PHP Sorting Array Using usort() Not Working

Question

I have this array :

Array
(
    [0] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 53.83600000
        )

    [1] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 53.83600000
        )

    [2] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 0.09188900
        )

    [3] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 0.09188900
        )

    [4] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 584.80000000
        )

)

and I'm expecting result like this (sort by robo DESC) :

Array
(
    [0] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 53.83600000
        )

    [1] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => RE
            [price] => 0.09188900
        )

    [2] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 53.83600000
        )

    [3] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 0.09188900
        )

    [4] => Array
        (
            [timestamp] => lm81-1527799632244
            [robo] => C4
            [price] => 584.80000000
        )

)

and I already done this :

    usort($dc_array_process, function($a, $b) {
        return $a['robo'] - $b['robo'];
    });

but my array still not in DESC order. any idea what did I do wrong?

To sort descending, use `$b <=> $a` instead of `$a <=> $b`. https://stackoverflow.com/a/54647220/2943403 — mickmackusa, Apr 14 '22 at 22:07

score 2 · Accepted Answer · answered Jun 03 '18 at 14:06

If you are sorting string values, you should use strcmp

usort($dc_array_process, function($a, $b) {
    return strcmp($a['robo'], $b['robo']);
});

or

usort($dc_array_process, function($a, $b) {
    return -strcmp($a['robo'], $b['robo']);  //negative to reverse
});

document:

int strcmp ( string $str1 , string $str2 )

Returns < 0 if str1 is less than str2; > 0 if str1 is greater than str2, and 0 if they are equal.

score 1 · Answer 2 · answered Jun 03 '18 at 14:06

1

Because arithmetic - causes values to be converted to int.

usort($dc_array_process, function($a, $b) {
    return strcmp($a['robo'], $b['robo']);
});

answered Jun 03 '18 at 14:06

u_mulder

54,101
5
48
64

score 0 · Answer 3 · answered Jun 03 '18 at 14:08

As per the manual, the comparison function passed into usort must…

return an integer less than, equal to, or greater than zero

…for the order to be correctly determined. As the values you're attempting to sort are strings, the minus operation you're using therefore won't work.

Try using strcmp in the return…

return strcmp( $a['robo'], $b['robo'] );

PHP Sorting Array Using usort() Not Working

3 Answers3