Recursion PHP - after one permutation why doesn't this recursive function return a value?

Question

function noOfPermutations($n, $permutation){
    $array = str_split($n);
    if(count($array)==1) {
        echo $permutation."\n";
        return $permutation;
    }else{
        $permutation++;
        noOfPermutations(array_sum($array), $permutation);
    }
}

echo "--".noOfPermutations(91,0)."--\n";

I'm trying to find the number of permutations for the sum of the digits in $n becomes single. e.g. 91 -> 9+1 = 10 -> 1+0 = 1. This took 2 rounds so would return 2. The function works fine if $n starts as a single digit number (it correctly returns 0) however more than one permutation and it just returns null. The first echo in the function outputs the correct value but the echo outside the function returns null.

Unless you need to use recursion, this sort of problem could be easily solved using a `while()` loop. — Nigel Ren, Jul 28 '20 at 14:52
Weird, I'm using an online sandbox so maybe there's something different with that compared to an actual server. — Guy, Jul 28 '20 at 15:12

AbraCadaver · Answer 1 · 2020-07-28T15:07:37.573

1

The solution is in your title after one permutation why doesn't this recursive function return a value?:

return noOfPermutations(array_sum($array), $permutation);

You might also look at array_reduce.

edited Jul 28 '20 at 15:07

answered Jul 28 '20 at 14:52

AbraCadaver

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score 1 · Answer 2 · answered Jul 28 '20 at 14:53

Because you're doing nothing with the result. You go one deeper in the else, but do nothing with the outcome of that. I'm guessing it should be something along the lines of:

function noOfPermutations(int $n, int $permutation = 0){
    $array = str_split($n);
    if( count($array) > 1 ) {
        $permutation += noOfPermutations(array_sum($array),  $permutation);
        //  ^^--- Now we actually do something with the result 
    }

    return $permutation; 
}

^{I've added a little codeformat and typehint as well}

Alternatively, you can make the $permutation by reference, that way you dont have to return it. I dont recommend it, but it might be a solution for someone sometime.

score 0 · Accepted Answer · answered Jul 28 '20 at 15:00

you are not doing anything with the returned value, I slightly re-elaborated your function in this way

function noOfPermutations($n, $permutation) {
    $array = str_split($n);
    if(count($array)==1) {
        echo "==>" . $permutation."\n";
    }else{
        $permutation++;
        $permutation = noOfPermutations(array_sum($array), $permutation);
    }
    return $permutation;
}

echo "--".noOfPermutations(91,0)."--\n";

Recursion PHP - after one permutation why doesn't this recursive function return a value?

3 Answers3