Sort array by key function in PHP

Question

Is there a way to sort a PHP array by return value of a key function? Like Python's sorted(arr, key=keyfn) or Clojure's (sort-by keyfn arr),

usort($array, function ($a, $b) {
     $key_a = keyfn($a);
     $key_b = keyfn($b);
     if ($key_a == $key_b) return 0;
     if ($key_a < $key_b) return -1;
     return 1;
});

does what I want, but it's verbose and it calls keyfn (which can be slow) much more often than needed

@Zyigh it sorts by _key_ as in the key of an associative array, I am looking for a way to sort by key _function_ — 0x60, Jul 23 '18 at 15:04
I suppose python's function calls predicate too, on each compare step. — u_mulder, Jul 23 '18 at 15:06
@u_mulder https://pasteboard.co/HvNO1Yb.png 1 call per list element — 0x60, Jul 23 '18 at 15:11
@Zyigh So do you suggest turning the array into an associative one and then sorting it by key? What if several elements have the same value of `keyfn`? — 0x60, Jul 23 '18 at 15:12
All array are associative in PHP. I don't get why you need this and how it would return the same result... — Zyigh, Jul 23 '18 at 15:15
Nice, but in php you have to create a cache with results manually( — u_mulder, Jul 23 '18 at 15:15
[uksort](http://php.net/manual/fr/function.uksort.php) allow you to sort by key with a home made function might be more appropriate maybe — Zyigh, Jul 23 '18 at 15:21
See this link https://stackoverflow.com/questions/1597736/how-to-sort-an-array-of-associative-arrays-by-value-of-a-given-key-in-php — Malki Mohamed, Jul 23 '18 at 15:25

score 1 · Answer 1 · answered Jul 23 '18 at 15:21

Some simple code for caching results of predicate and sorting (using spaceship operator which reduces lines where you return 0,1,-1). In case of predicate result as int you can even return $key_a - $key_b:

$array = [2,2,2,1,1,0,0,8];
$values_cache = [];
usort($array, function ($a, $b) use (&$values_cache) {
    $key_a = isset($values_cache[$a]) ? $values_cache[$a] : ($values_cache[$a] = keyfn($a));
    $key_b = isset($values_cache[$b]) ? $values_cache[$b] : ($values_cache[$b] = keyfn($b));
    return $key_a <=> $key_b;
});
echo '<pre>', print_r($array), '</pre>';

function keyfn($v) {
    echo 'call a keyfn' . PHP_EOL;
    return 2 * $v;
}

Simple fiddle https://3v4l.org/W1N7Y

score 1 · Accepted Answer · answered Jul 27 '18 at 10:18

I ended up implementing it this way:

function sort_by($key_f, $arr) {
    $values = array_map($key_f, $arr);
    asort($values);
    $sorted_arr = array();

    foreach ($values as $idx => $value) {
        $sorted_arr[] = $arr[$idx];
    }

    return $sorted_arr;
}

Sort array by key function in PHP

2 Answers2