Sort multi-dimensional array by an element [duplicate]

Question

I would like to sort this array by a value that is in a sub table. I tried with the method Usort (), but impossible to sort.

    foreach ($res as $r) {
        usort($r, function($a1, $a2) {
            $v1 = $a1['WeekNumDay'];
            $v2 = $a2['WeekNumDay'];
            return $v1 - $v2; // $v2 - $v1 to reverse direction
        });
    }

In my array I would like to sort by key ['WeekNumDay'].

Array
(
    [R422] => Array
        (

            [0] => Array
                (
                    [WeekNumDay] => 19321
                    [Day1DeliveryTime] => 52932000
                )

            [1] => Array
                (
                    [WeekNumDay] => 18331
                    [Day1DeliveryTime] => 42770000
                )

            [2] => Array
                (
                    [WeekNumDay] => 19305
                    [Day1DeliveryTime] => null
                )
        )

)

Can you help me ?

it seems to work. but you don't save results, use reference in foreach foreach ($res as & $r) { — splash58
– splash58, Commented Aug 7, 2019 at 13:43

Mark · Accepted Answer · 2019-08-07 14:05:16Z

1

Should work if you just change these lines:

foreach ($res as $r) {
    usort($r, function($a1, $a2) {
        // ...
    });
}

to the following:

foreach ($res as $k => $r) {

    usort($r, function($a1, $a2) {
        // ...
    });

    $res[$k] = $r; // Edit the original array.

}

The reason it should work is because usort accepts the array as a reference, once you have sorted $r into the correct order you will then need to alter the original array, hence:

$res[$k] = $r;

edited Aug 7, 2019 at 14:05

answered Aug 7, 2019 at 13:47

Mark

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1 Comment

Oussama Ben Ghorbel Over a year ago

"$res[$k] and $r essentially are the same thing, but one is the array and one is an element of the array." Heuh? No. By the way usort first argument is passed by reference.

Oussama Ben Ghorbel · Accepted Answer · 2019-08-07 13:59:16Z

1

foreach ($arr as $k => $v) {
    usort($v, function ($a1, $a2) {
        return $a1['WeekNumDay'] <=> $a2['WeekNumDay']; //spaceship operator only php 7+
    });
    $arr[$k] = $v; //important to return the sorted array to the original container array
}

Note: usort first argument is passed by reference, so passing $arr[$k] would work as well since php would figure where the container of $v.

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