Solution to the problem of usort changing the original position when the values are the same in PHP

Solution to the problem of usort changing the original position when the values are the same in PHP_PHP tutorial

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Release： 2016-07-21 15:22:18

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From PHP 4.1.0 onwards, the original position of usort may change when the compared values are the same. The documentation says this:
If two members compare as equal, their order in the sorted array is undefined.
That is, if the 2 values being compared are the same, their order in the sorted result is random. If you need to keep the original position of the same value, you can refer to the method in this article.
Demo data:

Copy code The code is as follows:

 
/* 
 Solve the problem of usort changing the original position when the values ​​​​are the same in PHP 
Author: Artlover http://www.CodeBit.cn 
*/ 
$arr = array( 
array('a' = > 5, 'b' => 3), 
array('a' => 5, 'b' => 1), 
array('a' => 5, 'b ' => 4), 
array('a' => 5, 'b' => 2), 
); 
?> 

array The value of the first element in is the same. The expected result is to keep the existing position unchanged, that is, the order of b is 3, 1, 4, 2
Use usort to sort, when the values of the compared fields are the same, The original order may change

Copy code The code is as follows:

 
/* 
Solution The problem of usort changing the original position when the values ​​​​are the same in PHP 
Author: Artlover http://www.CodeBit.cn 
*/ 
$callback = create_function('$a,$b', 'return ($a["a"] == $b["a"])?0:(($a["a"] > $b["a"]) ? 1 : -1);'); 
usort($arr, $callback); 
?> 

Result:

Copy code Code As follows:

 
Array 
( 
[0] => Array 
( 
[a] => 5 
[b] => 2 
) 
[1] => Array 
( 
[a] => 5 
[b] => 4 
) 
[2] => ; Array 
( 
[a] => 5 
[b] => 1 
) 
[3] => Array 
( 
[a] => 5 
[b] => 3 
) 
) 

Although the values of the sort fields are the same, usort disrupts the order of the entire array.
If you want to keep the original position when the compared values are the same, you can use array_multisort:

Copy the code The code is as follows:

 
/* 
Solve the problem of usort changing the original position when the values ​​​​are the same in PHP 
Author: Artlover http://www.CodeBit.cn 
*/ 
/ / Index counter
$i = 0; 
// Create 2 empty arrays, the first one holds the field to be sorted, and the second one holds the original index information
$a = $index = array() ; 
foreach ($arr as $key => $data) { 
$a[$key] = $data['a']; 
$index[] = $i++; 
 } 
// Sort the first array first, then sort by the original index 
array_multisort($a, SORT_ASC, $index, SORT_ASC, $arr); 
?> 

Result:

Copy code The code is as follows:

 
Array 
( 
[0] => Array 
( 
[a] => 5 
[b] => 3 
) 
[1] => Array 
( 
[a] = > 5 
[b] => 1 
) 
[2] => Array 
( 
[a] => 5 
[b] => 4 
) 
[3] => Array 
( 
[a] => 5 
[b] => 2 
) 
) 
 