PHP按值分组并随机排序

Question

I want to group an associative array by value and randomize the items per group.

I have the following array $result

Aray(
    [0] => Building Object
    (
        [id] => 285
        [formula] => 4
        [title] => test 1
    )
    [1] => Building Object
    (
        [id] => 120
        [formula] => 4
        [title] => test 2
    )
    [2] => Building Object
    (
        [id] => 199
        [formula] => 2
        [title] => test 3
    )
    [3] => Building Object
    (
        [id] => 231
        [formula] => 1
        [title] => test 4
    )
    [3] => Building Object
    (
        [id] => 230
        [formula] => 1
        [title] => test 5
    )
)

So I want to group the array by its formula so the objects with formula 4 should be on top. But the buildings should be per group random so first id 285 on top then id 120 on top... So I want randomly

Aray(
      [0] => Building Object
      (
          [id] => 285
          [formula] => 4
          [title] => test 1
      )

      [1] => Building Object
      (
          [id] => 120
          [formula] => 4
          [title] => test 2
      ) ..

How can I do this I tried:

shulffle($result);
usort($result, "cmp");

But that doesn't keep my array grouped by the formula.

Answer 1

usort is the right function, but you need to be more specific:

// drop the `shuffle`, we'll be shuffling in the sort
usort($result,function($a,$b) {
    // PHP 5.4 or newer:
    return ($a->formula - $b->formula) ?: rand(-1,1);
    // older PHP:
    if( $a->formula == $b->formula) return rand(-1,1);
    return $a->formula - $b->formula;
});

And before people say my shuffling "isn't really random", I say "it's random enough for this application".