问题
I'm using usort to sort an array with an associative array within each element.
When all of the values I am sorting on in the array are the same then it still changes the position of the elements in the array, is there a way to prevent this?
For example this:
array(
array('name' => 'Ben', 'authn_weight' => 85.3),
array('name' => 'Josh', 'authn_weight' => 85.3),
array('name' => 'Fred', 'authn_weight' => 85.3)
);
May be changed to this:
array(
array('name' => 'Josh', 'authn_weight' => 85.3),
array('name' => 'Ben', 'authn_weight' => 85.3),
array('name' => 'Fred', 'authn_weight' => 85.3)
);
This is the sort function:
private function weightSortImplementation($a, $b){
$aWeight = $a['autn_weight'];
$bWeight = $b['autn_weight'];
if ($aWeight == $bWeight) {
return 0;
}
return ($aWeight < $bWeight) ? 1 : -1;
}
I have checked that the weightSortImplementation
function is always returning 0 showing that they are the same. So why is this still reordering the array?
回答1:
Aha, a case for the Schwartzian Transform.
It basically consists of three steps:
- decorate; you turn every value into an array with the value as the first element and the key/index as the second
- sort (as per normal)
- undecorate; you reverse step 1
Here it is (I've tweaked it to your particular use case):
function decorate(&$v, $k)
{
$v['authn_weight'] = array($v['authn_weight'], $k);
}
function undecorate(&$v, $k)
{
$v['authn_weight'] = $v['authn_weight'][0];
}
array_walk($a, 'decorate');
usort($a, 'weightSortImplementation');
array_walk($a, 'undecorate');
The trick is in the following assertion:
array($x, 0) < array($x, 1)
This is what keeps the correct order of your array. And, no recursion required :)
回答2:
From the documentation:
If two members compare as equal, their relative order in the sorted array is undefined.
You can use this function [source] that preserves order in the case of two elements being equal:
function mergesort(&$array, $cmp_function = 'strcmp') {
// Arrays of size < 2 require no action.
if (count($array) < 2) return;
// Split the array in half
$halfway = count($array) / 2;
$array1 = array_slice($array, 0, $halfway);
$array2 = array_slice($array, $halfway);
// Recurse to sort the two halves
mergesort($array1, $cmp_function);
mergesort($array2, $cmp_function);
// If all of $array1 is <= all of $array2, just append them.
if (call_user_func($cmp_function, end($array1), $array2[0]) < 1) {
$array = array_merge($array1, $array2);
return;
}
// Merge the two sorted arrays into a single sorted array
$array = array();
$ptr1 = $ptr2 = 0;
while ($ptr1 < count($array1) && $ptr2 < count($array2)) {
if (call_user_func($cmp_function, $array1[$ptr1], $array2[$ptr2]) < 1) {
$array[] = $array1[$ptr1++];
}
else {
$array[] = $array2[$ptr2++];
}
}
// Merge the remainder
while ($ptr1 < count($array1)) $array[] = $array1[$ptr1++];
while ($ptr2 < count($array2)) $array[] = $array2[$ptr2++];
return;
}
来源:https://stackoverflow.com/questions/12163532/php-usort-reorders-array-the-sort-value-is-the-same-for-all