PHP: get permutations of nested array with recursion

Question

I have the nested array structure below.

$letters = array(
    $A = array(1,2),
    $B = array(3,4),
    $C = array(5,6)
);

My goal is to find all possible permutations of the numbers, while the letters must keep the order A-B-C. The expected output is:

1-3-5
1-3-6
1-4-5
1-4-6
2-3-5
2-3-6
2-4-5
2-4-6

Of course, this could easily be accomplished with foreach:

foreach($A as $a){
    foreach($B as $b){
        foreach($C as $c){
            echo $a.$b.$c.'<br>';
        }
    }
}

However, I want this to work dynamically, with a varying number of arrays. I already figured that a recursive approach could work, but I cannot wrap my head around it. Can someone help?

Your $letters array looks weird. Wouldn't $letters = array(array(1, 2), array(3, 4), array(5, 6)); be enough? — Cobra_Fast
– Cobra_Fast, Commented Feb 13, 2019 at 13:46

Cobra_Fast · Accepted Answer · 2019-02-13 14:01:05Z

3

Simply pass the string along into recursions:

function work($str, $arr, $i)
{
    $last = ($i == count($arr) - 1);
    foreach ($arr[$i] as $c)
    {
        if ($last)
            echo $str . $c . "\n"; // print whole line
        else
            work($str . $c, $arr, $i + 1); // recurse to next column
    }
}

$letters = array(
    array(1,2),
    array(3,4),
    array(5,6)
);

work("", $letters, 0);

produces

135
136
145
146
235
236
245
246

answered Feb 13, 2019 at 14:01

Cobra_Fast

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