PHP. How does user defined compare function work?

Question

Example:

<?php
function cmp($a, $b)
{
    if ($a == $b) {
        return 0;
    }
    return ($a < $b) ? -1 : 1;
}

$a = array(3, 2, 5, 6, 1);

usort($a, "cmp");

foreach ($a as $key => $value) {
    echo "$key: $value\n";
}
?>

Result

0: 1
1: 2
2: 3
3: 5
4: 6

in the user defined compare function 'cmd', it has two arguments, $a & $b.

What do the first and second argument represent? Are they array[x] and array[x+1] ?

Oliver Charlesworth · Accepted Answer · 2012-06-09 11:28:37Z

3

They are just two of the elements from the array being sorted, not necessarily neighbours. usort performs some sort algorithm (see http://en.wikipedia.org/wiki/Sort_algorithm#Comparison_of_algorithms), which involves comparing various pairs of elements to determine their order. It uses your callback function to determine this.

answered Jun 9, 2012 at 11:28

Oliver Charlesworth

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6 Comments

Mark Baker Over a year ago

As a general note: PHP uses a quicksort algorithm

gilzero Over a year ago

don't get it. "They are just two of the elements from the array being sorted". Say in this case, $a is which element, $b is which?

Oliver Charlesworth Over a year ago

@MarkBaker: I assumed it did, but couldn't prove it. Is that documented anywhere, or is that gleaned from looking at the source?

Oliver Charlesworth Over a year ago

@gilzero: It could be any pair, in general. Take a look at how e.g. the quicksort algorithm or the bubblesort algorithm work; they all involve performing comparisons on arbitrary pairs of elements.

NikiC Over a year ago

@MarkBaker @Oli PHP has a generic hashtable sorting function zend_hash_sort (lxr.php.net/xref/PHP_TRUNK/Zend/zend_hash.c#1421), but in practice it is always provided with a zend_qsort (lxr.php.net/xref/PHP_TRUNK/Zend/zend_qsort.c#56) sorting function (e.g. lxr.php.net/xref/PHP_TRUNK/ext/standard/array.c#536). So yeah, PHP uses quicksort :)

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