The `.hasClass()` check is not useful because jQuery [will also always check][1] from within `.addClass()`. It&#39;s just extra work.


  [1]: https://github.com/jquery/jquery/blob/master/src/attributes/classes.js#L43-L45

You can see at the source code : https://github.com/jquery/jquery/blob/master/src/attributes/classes.js#L38-L45 that they do check if the class exists when using `addClass`. 

So there is no reason to use the `.hasClass()` in this case.. (*an exception would be if you wanted to perform more actions if the element did not have the class..*)

For what it&#39;s worth, there is a performance improvement with `.hasClass()` in my limited testing: http://jsperf.com/jquery-hasclass-vs-addclass-and-removeclass

However, even when standalone `.removeClass()` reports several times slower in Chrome, it comes in at approximately 70,000 operations a second.

I was suggested, not long ago, to change my code to use PDO in order to parameterize my queries and safely save HTML in the database.

Well, here are the main problems:

 1. I looked at &lt;http://php.net/manual/en/ref.pdo-mysql.php&gt;, and I don&#39;t really get where I should put that `$ ./configure --with-pdo-mysql` string...

 2. The site I&#39;m building actually only requires PDO for one page. While I may consider re-writing it, it would take a while and I need the pages to be running soon, so I can&#39;t turn off MySQL completely. If I do install PDO, will I still be able to use `mysql_*` handlers?

The server in question is running PHP Version 5.4.6-1ubuntu1 and Apache/2.2.22 (Ubuntu). I&#39;m also running a phpMyAdmin database, if it matters.



Installing PDO driver on MySQL Linux server

How can 2 unsorted text files of different lengths be display side by side *(in columns)* in a `shell` 

Given `one.txt` and `two.txt`:

    $ cat one.txt
    apple
    pear
    longer line than the last two
    last line
                                      
    $ cat two.txt
    The quick brown fox..
    foo
    bar 
    linux
    
    skipped a line

Display:

    apple				      	        The quick brown fox..
    pear				                foo
    longer line than the last two	    bar 
    last line			      	        linux
    				     
                                        skipped a line

`paste one.txt two.txt` almost does the trick but doesn&#39;t align the columns nicely as it just prints one tab between column 1 and 2. *I know how to this with emacs and vim but want the output displayed to stdout for piping ect.*

The solution I came up with uses `sdiff` and then pipes to sed to remove the output `sdiff` adds.  
 
`sdiff one.txt two.txt | sed -r &#39;s/[&lt;&gt;|]//;s/(\t){3}//&#39;`

I could create a function and stick it in my `.bashrc` but surely a command for this exists already (or a *cleaner* solution potentially)?

Display two files side by side

I have come across the following script which checks whether an element has class `a`, and if not, adds it:

    if (!$(&quot;div&quot;).hasClass(&quot;a&quot;)){
       $(&quot;div&quot;).addClass(&quot;a&quot;);
    }

As jQuery won&#39;t add the class if it already exists, this could be changed to:

    $(&quot;div&quot;).addClass(&quot;a&quot;);

However, is there any performance improvements by using `hasClass` first, or is this using the same method which `addClass` does anyway, and therefore duplicating the logic?

Should I use &quot;hasClass&quot; before &quot;addClass&quot;?

I have come across the following script which checks whether an element has class <code>a</code>, and if not, adds it:
<pre><code class="hljs language-javascript">if (!$("div").hasClass("a")){
 $("div").addClass("a");
}
</code></pre>
As jQuery won't add the class if it already exists, this could be changed to:
<pre><code class="hljs language-javascript">$("div").addClass("a");
</code></pre>
However, is there any performance improvements by using <code>hasClass</code> first, or is this using the same method which <code>addClass</code> does anyway, and therefore duplicating the logic?

I am using `getJSON` to fetch the results from server side but facing browser cache problems. I want the cache to be false. I tried using this just before my `getJSON` call.


     $.ajaxSetup({
                    cache: false
                })


But I am not getting the expected results. It still shows the old results.

I also identified some other solutions such as using `.ajax` but I really don&#39;t want to use that.

How to set cache false for getJSON in jQuery?

&gt; **Possible Duplicate:**  
&gt; [jQuery, Select by attribute value, adding new attribute](https://stackoverflow.com/questions/3724245/jquery-select-by-attribute-value-adding-new-attribute)  
&gt; [jQuery - How to select by attribute](https://stackoverflow.com/questions/9210073/jquery-how-to-select-by-attribute)  

&lt;!-- End of automatically inserted text --&gt;

please consider this code:

    &lt;p&gt;11111111111111&lt;/p&gt;
    &lt;p MyTag=&quot;nima&quot;&gt;2222222222&lt;/p&gt;
    &lt;p&gt;33333333333&lt;/p&gt;
    &lt;p MyTag=&quot;Sara&quot;&gt;&gt;4444444444&lt;/p&gt;

how I can select All `p` tag with attribute `MyTag`?

thanks

jQuery : select all element with custom attribute

**JS:** 

    $(function(){
      $(&quot;#example&quot;).popover({
        placement: &#39;bottom&#39;,
        html: &#39;true&#39;,
        title : &#39;&lt;span class=&quot;text-info&quot;&gt;&lt;strong&gt;title&lt;/strong&gt;&lt;/span&gt; &lt;button type=&quot;button&quot; id=&quot;close&quot; class=&quot;close&quot;&gt;&amp;times;&lt;/button&gt;&#39;,
        content : &#39;test&#39;
      })
      $(&#39;html&#39;).click(function() {
        $(&#39;#close&#39;).popover(&#39;hide&#39;);
      });
    });

**HTML:**

    &lt;button type=&quot;button&quot; id=&quot;example&quot; class=&quot;btn btn-primary&quot; &gt;&lt;/button&gt;

i&#39;m write your code isn&#39;t show your popup.

anyone come across this problem?

How to insert close button in popover for Bootstrap

The problem is that I have some dynamically created sets of input tags and I also have a function that is meant to trigger any time an input value is changed. 

    $(&#39;input&#39;).on(&#39;change&#39;, function() {
      // Does some stuff and logs the event to the console
    });

However the `.on(&#39;change&#39;)` is not triggering for any dynamically created inputs, only for items that were present when the page was loaded. Unfortunately this leaves me in a bit of a bind as `.on` is meant to be the replacement for `.live()` and `.delegate()` all of which are wrappers for `.bind()` :/

Has anyone else had this problem or know of a solution?

jQuery .on(&#39;change&#39;, function() {} not triggering for dynamically created inputs

I am using the bootstrap Dropdown component in my application like this:

    &lt;div class=&quot;btn-group&quot;&gt;
        &lt;button class=&quot;btn&quot;&gt;Please Select From List&lt;/button&gt;
        &lt;button class=&quot;btn dropdown-toggle&quot; data-toggle=&quot;dropdown&quot;&gt;
            &lt;span class=&quot;caret&quot;&gt;&lt;/span&gt;
        &lt;/button&gt;
        &lt;ul class=&quot;dropdown-menu&quot; role=&quot;menu&quot; aria-labelledby=&quot;dropdownMenu&quot;&gt;
           &lt;li&gt;&lt;a tabindex=&quot;-1&quot; href=&quot;#&quot;&gt;Item I&lt;/a&gt;&lt;/li&gt;
           &lt;li&gt;&lt;a tabindex=&quot;-1&quot; href=&quot;#&quot;&gt;Item II&lt;/a&gt;&lt;/li&gt;
           &lt;li&gt;&lt;a tabindex=&quot;-1&quot; href=&quot;#&quot;&gt;Item III&lt;/a&gt;&lt;/li&gt;
           &lt;li class=&quot;divider&quot;&gt;&lt;/li&gt;
           &lt;li&gt;&lt;a tabindex=&quot;-1&quot; href=&quot;#&quot;&gt;Other&lt;/a&gt;&lt;/li&gt;
        &lt;/ul&gt;
    &lt;/div&gt;

I would like to display the selected item as the btn label. In other words, replace &quot;Please Select From List&quot; with the list item that has been selected(&quot;Item I&quot;, &quot;Item II&quot;, &quot;Item III&quot;).



How to Display Selected Item in Bootstrap Button Dropdown Title

**Background**

My question is motivated by simple observations, which somewhat undermine the beliefs/assumptions often held/made by experienced MATLAB users:

 - MATLAB is very well optimized when it comes to the built-in functions and the fundamental language features, such as indexing vectors and matrices.
 - Loops in MATLAB are slow (despite the JIT) and should generally be avoided if the algorithm can be expressed in a native, &#39;vectorized&#39; manner. 

The bottom line: core MATLAB functionality is efficient and trying to outperform it using MATLAB code is hard, if not impossible.


**Investigating performance of vector indexing**

The example codes shown below are as fundamental as it gets: I assign a scalar value to all vector entries. First, I allocate an empty vector `x`:

    tic; x = zeros(1e8,1); toc
    Elapsed time is 0.260525 seconds.

Having `x` I would like to set all its entries to the same value. In practice you would do it differently, e.g., `x = value*ones(1e8,1)`, but the point here is to investigate the performance of vector indexing. The simplest way is to write:

    tic; x(:) = 1; toc
    Elapsed time is 0.094316 seconds.

Let&#39;s call it method 1 (from the value assigned to `x`). It seems to be very fast (faster at least than memory allocation). Because the only thing I do here is operate on memory, I can estimate the efficiency of this code by calculating the obtained *effective memory bandwidth* and comparing it to the *hardware memory bandwidth* of my computer:

    eff_bandwidth = numel(x) * 8 bytes per double * 2 / time

In the above, I multiply by `2` because unless SSE streaming is used, setting values in memory requires that the vector is both read from and written to the memory. In the above example:

    eff_bandwidth(1) = 1e8*8*2/0.094316 = 17 Gb/s

[STREAM-benchmarked memory bandwidth](https://www.cs.virginia.edu/stream/) of my computer is around 17.9 Gb/s, so indeed - MATLAB delivers close to peak performance in this case! So far, so good.

Method 1 is suitable if you want to set *all* vector elements to some value. But if you want to access elements every `step` entries, you need to substitute the `:` with e.g., `1:step:end`. Below is a direct speed comparison with method 1:

    tic; x(1:end) = 2; toc
    Elapsed time is 0.496476 seconds.

While you would not expect it to perform any different, method 2 is clearly big trouble: factor 5 slowdown for no reason. My suspicion is that in this case MATLAB explicitly allocates the index vector (`1:end`). This is somewhat confirmed by using explicit vector size instead of `end`:

    tic; x(1:1e8) = 3; toc
    Elapsed time is 0.482083 seconds.

Methods 2 and 3 perform equally bad.

Another possibility is to explicitly create an index vector `id` and use it to index `x`. This gives you the most flexible indexing capabilities. In our case:

    tic;
    id = 1:1e8; % colon(1,1e8);
    x(id) = 4;
    toc
    Elapsed time is 1.208419 seconds.

Now that is really something - 12 times slowdown compared to method 1! I understand it should perform worse than method 1 because of the additional memory used for `id`, but why is it so much worse than methods 2 and 3?

Let&#39;s try to give the loops a try - as hopeless as it may sound.

    tic;
    for i=1:numel(x)
        x(i) = 5;
    end
    toc
    Elapsed time is 0.788944 seconds.

A big surprise - a loop beats a `vectorized` method 4, but is still slower than methods 1, 2 and 3. It turns out that in this particular case you can do it better:

    tic;
    for i=1:1e8
        x(i) = 6;
    end
    toc
    Elapsed time is 0.321246 seconds.

And that is the probably the most bizarre outcome of this study - **a MATLAB-written loop significantly outperforms native vector indexing**. That should certainly not be so. Note that the JIT&#39;ed loop is still 3 times slower than the theoretical peak almost obtained by method 1. So there is still plenty of room for improvement. It is just surprising (a stronger word would be more suitable) that usual &#39;vectorized&#39; indexing (`1:end`) is even slower.

**Questions**

 - is simple indexing in MATLAB **very** inefficient (methods 2, 3, and 4 are slower than method 1), or did I miss something?
 - why is method 4 **(so much) slower** than methods 2 and 3?
 - why does using `1e8` instead of `numel(x)` as a loop bound speed up the code by factor 2?

**Edit**
After reading Jonas&#39;s comment, here is another way to do that using logical indices:

    tic;
    id = logical(ones(1, 1e8));
    x(id) = 7;
    toc
    Elapsed time is 0.613363 seconds.

Much better than method 4.

For convenience:

    function test

    tic; x = zeros(1,1e8); toc

    tic; x(:) = 1; toc
    tic; x(1:end) = 2; toc
    tic; x(1:1e8) = 3; toc

    tic;
    id = 1:1e8; % colon(1,1e8);
    x(id) = 4;
    toc

    tic;
    for i=1:numel(x)
        x(i) = 5;
    end
    toc

    tic;
    for i=1:1e8
        x(i) = 6;
    end
    toc

    end

Is indexing vectors in MATLAB inefficient?

I aim to start opencv little by little but first I need to decide which API of OpenCV is more useful. I predict that Python implementation is shorter but running time will be more dense and slow compared to the native C++ implementations. Is there any know can comment about performance and coding differences between these two perspectives?

Does performance differ between Python or C++ coding of OpenCV?

This question is merely for me as I always like to write optimized code that can run also on cheap slow servers (or servers with A LOT of traffic)

I looked around and I was not able to find an answer.  I was wondering what is faster between those two examples keeping in mind that the array&#39;s keys in my case are not important (pseudo-code naturally):

    &lt;?php
    $a = array();
    while($new_val = &#39;get over 100k email addresses already lowercased&#39;){
        if(!in_array($new_val, $a){
            $a[] = $new_val;
            //do other stuff
        }
    }
    ?&gt;
    
    &lt;?php
    $a = array();
    while($new_val = &#39;get over 100k email addresses already lowercased&#39;){
        if(!isset($a[$new_val]){
            $a[$new_val] = true;
            //do other stuff
        }
    }
    ?&gt;

As the point of the question is not the array collision, I would like to add that if you are afraid of colliding inserts for `$a[$new_value]`, you can use `$a[md5($new_value)]`.  it can still cause collisions, but would take away from a possible DoS attack when reading from an user provided file (http://nikic.github.com/2011/12/28/Supercolliding-a-PHP-array.html)

what is faster: in_array or isset?

Here is how I check whether `mystring` begins with some string:

    &gt;&gt;&gt; mystring.lower().startswith(&quot;he&quot;)
    True

The problem is that `mystring` is very long (thousands of characters), so the `lower()` operation takes a lot of time.

**QUESTION:** Is there a more efficient way?

My unsuccessful attempt:

    &gt;&gt;&gt; import re;
    &gt;&gt;&gt; mystring.startswith(&quot;he&quot;, re.I)
    False


Case-insensitive string startswith in Python

I have a data.table with about 3 million rows and 40 columns. I would like to sort this table by descending order within groups like the following sql mock code:

    sort by ascending Year, ascending MemberID, descending Month 

Is there an equivalent way in data.table to do this? So far I have to break it down into 2 steps:

    setkey(X, Year, MemberID)

This is very fast and takes only a few second.

    X &lt;- X[,.SD[order(-Month)],by=list(Year, MemberID)]

This step takes so much longer (5 minutes).

Update:
Someone made a comment to do `X &lt;- X[sort(Year, MemberID, -Month)]` and later deleted. This approach seems to be much faster:

    user  system elapsed 
    5.560  11.242  66.236 

My approach: setkey() then order(-Month)

       user  system elapsed 
    816.144   9.648 848.798 

My question is now: if I want to summarize by Year, MemberId and Month after sort(Year, MemberID, Month), does data.table recognize the sort order?

Update 2: to response to Matthew Dowle:

After setkey with Year, MemberID and Month, I still have multiple records per group. What I would like is to summarize for each of the groups. What I meant was: if I use X[order(Year, MemberID, Month)], does the summation utilizes binary search functionality of data.table:

    monthly.X &lt;- X[, lapply(.SD[], sum), by = list(Year, MemberID, Month)]

Update 3: Matthew D proposed several approaches. Run time for the first approach is faster than order() approach:

       user  system elapsed 
      7.910   7.750  53.916 

Matthew: what surprised me was converting the sign of Month takes most of the time. Without it, setkey is blazing fast.



Content Type	Original Author	Original Content on Stackoverflow
Question	Curtis	View Question on Stackoverflow
Solution 1 - Jquery	Jon	View Answer on Stackoverflow
Solution 2 - Jquery	Gabriele Petrioli	View Answer on Stackoverflow
Solution 3 - Jquery	chrisM	View Answer on Stackoverflow

Should I use "hasClass" before "addClass"?

Jquery Problem Overview

Jquery Solutions

Solution 1 - Jquery

Solution 2 - Jquery

Solution 3 - Jquery

Display two files side by side

Installing PDO driver on MySQL Linux server

Attributions