PHP's USORT Callback Function Parameters

Question

This is a really esoteric question, but I\'m genuinely curious. I\'m using usort for the first time today in years, and I\'m particularly interested in what exactly is going on.

Answer 1

usort() or uasort() have a human-feeling bug on sorted result. See the code segment:

function xxx($a,$b) { if ($a==$b) return 0; else return $a<$b?-1:1; }
$x=array(1=>10,2=>9,3=>9,4=>9,5=>6,6=>38);
uasort($x,'xxx');
print_r($x);

the result is:

Array ( [5] => 6 [4] => 9 [3] => 9 [2] => 9 [1] => 10 [6] => 38 )

Do you see the bug? No? Ok, let me explain it. The original three '9' elments are in key order: 2,3,4. But in the result, the three '9' elements are now in key order: 4,3,2, i.e. equal-value elements are in reverse key order after sorting.

If the element is only single value, as in above example,it's fine with us. However, if the element is compound value, then it could cause human-feeling bug. See another code segments. We are to sort many points horizontally, i.e. sort them based on ascending x-coordinate value order :

function xxx($a,$b) { if ($a['x']==$b['x']) return 0; else return $a['x']<$b['x']?-1:1; }
$x=array(1=>array('x'=>1, 'v'=>'l'),2=>array('x'=>9, 'v'=>'love'),
       3=>array('x'=>9,  'v'=>'Lara'),4=>array('x'=>9,  'v'=>'Croft'),
       5=>array('x'=>15,  'v'=>'and'),6=>array('x'=>38,  'v'=>'Tombraider'));
uasort($x,'xxx');
print_r($x);

the result is:

Array ( [1] => Array ( [x] => 1 [v] => l ) [4] => Array ( [x] => 9 [v] => croft ) 
             [3] => Array ( [x] => 9 [v] => Lara ) [2] => Array ( [x] => 9 [v] => love )
             [5] => Array ( [x] => 15 [v] => and ) [6] => Array ( [x] => 38 [v] => Tombraider ) )

You see 'I love Lara Croft and Tombraider' becomes 'I Croft Lara love and Tombraider'.

I call it human-feeling bug because it depends on what case you use and how you feel it should be sorted in real world when the compared values are same.