PHP按两个条件对数组进行排序

Question

I have an array like this:

$arr = [
    0 => [
        'a' => 0,
        'b' => 150
    ],
    1 => [
        'a' => 1,
        'b' => 155
    ],
    2 => [
        'a' => 0,
        'b' => 131
    ],
    3 => [
        'a' => 2,
        'b' => 241
    ],
    4 => [
        'a' => 2,
        'b' => 45
    ],
    5 => [
        'a' => 1,
        'b' => 23
    ],
    6 => [
        'a' => 0,
        'b' => 78
    ],
    7 => [
        'a' => 0,
        'b' => 123
    ],
    8 => [
        'a' => 1,
        'b' => 412
    ],
    9 => [
        'a' => 0,
        'b' => 22
    ],
    10 => [
        'a' => 0,
        'b' => 11
    ],
    11 => [
        'a' => 2,
        'b' => 100
    ],
    12 => [
        'a' => 2,
        'b' => 105
    ],
    13 => [
        'a' => 1,
        'b' => 18
    ]
];

I want to sort the array first by the 'a' property, so all 0s, all 1s and all 2s should be together, and then all the 'b's should be ordered from lowest to highest. I hope you know what I mean. I could separate it into three arrays and then sort each one and then put them back together but I hope there's a shorter and better solution.

Answer 1

Just wanted to explain why the thing you originally tried didn't work.

return a['a'] - b['a'] || a['b'] - b['b']

|| is a logical operator , so when you use that the entire expression will be evaluated as a boolean, so the only possible values are true or false (or numerically, as the usort comparison function should return, 0 or 1). Since none of the comparisons ended up with zeroes on both sides of the || , all of them evaluated to true , so no sorting happened.

Using ?: instead worked because it doesn't evaluate the whole expression as a boolean, it evaluates the first part and returns the result of that, unless it's zero (equal values) in which case it goes on to evaluate the second part and return the result of that.

Example showing the difference between the two comparisons at 3v4l.org: https://3v4l.org/32IPa

---

You can actually sort that array more simply, though.

sort($arr);

will work. You can't always count on that working for sorting arrays, though. It just works in this case because all the inner arrays have the same keys, and you're wanting to sort by the first key, then the second. (You can see how arrays are compared in "Example #2 Transcription of standard array comparison" here .)