Get all combinations of one element from a dynamic amount of arrays

Question

I want to merge every array value of an array with every array value of another array. Like so:

$a1 = ['a', 'b'];
$a2 = ['c', 'd', 'e', 'f'];
$a3 = ['g', 'h'];
$result = [];

foreach($a1 as $a) {
    foreach($a2 as $b) {
        foreach($a3 as $c) {
            $result[] = [$a, $b, $c];
        }
    }
}

The result should look like this:

array(16) {
  [0]=>
  array(3) {
    [0]=>
    string(1) "a"
    [1]=>
    string(1) "c"
    [2]=>
    string(1) "g"
  }
  [1]=>
  array(3) {
    [0]=>
    string(1) "a"
    [1]=>
    string(1) "c"
    [2]=>
    string(1) "h"
  }
  .... 
}

But I don't know how I can do it dynamically, because the number of arrays (e.g. $a1 - $a3) should be dynamically as well. So that I could merge for example $a1 - $a6 or $a1 - $a2, too.

I hope someone can help me. :)

Answer 1

Use the following recursive function for the cartesian product of a variable number of arrays:

function array_cartesian_product(...$array)
{
    if (empty($array)) return [[]];

    $column = array_shift($array);
    $cartesian = array_cartesian_product(...$array);

    $result = [];
    foreach ($column as $item) {
        foreach ($cartesian as $row) {
            array_unshift($row, $item);
            array_push($result, $row);
        }
    }
    return $result;        
}

// Usage:

$a1 = ['a', 'b'];
$a2 = ['c', 'd', 'e', 'f'];
$a3 = ['g', 'h'];

$result = array_cartesian_product($a1, $a2, $a3);

print_r($result);

$a = [ ['a', 'b'], ['c', 'd', 'e', 'f'], ['g', 'h'] ];

$result = array_cartesian_product(...$a);

print_r($result);