Skip to main content

PHP"s count(), O(1) or O(n) for arrays?


do you know if count() in PHP really counts the all elements of a PHP-array, or if this value is cached somewhere and just needs to be retrieved?



The docs don't say much about this and various blog posts that measure the performance of count() don't talk about it either.



(Sorry for the title didn't know how to describe it more precisely.)


Source: Tips4allCCNA FINAL EXAM

Comments

  1. Well, we can look at the source:

    /ext/standard/array.c


    PHP_FUNCTION(count) calls php_count_recursive(), which in turn calls zend_hash_num_elements() for non-recursive array, which is implemented this way:

    ZEND_API int zend_hash_num_elements(const HashTable *ht)
    {
    IS_CONSISTENT(ht);

    return ht->nNumOfElements;
    }


    So you can see, it's O(1) for $mode = COUNT_NORMAL.

    ReplyDelete
  2. In PHP 5+ the length is stored in the array so the counting is not done each time.

    EDIT: You also might find this analysis interesting: PHP Count Performance. Although the length of the array is maintained by the array, it still seems as though it is faster to hold on to it if you are going to call count() many times.

    ReplyDelete
  3. PHP stores the size of an array internally, but you're still making a function call when which is slower than not making one, so you'll want to store the result in a variable if you're doing something like using it in a loop:

    For example,

    $cnt = count($array);
    for ($i =0; $i < $cnt; $i++) {
    foo($array[$i]);
    }


    Additionally, you can't always be sure count is being called on an array. If it's called on an object that implements Countable for example, the count method of that object will be called.

    ReplyDelete

Post a Comment

Popular posts from this blog

Why is this Javascript much *slower* than its jQuery equivalent?

I have a HTML list of about 500 items and a "filter" box above it. I started by using jQuery to filter the list when I typed a letter (timing code added later): $('#filter').keyup( function() { var jqStart = (new Date).getTime(); var search = $(this).val().toLowerCase(); var $list = $('ul.ablist > li'); $list.each( function() { if ( $(this).text().toLowerCase().indexOf(search) === -1 ) $(this).hide(); else $(this).show(); } ); console.log('Time: ' + ((new Date).getTime() - jqStart)); } ); However, there was a couple of seconds delay after typing each letter (particularly the first letter). So I thought it may be slightly quicker if I used plain Javascript (I read recently that jQuery's each function is particularly slow). Here's my JS equivalent: document.getElementById('filter').addEventListener( 'keyup', function () { var jsStart = (new Date).getTime()...

Is it possible to have IF statement in an Echo statement in PHP

Thanks in advance. I did look at the other questions/answers that were similar and didn't find exactly what I was looking for. I'm trying to do this, am I on the right path? echo " <div id='tabs-".$match."'> <textarea id='".$match."' name='".$match."'>". if ($COLUMN_NAME === $match) { echo $FIELD_WITH_COLUMN_NAME; } else { } ."</textarea> <script type='text/javascript'> CKEDITOR.replace( '".$match."' ); </script> </div>"; I am getting the following error message in the browser: Parse error: syntax error, unexpected T_IF Please let me know if this is the right way to go about nesting an IF statement inside an echo. Thank you.