An interpretation of the Usort () function in PHP

Recently learned to encounter a custom array sort function usort () Some do not understand, search a lot of places are not very good explanation, their research for a long time, hair and I like the same beginners share ~

BOOL usort ( array &$array , callable $cmp_function )

The function makes its own custom sort for the array, and the collation consists $cmp_function 定义。 of a return value of ture or false.

Now let's analyze a simple function:

1<?PHP2 functionRe$a,$b){3     return($a<$b)? 1:-1;4 }5 $x=Array(1,3,2,5,9);6 Usort($x, ' re ');7 Print_r($x);8?>

Printing results are:

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

The array is implemented in reverse order. The analysis is as follows:

Usort 22 extracts the values in the array and enters the custom function sequentially, the custom function returns 1 according to the content, or -1;usort according to the return value of 1 or-1, gets the value passed in 1 "greater than" or "less than" the value 2, and then the value is ordered from small to large. that is: The return value is 1, the value 1 "is greater than" the value 2, and then sort: The value 2-> the value 1; The return value is-1, the value 1 "is less than" the value 2, and then sorts: the value 1-> the value 2.

In the above custom function, $a < $b If 1 is returned correctly, the $ A "greater than" $b is sorted by the order $b-> $a, and if the error returns-1, the description of $ A "less than" $b is sorted by the order $a-> $b.

The following is a more complex sort: An array is preceded by odd, then the order is made from large to small.

1 functionCompare ($str 1,$str 2) {2     if(($str 1% 2 = = 0) && ($str 2%2 = = 0)) {3         if($str 1>$str 2)4             return-1;5         Else6             return1;7     }8     if($str 1% 2 = = 0)9         return1;Ten     if($str 2% 2 = = 0) One         return-1; A     return($str 2>$str 1) ? 1:-1; - } - $scores=Array(22,57,55,12,87,56,54,11); the Usort($scores, ' Compare ' ); - Print_r($scores);

The implementation steps are:

1 "Determine whether the input two values are even, are even, from large to small sorting;

2 If not all are even, then at least one is odd, first determine whether $STR1 is an even number, if an even number, namely: if ($str 1%2==0) is established, then return 1, meaning $str1 "greater than" $str 2, the Usort function is ordered as "small" $str 2-> " Large "$str 1 (even);

3 "If the $str1 is odd, the above does not return any value, and then determine whether $STR2 is even, if it is even, then return-1, meaning $str1" less than "$str 2, the Usort function is ordered to" small "$str 1 (odd)," big "$ STR2 (even);

4 "If two values are odd, then no value is returned above, then the $STR1 and $str2 are sorted from large to small;

The output is:

Array (    [0] [+] [    1] = [2] = [    3    ] = [4] [5] = [6] [+]/[+]/+]    [7] = 12)

Above, the whole function finished, smooth to the ideal result.

Although a little pediatrics for the great God, but run through this function, still full of excitement!

An interpretation of the Usort () function in PHP