usort changing Array's order
I have a usort function with a single line: return 0.
I tried to use it on an Array 开发者_开发知识库of stdClass objects, and it changes their order, how is that possible?The property you assume is called stability: A stable sorting algorithm will not change the order of elements that are equal.
php's sorting functions are not stable (because non-stable sorts may be slightly faster). From the documentation of usort
:
If two members compare as equal, their order in the sorted array is undefined.
If you want a stable sorting algorithm, you must implement it yourself.
It's because that function means "I really don't care how they are sorted, they are equal to me". With this simple sample I receive reversed array:
function sortaaa($a,$b) {return 0;}
$array = array(1,2,3,4,5);
usort($array,"sortaaa");
var_dump($array);
//prints array(5) { [0]=> int(5) [1]=> int(4) [2]=> int(3) [3]=> int(2) [4]=> int(1) }
So it looks like PHP loops the array in reverse order in function usort
. So, note the usort
manual states that
If two members compare as equal, their order in the sorted array is undefined.
If you're looking for a quick solution for a stable usort
, you could use uksort
like in the following example:
<?php
uksort($array, function ($ak, $bk) use ($array) {
$a = $array[$ak];
$b = $array[$bk];
if ($a['foo'] === $b['foo'])
return $ak - $bk;
return $a['foo'] > $b['foo'] ? 1 : -1;
});
This works as expected only if the initial indices (keys) of $array
are in ascending order.
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