How to use usort() in PHP

This article mainly introduces how to use usort() in PHP. Interested friends can refer to it. I hope it will be helpful to everyone.

The function performs its own custom sorting of the array, and the sorting rules are defined by $cmp_function. The return value is true or false.

Now let’s analyze a simple function:

<?php
function re($a,$b){
           return ($a<$b)?1:-1;
          }5 $x=array(1,3,2,5,9);
      usort($x, &#39;re&#39;)    
      print_r($x);
?>

The print result is :

Array(
    [0] => 9
    [1] => 5
    [2] => 3
    [3] => 2
    [4] => 1<br>)

Implements the reverse order of the array. The analysis is as follows:

usort extracts the values ​​​​in the array in pairs and inputs them into the custom function in order. The custom function returns 1 or -1 according to the content; usort returns 1 or -1 according to the return value , get the incoming value 1 to be "greater than" or "less than" the value 2, and then sort the values ​​from small to large. That is: The return value is 1, indicating that value 1 is "greater" than value 2, and then sorting: value 2 -> value 1; the return value is -1, indicating that value 1 is "less than" value 2, and then sorting : Value 1-> Value 2.

In the above custom function, if $a$a; if it returns -1 in error, Description: $a is "less than" $b, then sort in the order $a->$b.

The following is a more complex sorting: first odd and then even, and then sort from large to small.

function Compare($str1, $str2) { 
    if (($str1 % 2 == 0) && ($str2 %2 == 0)) { 
        if ($str1 > $str2) 
            return - 1; 
        else 
            return 1; 
    } 
    if ($str1 % 2 == 0) 
        return 1;
    if ($str2 % 2 == 0)
        return -1;
    return ($str2 > $str1) ? 1 : - 1;
}
$scores = array (22,57,55,12,87,56,54,11);
usort ( $scores, 'Compare' );
print_r ( $scores );

The implementation steps are:

1》Determine whether the two input values ​​​​are both even numbers, both are even numbers, and sort from large to small;

2》If If not all are even numbers, then at least one is an odd number. First determine whether $str1 is an even number. If it is an even number, that is: if ($str1%2==0) is established, then return 1, which means $str1 is "greater than" $str2 , then the usort function sorts into "small" $str2-> "large" $str1 (even number);

3》If $str1 is an odd number, the above does not return any value, and then determines $str2 Whether it is an even number, if it is an even number, it returns -1, which means $str1 is "less than" $str2, and the usort function sorts it into "small" $str1 (odd number) -> "large" $str2 (even number) ;

4》If both values ​​​​are odd numbers, no value will be returned above, and then $str1 and $str2 will be sorted from large to small;

The output result is:

Array(
    [0] => 87
    [1] => 57
    [2] => 55
    [3] => 11
    [4] => 56
    [5] => 54
    [6] => 22
    [7] => 12
)

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