Unflattening a simple array with recursive lambda function

Question

I've been trying to unflatten an array with a recursive lambda function but I can't seem to get my head around it.

$test = function( $a, $b ) use ( &$test ) {
    if ( ! count($a) ) return $b;

    $b[array_shift($a)] = []; // Missing logic here.

    return $test( $a, $b );
};

$newArr = $test( [0, 1, 2], []  );

echo "<pre>";
print_r($newArr);
echo "</pre>";

That's the code but I have no idea what to do in the "// Missing logic here." part.

I'd like this recursive lambda function to convert

[0, 1, 2]

Into:

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

You'd have to explain what exactly that "// Missing logic here" should do. There's not an obvious route from [0, 1, 2] to [0 => [1 => [2 => 'test']]], when "test" is not specified anywhere. — cHao
– cHao, Commented Nov 29, 2017 at 21:04

splash58 · Accepted Answer · 2017-11-29 21:26:51Z

1

You don't need the 2nd argument

$test = function( $a) use ( &$test ) {
    // the leaf element
    if ( ! count($a) ) return 'test';
    $c = array_shift($a);
    return [$c => $test($a)];
};

$newArr = $test( [0, 1, 2] );

demo

answered Nov 29, 2017 at 21:26

splash58

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4 Comments

Don't Panic Over a year ago

I guess count and $c aren't needed either: return $a ? [array_shift($a) => $test($a)] : 'test';

splash58 Over a year ago

Yes, you are right. But if OP's code is almost working, I usually save as much as possible.

Don't Panic Over a year ago

That makes sense. I wasn't necessarily suggesting you should change your answer, though. Just commenting.

splash58 Over a year ago

@Don'tPanic Well, that's right, especially if take into account that your code is very elegant :)

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