/vzcYjTkhhTkljT87tz03sRkZmS8mnT03tT89vTsvoTk1sz86uTkekzkjmzkwpT01rTsmnzsplTUwqz89uy0jmzsrmTknkT0zqT3X4fRAAAAbklEQVR4XnXOVw6FIBBAUafQsZfX9r/PB8JoTPT+QE4o01AtMoS8HkALcH8BGmGIAvaXLw0wCqxKz0Q9w1LBfFSiJBzljVerlbYhlBO4dZHM/F3llybncbIC6N+70Q7OlUm7DdO+gKs9gyRwdgd/LOcGXHzLN5gAAAAASUVORK5CYII=

data:image/x-icon;base64,AAABAAEAEBAAAAEAIABoBAAAFgAAACgAAAAQAAAAIAAAAAEAIAAAAAAAAAQAAAAAAAAAAAAAAAAAAAAAAAD/////ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv///////////2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb///////////9mZmb/ZmZm//////////////////////////////////////////////////////9mZmb/ZmZm////////////ZmZm/2ZmZv//////ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv//////ZmZm/2ZmZv///////////2ZmZv9mZmb//////2ZmZv9mZmb/ZmZm/2ZmZv9mZmb/ZmZm/2ZmZv9mZmb//////2ZmZv9mZmb///////////9mZmb/ZmZm////////////////////////////8fX4/8nW5P+twtb/oLjP//////9mZmb/ZmZm////////////////////////////oLjP/3eZu/9pj7T/M2aZ/zNmmf8zZpn/M2aZ/zNmmf///////////////////////////////////////////zNmmf8zZpn/M2aZ/zNmmf8zZpn/d5m7/6C4z/+WwuH/wN/3//////////////////////////////////////+guM//rcLW/8nW5P/x9fj//////9/v+/+w1/X/QZ7m/1Cm6P//////////////////////////////////////////////////////7/f9/4C+7v8xluT/EYbg/zGW5P/A3/f/0933/9Pd9//////////////////////////////////f7/v/YK7q/xGG4P8RhuD/MZbk/7DX9f//////4uj6/zJh2/8yYdv/8PT8////////////////////////////UKbo/xGG4P8xluT/sNf1////////////4uj6/zJh2/8jVtj/e5ro/////////////////////////////////8Df9/+gz/P/////////////////8PT8/0944P8jVtj/bI7l/////////////////////////////////////////////////////////////////2yO5f8jVtj/T3jg//D0/P///////////////////////////////////////////////////////////3ua6P8jVtj/MmHb/+Lo+v////////////////////////////////////////////////////////////D0/P8yYdv/I1bY/9Pd9///////////////////////AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA==

How can a Java program get its own process ID?

There exists no platform-independent way that can be guaranteed to work in all jvm implementations. ManagementFactory.getRuntimeMXBean().getName() looks like the best (closest) solution, and typically includes the PID. It's short, and probably works in every implementation in wide use.

On linux+windows it returns a value like 12345@hostname (12345 being the process id). Beware though that according to the docs, there are no guarantees about this value:

Returns the name representing the running Java virtual machine. The returned name string can be any arbitrary string and a Java virtual machine implementation can choose to embed platform-specific useful information in the returned name string. Each running virtual machine could have a different name.

In Java 9 the new process API can be used:

long pid = ProcessHandle.current().pid();

What is the Difference Between Mercurial and Git?

Yet another interesting comparison of mercurial and git: Mercurial vs Git. Main focus is on internals and their influence on branching process.

XML serialization in Java?

XStream is pretty good at serializing object to XML without much configuration and money! (it's under BSD license).

We used it in one of our project to replace the plain old java-serialization and it worked almost out of the box.

Internet Access in Ubuntu on VirtualBox

I had the same problem.

Solved by sharing internet connection (on the hosting OS).

Network Connection Properties -> advanced -> Allow other users to connect...

What are the main performance differences between varchar and nvarchar SQL Server data types?

Always use nvarchar.

You may never need the double-byte characters for most applications. However, if you need to support double-byte languages and you only have single-byte support in your database schema it's really expensive to go back and modify throughout your application.

The cost of migrating one application from varchar to nvarchar will be much more than the little bit of extra disk space you'll use in most applications.

What is the difference between the | and || or operators?

The | operator performs a bitwise OR of its two operands (meaning both sides must evaluate to false for it to return false) while the || operator will only evaluate the second operator if it needs to.

http://msdn.microsoft.com/en-us/library/kxszd0kx(VS.71).aspx

http://msdn.microsoft.com/en-us/library/6373h346(VS.71).aspx

How do I fix a NoSuchMethodError?

Try this way: remove all .class files under your project directories (and, of course, all subdirectories). Rebuild.

Sometimes mvn clean (if you are using maven) does not clean .class files manually created by javac. And those old files contain old signatures, leading to NoSuchMethodError.

How to declare an array of strings in C++?

Declare an array of strings in C++ like this : char array_of_strings[][]

For example : char array_of_strings[200][8192];

will hold 200 strings, each string having the size 8kb or 8192 bytes.

use strcpy(line[i],tempBuffer); to put data in the array of strings.

How do you create optional arguments in php?

Give the optional argument a default value.

function date ($format, $timestamp='') {
}

How do I list the symbols in a .so file

You can use the nm -g tool from the binutils toolchain. However, their source is not always readily available. and I'm not actually even sure that this information can always be retrieved. Perhaps objcopy reveals further information.

/EDIT: The tool's name is of course nm. The flag -g is used to show only exported symbols.

Subversion ignoring "--password" and "--username" options

Best I can give you is a "works for me" on SVN 1.5. You may try adding --no-auth-cache to your svn update to see if that lets you override more easily.

If you want to permanently switch from user2 to user1, head into ~/.subversion/auth/ on *nix and delete the auth cache file for domain.com (most likely in ~/.subversion/auth/svn.simple/ -- just read through them and you'll find the one you want to drop). While it is possible to update the current auth cache, you have to make sure to update the length tokens as well. Simpler just to get prompted again next time you update.

What is a good Hash Function?

For doing "normal" hash table lookups on basically any kind of data - this one by Paul Hsieh is the best I've ever used.

http://www.azillionmonkeys.com/qed/hash.html

If you care about cryptographically secure or anything else more advanced, then YMMV. If you just want a kick ass general purpose hash function for a hash table lookup, then this is what you're looking for.

How do I change the number of open files limit in Linux?

You could always try doing a ulimit -n 2048. This will only reset the limit for your current shell and the number you specify must not exceed the hard limit

Each operating system has a different hard limit setup in a configuration file. For instance, the hard open file limit on Solaris can be set on boot from /etc/system.

set rlim_fd_max = 166384
set rlim_fd_cur = 8192

On OS X, this same data must be set in /etc/sysctl.conf.

kern.maxfilesperproc=166384
kern.maxfiles=8192

Under Linux, these settings are often in /etc/security/limits.conf.

There are two kinds of limits:

  • soft limits are simply the currently enforced limits
  • hard limits mark the maximum value which cannot be exceeded by setting a soft limit

Soft limits could be set by any user while hard limits are changeable only by root. Limits are a property of a process. They are inherited when a child process is created so system-wide limits should be set during the system initialization in init scripts and user limits should be set during user login for example by using pam_limits.

There are often defaults set when the machine boots. So, even though you may reset your ulimit in an individual shell, you may find that it resets back to the previous value on reboot. You may want to grep your boot scripts for the existence ulimit commands if you want to change the default.

How do I test a private function or a class that has private methods, fields or inner classes?

Hey use this utility class if you are on spring.

ReflectionTestUtils.invokeMethod(new ClassName(), "privateMethodName");

What is a mutex?

What is a Mutex?

The mutex (In fact, the term mutex is short for mutual exclusion) also known as spinlock is the simplest synchronization tool that is used to protect critical regions and thus prevent race conditions. That is a thread must acquire a lock before entering into a critical section (In critical section multi threads share a common variable, updating a table, writing a file and so on), it releases the lock when it leaves critical section.

What is a Race Condition?

A race condition occurs when two or more threads can access shared data and they try to change it at the same time. Because the thread scheduling algorithm can swap between threads at any time, you don't know the order in which the threads will attempt to access the shared data. Therefore, the result of the change in data is dependent on the thread scheduling algorithm, i.e. both threads are "racing" to access/change the data.

Real life example:

When I am having a big heated discussion at work, I use a rubber chicken which I keep in my desk for just such occasions. The person holding the chicken is the only person who is allowed to talk. If you don't hold the chicken you cannot speak. You can only indicate that you want the chicken and wait until you get it before you speak. Once you have finished speaking, you can hand the chicken back to the moderator who will hand it to the next person to speak. This ensures that people do not speak over each other, and also have their own space to talk.

Replace Chicken with Mutex and person with thread and you basically have the concept of a mutex.

@Xetius

Usage in C#:

This example shows how a local Mutex object is used to synchronize access to a protected resource. Because each calling thread is blocked until it acquires ownership of the mutex, it must call the ReleaseMutex method to release ownership of the thread.

using System;
using System.Threading;

class Example
{
    // Create a new Mutex. The creating thread does not own the mutex.
    private static Mutex mut = new Mutex();
    private const int numIterations = 1;
    private const int numThreads = 3;

    static void Main()
    {
        // Create the threads that will use the protected resource.
        for(int i = 0; i < numThreads; i++)
        {
            Thread newThread = new Thread(new ThreadStart(ThreadProc));
            newThread.Name = String.Format("Thread{0}", i + 1);
            newThread.Start();
        }

        // The main thread exits, but the application continues to
        // run until all foreground threads have exited.
    }

    private static void ThreadProc()
    {
        for(int i = 0; i < numIterations; i++)
        {
            UseResource();
        }
    }

    // This method represents a resource that must be synchronized
    // so that only one thread at a time can enter.
    private static void UseResource()
    {
        // Wait until it is safe to enter.
        Console.WriteLine("{0} is requesting the mutex", 
                          Thread.CurrentThread.Name);
        mut.WaitOne();

        Console.WriteLine("{0} has entered the protected area", 
                          Thread.CurrentThread.Name);

        // Place code to access non-reentrant resources here.

        // Simulate some work.
        Thread.Sleep(500);

        Console.WriteLine("{0} is leaving the protected area", 
            Thread.CurrentThread.Name);

        // Release the Mutex.
        mut.ReleaseMutex();
        Console.WriteLine("{0} has released the mutex", 
            Thread.CurrentThread.Name);
    }
}
// The example displays output like the following:
//       Thread1 is requesting the mutex
//       Thread2 is requesting the mutex
//       Thread1 has entered the protected area
//       Thread3 is requesting the mutex
//       Thread1 is leaving the protected area
//       Thread1 has released the mutex
//       Thread3 has entered the protected area
//       Thread3 is leaving the protected area
//       Thread3 has released the mutex
//       Thread2 has entered the protected area
//       Thread2 is leaving the protected area
//       Thread2 has released the mutex

MSDN Reference Mutex

What is a semaphore?

The article Mutexes and Semaphores Demystified by Michael Barr is a great short introduction into what makes mutexes and semaphores different, and when they should and should not be used. I've excerpted several key paragraphs here.

The key point is that mutexes should be used to protect shared resources, while semaphores should be used for signaling. You should generally not use semaphores to protect shared resources, nor mutexes for signaling. There are issues, for instance, with the bouncer analogy in terms of using semaphores to protect shared resources - you can use them that way, but it may cause hard to diagnose bugs.

While mutexes and semaphores have some similarities in their implementation, they should always be used differently.

The most common (but nonetheless incorrect) answer to the question posed at the top is that mutexes and semaphores are very similar, with the only significant difference being that semaphores can count higher than one. Nearly all engineers seem to properly understand that a mutex is a binary flag used to protect a shared resource by ensuring mutual exclusion inside critical sections of code. But when asked to expand on how to use a "counting semaphore," most engineers—varying only in their degree of confidence—express some flavor of the textbook opinion that these are used to protect several equivalent resources.

...

At this point an interesting analogy is made using the idea of bathroom keys as protecting shared resources - the bathroom. If a shop has a single bathroom, then a single key will be sufficient to protect that resource and prevent multiple people from using it simultaneously.

If there are multiple bathrooms, one might be tempted to key them alike and make multiple keys - this is similar to a semaphore being mis-used. Once you have a key you don't actually know which bathroom is available, and if you go down this path you're probably going to end up using mutexes to provide that information and make sure you don't take a bathroom that's already occupied.

A semaphore is the wrong tool to protect several of the essentially same resource, but this is how many people think of it and use it. The bouncer analogy is distinctly different - there aren't several of the same type of resource, instead there is one resource which can accept multiple simultaneous users. I suppose a semaphore can be used in such situations, but rarely are there real-world situations where the analogy actually holds - it's more often that there are several of the same type, but still individual resources, like the bathrooms, which cannot be used this way.

...

The correct use of a semaphore is for signaling from one task to another. A mutex is meant to be taken and released, always in that order, by each task that uses the shared resource it protects. By contrast, tasks that use semaphores either signal or wait—not both. For example, Task 1 may contain code to post (i.e., signal or increment) a particular semaphore when the "power" button is pressed and Task 2, which wakes the display, pends on that same semaphore. In this scenario, one task is the producer of the event signal; the other the consumer.

...

Here an important point is made that mutexes interfere with real time operating systems in a bad way, causing priority inversion where a less important task may be executed before a more important task because of resource sharing. In short, this happens when a lower priority task uses a mutex to grab a resource, A, then tries to grab B, but is paused because B is unavailable. While it's waiting, a higher priority task comes along and needs A, but it's already tied up, and by a process that isn't even running because it's waiting for B. There are many ways to resolve this, but it most often is fixed by altering the mutex and task manager. The mutex is much more complex in these cases than a binary semaphore, and using a semaphore in such an instance will cause priority inversions because the task manager is unaware of the priority inversion and cannot act to correct it.

...

The cause of the widespread modern confusion between mutexes and semaphores is historical, as it dates all the way back to the 1974 invention of the Semaphore (capital "S", in this article) by Djikstra. Prior to that date, none of the interrupt-safe task synchronization and signaling mechanisms known to computer scientists was efficiently scalable for use by more than two tasks. Dijkstra's revolutionary, safe-and-scalable Semaphore was applied in both critical section protection and signaling. And thus the confusion began.

However, it later became obvious to operating system developers, after the appearance of the priority-based preemptive RTOS (e.g., VRTX, ca. 1980), publication of academic papers establishing RMA and the problems caused by priority inversion, and a paper on priority inheritance protocols in 1990, 3 it became apparent that mutexes must be more than just semaphores with a binary counter.

Mutex: resource sharing

Semaphore: signaling

Don't use one for the other without careful consideration of the side effects.

What is a deadlock?

Deadlock occurs when a thread is waiting for other thread to finish and vice versa.

How to avoid?
- Avoid Nested Locks
- Avoid Unnecessary Locks
- Use thread join()

How do you detect it?
run this command in cmd:

jcmd $PID Thread.print

reference : geeksforgeeks

What is a race condition?

You can prevent race condition, if you use "Atomic" classes. The reason is just the thread don't separate operation get and set, example is below:

AtomicInteger ai = new AtomicInteger(2);
ai.getAndAdd(5);

As a result, you will have 7 in link "ai". Although you did two actions, but the both operation confirm the same thread and no one other thread will interfere to this, that means no race conditions!

Finding what methods a Python object has

To check if it has a particular method:

hasattr(object,"method")

Why am I getting a NoClassDefFoundError in Java?

NoClassDefFoundError In Java

Definition:

  1. Java Virtual Machine is not able to find a particular class at runtime which was available at compile time.

  2. If a class was present during compile time but not available in java classpath during runtime.

enter image description here

Examples:

  1. The class is not in Classpath, there is no sure shot way of knowing it but many times you can just have a look to print System.getproperty("java.classpath") and it will print the classpath from there you can at least get an idea of your actual runtime classpath.
  2. A simple example of NoClassDefFoundError is class belongs to a missing JAR file or JAR was not added into classpath or sometimes jar's name has been changed by someone like in my case one of my colleagues has changed tibco.jar into tibco_v3.jar and the program is failing with java.lang.NoClassDefFoundError and I were wondering what's wrong.

  3. Just try to run with explicitly -classpath option with the classpath you think will work and if it's working then it's a sure short sign that someone is overriding java classpath.

  4. Permission issue on JAR file can also cause NoClassDefFoundError in Java.
  5. Typo on XML Configuration can also cause NoClassDefFoundError in Java.
  6. when your compiled class which is defined in a package, doesn’t present in the same package while loading like in the case of JApplet it will throw NoClassDefFoundError in Java.

Possible Solutions:

  1. The class is not available in Java Classpath.
  2. If you are working in J2EE environment than the visibility of Class among multiple Classloader can also cause java.lang.NoClassDefFoundError, see examples and scenario section for detailed discussion.
  3. Check for java.lang.ExceptionInInitializerError in your log file. NoClassDefFoundError due to the failure of static initialization is quite common.
  4. Because NoClassDefFoundError is a subclass of java.lang.LinkageError it can also come if one of it dependency like native library may not available.
  5. Any start-up script is overriding Classpath environment variable.
  6. You might be running your program using jar command and class was not defined in manifest file's ClassPath attribute.

Resources:

3 ways to solve NoClassDefFoundError

java.lang.NoClassDefFoundError Problem patterns

How to apply an XSLT Stylesheet in C#

Based on Daren's excellent answer, note that this code can be shortened significantly by using the appropriate XslCompiledTransform.Transform overload:

var myXslTrans = new XslCompiledTransform(); 
myXslTrans.Load("stylesheet.xsl"); 
myXslTrans.Transform("source.xml", "result.html"); 

(Sorry for posing this as an answer, but the code block support in comments is rather limited.)

In VB.NET, you don't even need a variable:

With New XslCompiledTransform()
    .Load("stylesheet.xsl")
    .Transform("source.xml", "result.html")
End With

How to read a value from the Windows registry

Since Windows >=Vista/Server 2008, RegGetValue is available, which is a safer function than RegQueryValueEx. No need for RegOpenKeyEx, RegCloseKey or NUL termination checks of string values (REG_SZ, REG_MULTI_SZ, REG_EXPAND_SZ).

#include <iostream>
#include <string>
#include <exception>
#include <windows.h>

/*! \brief                          Returns a value from HKLM as string.
    \exception  std::runtime_error  Replace with your error handling.
*/
std::wstring GetStringValueFromHKLM(const std::wstring& regSubKey, const std::wstring& regValue)
{
    size_t bufferSize = 0xFFF; // If too small, will be resized down below.
    std::wstring valueBuf; // Contiguous buffer since C++11.
    valueBuf.resize(bufferSize);
    auto cbData = static_cast<DWORD>(bufferSize * sizeof(wchar_t));
    auto rc = RegGetValueW(
        HKEY_LOCAL_MACHINE,
        regSubKey.c_str(),
        regValue.c_str(),
        RRF_RT_REG_SZ,
        nullptr,
        static_cast<void*>(valueBuf.data()),
        &cbData
    );
    while (rc == ERROR_MORE_DATA)
    {
        // Get a buffer that is big enough.
        cbData /= sizeof(wchar_t);
        if (cbData > static_cast<DWORD>(bufferSize))
        {
            bufferSize = static_cast<size_t>(cbData);
        }
        else
        {
            bufferSize *= 2;
            cbData = static_cast<DWORD>(bufferSize * sizeof(wchar_t));
        }
        valueBuf.resize(bufferSize);
        rc = RegGetValueW(
            HKEY_LOCAL_MACHINE,
            regSubKey.c_str(),
            regValue.c_str(),
            RRF_RT_REG_SZ,
            nullptr,
            static_cast<void*>(valueBuf.data()),
            &cbData
        );
    }
    if (rc == ERROR_SUCCESS)
    {
        cbData /= sizeof(wchar_t);
        valueBuf.resize(static_cast<size_t>(cbData - 1)); // remove end null character
        return valueBuf;
    }
    else
    {
        throw std::runtime_error("Windows system error code: " + std::to_string(rc));
    }
}

int main()
{
    std::wstring regSubKey;
#ifdef _WIN64 // Manually switching between 32bit/64bit for the example. Use dwFlags instead.
    regSubKey = L"SOFTWARE\\WOW6432Node\\Company Name\\Application Name\\";
#else
    regSubKey = L"SOFTWARE\\Company Name\\Application Name\\";
#endif
    std::wstring regValue(L"MyValue");
    std::wstring valueFromRegistry;
    try
    {
        valueFromRegistry = GetStringValueFromHKLM(regSubKey, regValue);
    }
    catch (std::exception& e)
    {
        std::cerr << e.what();
    }
    std::wcout << valueFromRegistry;
}

Its parameter dwFlags supports flags for type restriction, filling the value buffer with zeros on failure (RRF_ZEROONFAILURE) and 32/64bit registry access (RRF_SUBKEY_WOW6464KEY, RRF_SUBKEY_WOW6432KEY) for 64bit programs.

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

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

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

>>> mfcc = MelFrequencyCepstrum(filepath, params)
>>> data = mfcc.X[:]
>>> sys.getsizeof(mfcc)
64
>>> sys.getsizeof(mfcc.X)
>>>80
>>> sys.getsizeof(data)
80
>>> mfcc
<bregman.features.MelFrequencyCepstrum object at 0x104ad3e90>

What is tail recursion?

It means that rather than needing to push the instruction pointer on the stack, you can simply jump to the top of a recursive function and continue execution. This allows for functions to recurse indefinitely without overflowing the stack.

I wrote a blog post on the subject, which has graphical examples of what the stack frames look like.

Any way to write a Windows .bat file to kill processes?

You can do this with 'taskkill'. With the /IM parameter, you can specify image names.

Example:

taskkill /im somecorporateprocess.exe

You can also do this to 'force' kill:

Example:

taskkill /f /im somecorporateprocess.exe

Just add one line per process you want to kill, save it as a .bat file, and add in your startup directory. Problem solved!

If this is a legacy system, PsKill will do the same.

How does GPS in a mobile phone work exactly?

There's 3 satellites at least that you must be able to receive from of the 24-32 out there, and they each broadcast a time from a synchronized atomic clock. The differences in those times that you receive at any one time tell you how long the broadcast took to reach you, and thus where you are in relation to the satellites. So, it sort of reads from something, but it doesn't connect to that thing. Note that this doesn't tell you your orientation, many GPSes fake that (and speed) by interpolating data points.

If you don't count the cost of the receiver, it's a free service. Apparently there's higher resolution services out there that are restricted to military use. Those are likely a fixed cost for a license to decrypt the signals along with a confidentiality agreement.

Now your device may support GPS tracking, in which case it might communicate, say via GPRS, to a database which will store the location the device has found itself to be at, so that multiple devices may be tracked. That would require some kind of connection.

Maps are either stored on the device or received over a connection. Navigation is computed based on those maps' databases. These likely are a licensed item with a cost associated, though if you use a service like Google Maps they have the license with NAVTEQ and others.

How exactly do you configure httpOnlyCookies in ASP.NET?

If you want to do it in code, use the System.Web.HttpCookie.HttpOnly property.

This is directly from the MSDN docs:

// Create a new HttpCookie.
HttpCookie myHttpCookie = new HttpCookie("LastVisit", DateTime.Now.ToString());
// By default, the HttpOnly property is set to false 
// unless specified otherwise in configuration.
myHttpCookie.Name = "MyHttpCookie";
Response.AppendCookie(myHttpCookie);
// Show the name of the cookie.
Response.Write(myHttpCookie.Name);
// Create an HttpOnly cookie.
HttpCookie myHttpOnlyCookie = new HttpCookie("LastVisit", DateTime.Now.ToString());
// Setting the HttpOnly value to true, makes
// this cookie accessible only to ASP.NET.
myHttpOnlyCookie.HttpOnly = true;
myHttpOnlyCookie.Name = "MyHttpOnlyCookie";
Response.AppendCookie(myHttpOnlyCookie);
// Show the name of the HttpOnly cookie.
Response.Write(myHttpOnlyCookie.Name);

Doing it in code allows you to selectively choose which cookies are HttpOnly and which are not.

How do you configure HttpOnly cookies in tomcat / java webapps?

Please be careful not to overwrite the ";secure" cookie flag in https-sessions. This flag prevents the browser from sending the cookie over an unencrypted http connection, basically rendering the use of https for legit requests pointless.

private void rewriteCookieToHeader(HttpServletRequest request, HttpServletResponse response) {
    if (response.containsHeader("SET-COOKIE")) {
        String sessionid = request.getSession().getId();
        String contextPath = request.getContextPath();
        String secure = "";
        if (request.isSecure()) {
            secure = "; Secure"; 
        }
        response.setHeader("SET-COOKIE", "JSESSIONID=" + sessionid
                         + "; Path=" + contextPath + "; HttpOnly" + secure);
    }
}

How do I load an org.w3c.dom.Document from XML in a string?

Just had a similar problem, except i needed a NodeList and not a Document, here's what I came up with. It's mostly the same solution as before, augmented to get the root element down as a NodeList and using erickson's suggestion of using an InputSource instead for character encoding issues.

private String DOC_ROOT="root";
String xml=getXmlString();
Document xmlDoc=loadXMLFrom(xml);
Element template=xmlDoc.getDocumentElement();
NodeList nodes=xmlDoc.getElementsByTagName(DOC_ROOT);

public static Document loadXMLFrom(String xml) throws Exception {
        InputSource is= new InputSource(new StringReader(xml));
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        factory.setNamespaceAware(true);
        DocumentBuilder builder = null;
        builder = factory.newDocumentBuilder();
        Document doc = builder.parse(is);
        return doc;
    }

Setting the height of a DIV dynamically

inspired by @jason-bunting, same thing for either height or width:

function resizeElementDimension(element, doHeight) {
  dim = (doHeight ? 'Height' : 'Width')
  ref = (doHeight ? 'Top' : 'Left')

  var x = 0;
  var body = window.document.body;
  if(window['inner' + dim])
    x = window['inner' + dim]
  else if (body.parentElement['client' + dim])
    x = body.parentElement['client' + dim]
  else if (body && body['client' + dim])
    x = body['client' + dim]

  element.style[dim.toLowerCase()] = ((x - element['offset' + ref]) + "px");
}

SVN repository backup strategies

Detailed reference in the SVNBook: "Repository data migration using svnadmin"

How do you generate dynamic (parameterized) unit tests in Python?

I'd been having trouble with a very particular style of parameterized tests. All our Selenium tests can run locally, but they also should be able to be run remotely against several platforms on SauceLabs. Basically, I wanted to take a large amount of already-written test cases and parameterize them with the fewest changes to code possible. Furthermore, I needed to be able to pass the parameters into the setUp method, something which I haven't seen any solutions for elsewhere.

Here's what I've come up with:

import inspect
import types

test_platforms = [
    {'browserName': "internet explorer", 'platform': "Windows 7", 'version': "10.0"},
    {'browserName': "internet explorer", 'platform': "Windows 7", 'version': "11.0"},
    {'browserName': "firefox", 'platform': "Linux", 'version': "43.0"},
]


def sauce_labs():
    def wrapper(cls):
        return test_on_platforms(cls)
    return wrapper


def test_on_platforms(base_class):
    for name, function in inspect.getmembers(base_class, inspect.isfunction):
        if name.startswith('test_'):
            for platform in test_platforms:
                new_name = '_'.join(list([name, ''.join(platform['browserName'].title().split()), platform['version']]))
                new_function = types.FunctionType(function.__code__, function.__globals__, new_name,
                                                  function.__defaults__, function.__closure__)
                setattr(new_function, 'platform', platform)
                setattr(base_class, new_name, new_function)
            delattr(base_class, name)

    return base_class

With this, all I had to do was add a simple decorator @sauce_labs() to each regular old TestCase, and now when running them, they're wrapped up and rewritten, so that all the test methods are parameterized and renamed. LoginTests.test_login(self) runs as LoginTests.test_login_internet_explorer_10.0(self), LoginTests.test_login_internet_explorer_11.0(self), and LoginTests.test_login_firefox_43.0(self), and each one has the parameter self.platform to decide what browser/platform to run against, even in LoginTests.setUp, which is crucial for my task since that's where the connection to SauceLabs is initialized.

Anyway, I hope this might be of help to someone looking to do a similar "global" parameterization of their tests!

Is there a constraint that restricts my generic method to numeric types?

There's no constraint for this. It's a real issue for anyone wanting to use generics for numeric calculations.

I'd go further and say we need

static bool GenericFunction<T>(T value) 
    where T : operators( +, -, /, * )

Or even

static bool GenericFunction<T>(T value) 
    where T : Add, Subtract

Unfortunately you only have interfaces, base classes and the keywords struct (must be value-type), class (must be reference type) and new() (must have default constructor)

You could wrap the number in something else (similar to INullable<T>) like here on codeproject.


You could apply the restriction at runtime (by reflecting for the operators or checking for types) but that does lose the advantage of having the generic in the first place.

Creating a LINQ select from multiple tables

If the anonymous type causes trouble for you, you can create a simple data class:

public class PermissionsAndPages
{
     public ObjectPermissions Permissions {get;set}
     public Pages Pages {get;set}
}

and then in your query:

select new PermissionsAndPages { Permissions = op, Page = pg };

Then you can pass this around:

return queryResult.SingleOrDefault(); // as PermissionsAndPages

How can I force clients to refresh JavaScript files?

One simple way. Edit htaccess

RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} \.(jpe?g|bmp|png|gif|css|js|mp3|ogg)$ [NC]
RewriteCond %{QUERY_STRING} !^(.+?&v33|)v=33[^&]*(?:&(.*)|)$ [NC]
RewriteRule ^ %{REQUEST_URI}?v=33 [R=301,L]

How do you run a Python script as a service in Windows?

Yes you can. I do it using the pythoncom libraries that come included with ActivePython or can be installed with pywin32 (Python for Windows extensions).

This is a basic skeleton for a simple service:

import win32serviceutil
import win32service
import win32event
import servicemanager
import socket


class AppServerSvc (win32serviceutil.ServiceFramework):
    _svc_name_ = "TestService"
    _svc_display_name_ = "Test Service"

    def __init__(self,args):
        win32serviceutil.ServiceFramework.__init__(self,args)
        self.hWaitStop = win32event.CreateEvent(None,0,0,None)
        socket.setdefaulttimeout(60)

    def SvcStop(self):
        self.ReportServiceStatus(win32service.SERVICE_STOP_PENDING)
        win32event.SetEvent(self.hWaitStop)

    def SvcDoRun(self):
        servicemanager.LogMsg(servicemanager.EVENTLOG_INFORMATION_TYPE,
                              servicemanager.PYS_SERVICE_STARTED,
                              (self._svc_name_,''))
        self.main()

    def main(self):
        pass

if __name__ == '__main__':
    win32serviceutil.HandleCommandLine(AppServerSvc)

Your code would go in the main() method—usually with some kind of infinite loop that might be interrupted by checking a flag, which you set in the SvcStop method

Disable browser 'Save Password' functionality

I had been struggling with this problem a while, with a unique twist to the problem. Privileged users couldn't have the saved passwords work for them, but normal users needed it. This meant privileged users had to log in twice, the second time enforcing no saved passwords.

With this requirement, the standard autocomplete="off" method doesn't work across all browsers, because the password may have been saved from the first login. A colleague found a solution to replace the password field when it was focused with a new password field, and then focus on the new password field (then hook up the same event handler). This worked (except it caused an infinite loop in IE6). Maybe there was a way around that, but it was causing me a migraine.

Finally, I tried to just have the username and password outside of the form. To my surprise, this worked! It worked on IE6, and current versions of Firefox and Chrome on Linux. I haven't tested it further, but I suspect it works in most if not all browsers (but it wouldn't surprise me if there was a browser out there that didn't care if there was no form).

Here is some sample code, along with some jQuery to get it to work:

<input type="text" id="username" name="username"/>
<input type="password" id="password" name="password"/>

<form id="theForm" action="/your/login" method="post">
  <input type="hidden" id="hiddenUsername" name="username"/>
  <input type="hidden" id="hiddenPassword" name="password"/>
  <input type="submit" value="Login"/>
</form>

<script type="text/javascript" language="JavaScript">
  $("#theForm").submit(function() {
    $("#hiddenUsername").val($("#username").val());
    $("#hiddenPassword").val($("#password").val());
  });
  $("#username,#password").keypress(function(e) {
    if (e.which == 13) {
      $("#theForm").submit();
    }
  });
</script>

Sending email in .NET through Gmail

To avoid security issues in Gmail, you should generate an app password first from your Gmail settings and you can use this password instead of a real password to send an email even if you use two steps verification.

What is the simplest SQL Query to find the second largest value?

Use this query.

SELECT MAX( colname ) 
FROM Tablename 
where colname < (
    SELECT MAX( colname ) 
    FROM Tablename)

UDP vs TCP, how much faster is it?

Each TCP connection requires an initial handshake before data is transmitted. Also, the TCP header contains a lot of overhead intended for different signals and message delivery detection. For a message exchange, UDP will probably suffice if a small chance of failure is acceptable. If receipt must be verified, TCP is your best option.

What is a magic number, and why is it bad?

A Magic Number is a hard-coded value that may change at a later stage, but that can be therefore hard to update.

For example, let's say you have a Page that displays the last 50 Orders in a "Your Orders" Overview Page. 50 is the Magic Number here, because it's not set through standard or convention, it's a number that you made up for reasons outlined in the spec.

Now, what you do is you have the 50 in different places - your SQL script (SELECT TOP 50 * FROM orders), your Website (Your Last 50 Orders), your order login (for (i = 0; i < 50; i++)) and possibly many other places.

Now, what happens when someone decides to change 50 to 25? or 75? or 153? You now have to replace the 50 in all the places, and you are very likely to miss it. Find/Replace may not work, because 50 may be used for other things, and blindly replacing 50 with 25 can have some other bad side effects (i.e. your Session.Timeout = 50 call, which is also set to 25 and users start reporting too frequent timeouts).

Also, the code can be hard to understand, i.e. "if a < 50 then bla" - if you encounter that in the middle of a complicated function, other developers who are not familiar with the code may ask themselves "WTF is 50???"

That's why it's best to have such ambiguous and arbitrary numbers in exactly 1 place - "const int NumOrdersToDisplay = 50", because that makes the code more readable ("if a < NumOrdersToDisplay", it also means you only need to change it in 1 well defined place.

Places where Magic Numbers are appropriate is everything that is defined through a standard, i.e. SmtpClient.DefaultPort = 25 or TCPPacketSize = whatever (not sure if that is standardized). Also, everything only defined within 1 function might be acceptable, but that depends on Context.

How do you remove all the options of a select box and then add one option and select it with jQuery?

How about just changing the html to new data.

$('#mySelect').html('<option value="whatever">text</option>');

Another example:

$('#mySelect').html('
    <option value="1" selected>text1</option>
    <option value="2">text2</option>
    <option value="3" disabled>text3</option>
');

Generator expressions vs. list comprehensions

The benefit of a generator expression is that it uses less memory since it doesn't build the whole list at once. Generator expressions are best used when the list is an intermediary, such as summing the results, or creating a dict out of the results.

For example:

sum(x*2 for x in xrange(256))

dict( (k, some_func(k)) for k in some_list_of_keys )

The advantage there is that the list isn't completely generated, and thus little memory is used (and should also be faster)

You should, though, use list comprehensions when the desired final product is a list. You are not going to save any memeory using generator expressions, since you want the generated list. You also get the benefit of being able to use any of the list functions like sorted or reversed.

For example:

reversed( [x*2 for x in xrange(256)] )

Remove duplicates from a List<T> in C#

If you don't care about the order you can just shove the items into a HashSet, if you do want to maintain the order you can do something like this:

var unique = new List<T>();
var hs = new HashSet<T>();
foreach (T t in list)
    if (hs.Add(t))
        unique.Add(t);

Or the Linq way:

var hs = new HashSet<T>();
list.All( x =>  hs.Add(x) );

Edit: The HashSet method is O(N) time and O(N) space while sorting and then making unique (as suggested by @lassevk and others) is O(N*lgN) time and O(1) space so it's not so clear to me (as it was at first glance) that the sorting way is inferior (my apologies for the temporary down vote...)

String concatenation: concat() vs "+" operator

Tom is correct in describing exactly what the + operator does. It creates a temporary StringBuilder, appends the parts, and finishes with toString().

However, all of the answers so far are ignoring the effects of HotSpot runtime optimizations. Specifically, these temporary operations are recognized as a common pattern and are replaced with more efficient machine code at run-time.

@marcio: You've created a micro-benchmark; with modern JVM's this is not a valid way to profile code.

The reason run-time optimization matters is that many of these differences in code -- even including object-creation -- are completely different once HotSpot gets going. The only way to know for sure is profiling your code in situ.

Finally, all of these methods are in fact incredibly fast. This might be a case of premature optimization. If you have code that concatenates strings a lot, the way to get maximum speed probably has nothing to do with which operators you choose and instead the algorithm you're using!

How do you list all triggers in a MySQL database?

You can use below to find a particular trigger definition.

SHOW TRIGGERS LIKE '%trigger_name%'\G

or the below to show all the triggers in the database. It will work for MySQL 5.0 and above.

SHOW TRIGGERS\G

How do I monitor the computer's CPU, memory, and disk usage in Java?

A lot of this is already available via JMX. With Java 5, JMX is built-in and they include a JMX console viewer with the JDK.

You can use JMX to monitor manually, or invoke JMX commands from Java if you need this information in your own run-time.

Best way in asp.net to force https for an entire site?

I'm going to throw my two cents in. IF you have access to IIS server side, then you can force HTTPS by use of the protocol bindings. For example, you have a website called Blah. In IIS you'd setup two sites: Blah, and Blah (Redirect). For Blah only configure the HTTPS binding (and FTP if you need to, make sure to force it over a secure connection as well). For Blah (Redirect) only configure the HTTP binding. Lastly, in the HTTP Redirect section for Blah (Redirect) make sure to set a 301 redirect to https://blah.com, with exact destination enabled. Make sure that each site in IIS is pointing to it's own root folder otherwise the Web.config will get all screwed up. Also make sure to have HSTS configured on your HTTPSed site so that subsequent requests by the browser are always forced to HTTPS and no redirects occur.