Flip two-dimensional associative array in PHP

Question

I need to transform an array of parents to children into an array of children to parents. For example, I have an array like this:

[
    1 => [a,b,c],
    2 => [b,c,d],
    3 => [c,d,e],
]

And I want to turn it into this:

[
    a => [1],
    b => [1,2],
    c => [1,2,3],
    d => [2,3],
    e => [3]

]

Is there a way to accomplish this task without using nested foreach loops? If not, what is the most efficient way to do so?

Thanks in advance!

Answer 1

Short solution using array_merge_recursive, array_combine and array_fill functions:

$arr = [
    1 => ['a','b','c'],
    2 => ['b','c','d'],
    3 => ['c','d','e'],
];

$result = [];
foreach ($arr as $k => $v) {
    $result = array_merge_recursive($result, array_combine($v, array_fill(0, count($v), [$k])));
}

print_r($result);

The output:

Array
(
    [a] => Array
        (
            [0] => 1
        )

    [b] => Array
        (
            [0] => 1
            [1] => 2
        )

    [c] => Array
        (
            [0] => 1
            [1] => 2
            [2] => 3
        )

    [d] => Array
        (
            [0] => 2
            [1] => 3
        )

    [e] => Array
        (
            [0] => 3
        )
)

Answer 2

In terms of "efficiency" I believe a nested loop, in this case, is better:

$arr = [1 => ['a','b','c'],
        2 => ['b','c','d'],
        3 => ['c','d','e']];

$result = [];
foreach ($arr as $key => $value) {
    foreach ($value as $v) {
        $result[$v][] = $key;
    }
}

var_dump($result);

Attempting to get creative with other functions like array_map may turn out to be slower, at least according to this answer. Might be worth running some of your own benchmarks.