# Finding unique array combinations with PHP (permutations)

I was developing a website where people would place horse bets and the system should be able to calculate all the possible permutations based on the user selection. That means: finding all the possible combinations in a two dimensional array. I found a couple of functions but none of them would keep the array keys intact. So, I came up with this idea after a few gray hairs.

It is a mix of recursive array iterations. Consider you have an array like this:

$traits = array ( '52' => array('Happy', 'Sad', 'Angry', 'Hopeful'), '21' => array('Outgoing', 'Introverted'), '12' => array('Tall', 'Short', 'Medium'), '36' => array('Handsome', 'Plain', 'Ugly') );

The function should return all the possible combination of traits, exactly like this (keeping the keys intact):

Array ( [0] => Array ( [52] => Happy [21] => Outgoing [12] => Tall [36] => Handsome ) [1] => Array ( [52] => Happy [21] => Outgoing [12] => Tall [36] => Plain ) [2] => Array ( [52] => Happy [21] => Outgoing [12] => Tall [36] => Ugly ) [3] => Array ( [52] => Happy [21] => Outgoing [12] => Short [36] => Handsome ) [4] => Array ( [52] => Happy [21] => Outgoing [12] => Short [36] => Plain ) --- and so on....

So, here is the function:

function permutations(array $array, $inb=false) { switch (count($array)) { case 1: // Return the array as-is; returning the first item // of the array was confusing and unnecessary return $array[0]; break; case 0: throw new InvalidArgumentException('Requires at least one array'); break; } // We 'll need these, as array_shift destroys them $keys = array_keys($array); $a = array_shift($array); $k = array_shift($keys); // Get the key that $a had $b = permutations($array, 'recursing'); $return = array(); foreach ($a as $v) { if($v) { foreach ($b as $v2) { // array($k => $v) re-associates $v (each item in $a) // with the key that $a originally had // array_combine re-associates each item in $v2 with // the corresponding key it had in the original array // Also, using operator+ instead of array_merge // allows us to not lose the keys once more if($inb == 'recursing') $return[] = array_merge(array($v), (array) $v2); else $return[] = array($k => $v) + array_combine($keys, $v2); } } } return $return; } $x = permutations($traits);

Currently, it only works with numeric array keys. That’s the solution I needed at the moment and I haven’t really tested it with literal keys. Ideas and comments are always welcome.

Thanks to Jon from Stack Overflow for his suggestions and ideas.