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I need to iterate over an array that looks like this:

$myArray = array( 'key1' => array('subkey' => 'subvalue'), 'key2' => array('subkey' => 'subvalue'), )

Into each nested associative array, I want to add a key-value pair based on the outside key, like this:

$myNewArray = array( 'key1' => array('subkey' => 'subvalue', 'newkey' => 'only for key1'), 'key2' => array('subkey' => 'subvalue'), )

Ideally, I'm looking for something like:

$myNewArray = array_map(function($key, $value) { if ($key == 'key1') { $value['newkey'] = 'only for key1'; } return $value; }, $myArray);

However, that obviously doesn't work as callback isn't given two parameters but only one. I could do something like this:

$myNewArray = array_map(function($key, $value) { if ($key == 'key1') { $value['newkey'] = 'only for key1'; } return WHAT?? }, array_keys($myArray), $myArray);

However, what do I return here? It seems to always construct a new array, i.e. discarding my string keys (key1 and key2), while a single-array array_map() keeps them.

I can use array_walk() but it has a rather strange API (flipped parameter order, array passed by reference etc.) so I would generally prefer if this could be achieved using array_map(). Can it?

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5 Answers 5

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I'm afraid array_walk() IS the way to do this.

If you don't like array_walk() and insist on doing it with array_map(), well, it's possible. It involves using also array_keys(), array_values() and array_combine(), it is long and ugly but doable:

$myNewArray = array_combine( array_keys($myArray), array_map( function($key, $value) { if ($key == 'key1') { $value['newkey'] = 'only for key1'; } return $value; }, array_keys($myArray), array_values($myArray) // can omit array_values() and use $myArray ) );

You can also do it using array_reduce() but it's the same mess:

$myNewArray = array_reduce( array_keys($myArray), function (array $carry, $key) use ($myArray) { $value = $myArray[$key]; if ($key == 'key1') { $value['newkey'] = 'only for key1'; } $carry[$key] = $value; return $carry; }, array() );

I hope you have a condition more complex than $key == 'key1' because only for this it is not worth it writing complex traversal. Isn't it easier to just access the right element and modify it?

$myArray['key1']['newkey'] = 'only for key1';
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Yes, the transformation is far more complex, I just gave the minimal example that shows the challenge. Thanks for your answer, the first method actually doesn't look too ugly to me, though the array_walk() method will certainly be shorter. I just don't like passing arrays by reference at all.
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array_map only passes the value not the key. array_walk is very similar to the array_map. Just define the value as a reference. It takes the key where array_map does not. ALso, this modifies $myArray:

array_walk($myArray, function(&$value, $key) { if ($key == 'key1') { $value['newkey'] = 'only for key1'; } });

From the PHP manual, there does seem to be an array_map method using array_keys as another array:

$myNewArray = array_combine(array_keys($myArray), array_map(function($key, $value) { if ($key == 'key1') { $value['newkey'] = 'only for key1'; } return $value; }, array_keys($myArray), $myArray));
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Yes, this works, however, as your example shows, the API is confusing and it's easy to make a mistake - $myNewArray in your example will be true :) But I get it, thanks.
I just spotted that when I was working with array_map. Edited.
Your second example will return an array of associative arrays, which is different from just associative array.
Fixed, just got longer and longer... Whether it's confusing or not, array_walk is the proper and standard way.
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Array transformation function variant, supporting keys:

/** * Transform array elements. * * @param array $a Original array. * @param callable(V, K, $a): V|null $vTransformer Value transformation function. * @param callable(K, V, $a): K|null $kTransformer Key transformation function. * * @template K Array key * @template V Array value * * @return array Transformed array. */ function transformArray(array $a, ?callable $vTransformer = null, ?callable $kTransformer = null): array { $arrayKeys = array_keys($a); $arrayValues = array_values($a); return array_combine( is_callable($kTransformer) ? array_map(fn ($k) => $kTransformer($k, $a[$k], $a), $arrayKeys, $arrayValues) : $arrayKeys, is_callable($vTransformer) ? array_map(fn ($k) => $vTransformer($a[$k], $k, $a), $arrayKeys, $arrayValues) : $arrayValues ); }

Example

$a = [ 'k1' => 100, 'k2' => 101, ]; $b = transformArray($a, fn ($v) => $v + 10 ); $c = transformArray($a, null, fn ($k) => "k-$k" ); $d = transformArray($a, fn ($v) => $v + 10, fn ($k) => "k-$k" ); var_dump([ 'a' => $a, 'b' => $b, 'c' => $c, 'd' => $d, ]); 
array(2) { ["a"] => array(2) { ["k1"] => int(100) ["k2"] => int(101) } ["b"]=> array(2) { ["k1"] => int(110) ["k2"] => int(111) } ["c"]=> array(2) { ["k-k1"] => int(100) ["k-k2"] => int(101) } ["d"]=> array(2) { ["k-k1"] => int(110) ["k-k2"] => int(111) } }
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0

Use a custom fucntion.

I know I am late to the party but, the answer is so simple if you can define your own function.

function array_map_kv(array $array, callable $callback): array { $newArray = []; foreach ($array as $key => $value) { $newArray += $callback($key, $value); } return $newArray; }

Super easy to understand and use, just return an array in your callback:

//DEMO of associative array map function $oldArray = [ 'Chicky!' => null, 'not part of the song' => 'blah, blah, blah', 'My name is Cha-Cha!' => 'Clap, clap, cha-cha-cha' ]; $newArray = array_map_kv($oldArray, function ($key, $value) { //modify key or value or both at once. if (empty($value)) return ['My name is Chicky!' => 'Chicky, Chicky, Chicky']; // you can exclude entries giving you the power of array_filter at the same time 😉. if ($key === 'not part of the song') return []; // or change nothing. return [$key => $value]; }); //the expected result. assert($newArray == [ 'My name is Chicky!' => 'Chicky, Chicky, Chicky', 'My name is Cha-Cha!' => 'Clap, clap, cha-cha-cha', ]);
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Not the best approach performance wise but if you still want to do it with array_map, you can just pass the keys as the third parameter instead of second using array_keys . And then return the value itself.

$myArray = array( 'key1' => array('subkey' => 'subvalue'), 'key2' => array('subkey' => 'subvalue'), ); $myNewArray = array_map(function($value,$key){ if($key == 'key1'){ $value['newkey'] = 'only for key1'; } return $value; },$myArray,array_keys($myArray));

Here's the result with print_r($myNewArray)

Array ( [0] => Array ( [subkey] => subvalue [newkey] => only for key1 ) [1] => Array ( [subkey] => subvalue ) )

Simple enough for small arrays but consider the alternatives for better performance.

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