Prune/reduce a multi-dimensional array in PHP to a certain depth - php

Question

I've searched for this on and off for a couple of years to no avail. Occasionally I'd like to prune a multi-dimension to a certain depth. This would be useful when storing large recursive data sets when you only need data to a certain depth and the rest should be discarded.
For example, given the following array:
$arr = array(
'1' => array(
'1.1' => '1.1.1'),
'2',
'3' => array(
'3.1' => array(
'3.1.1' => '3.1.1.1')
)
);
I would want to prune it back to x number of dimensions calling something like this:
some_prune_method($arr array, $dimensions integer)
...and expect data to be returned something like this:
print_r(some_prune_method($arr, 1));
Array(
[
'1',
'2',
'3'
]
)
print_r(some_prune_method($arr, 2));
Array(
[
'1' => ['1.1'],
'2',
'3' => ['3.1']
]
)
print_r(some_prune_method($arr, 3));
Array(
[
'1' => ['1.1'],
'2',
'3' => ['3.1' => '3.1.1']
]
)
I can think how to do this manually using a callback to iterate through the array but that's slow. Ideally this would need to be done WITHOUT iterating through the entire array.
Maybe I'm missing a simple way to do this?

Answer 1

Think this is what your after. The main concept is just a recursive method to copy the input array which keeps track of the depth. Each time it goes back round it takes 1 off the depth and just before checking if it needs to call the recursive loop it checks if the depth will allow it. If not then it just adds the key to the output loop instead.
$arr = array(
'1' => array(
'1.1' => '1.1.1'),
'2',
'3' => array(
'3.1' => array(
'3.1.1' => '3.1.1.1')
)
);
function some_prune_method ( $array, $depth ) {
$output = [];
foreach ( $array as $key => $element ) {
if ( is_array($element)) {
if ( $depth > 1 ) {
$output [$key] = some_prune_method ( $element, $depth-1 );
}
else {
$output[] = $key;
}
}
else {
$output[] = $element;
}
}
return $output;
}
print_r(some_prune_method($arr, 2));
gives...
Array
(
[1] => Array
(
[0] => 1.1.1
)
[2] => 2
[3] => Array
(
[0] => 3.1
)
)