Get values from an array using array of indexes

user_18967009 0
Viewed 30

I have an array that I need key/values from, and an array of indexes:

$reference = [
    'a' => 'this is A',
    'b' => 'here is B',
    'c' => 'C here',
    'd' => 'finally, d',
];

$indexArray = [1, 3];

I want my result to be:

$result = [
    'b' => 'here is B',
    'd' => 'finally, d',
];

Currently what I have is:

$result = array_intersect(
    $reference,
    array_map(
        function ($index) { return array_values($reference)[$index]; },
        $indexArray
    )
);

This works, but I am wondering if there is a built-in function that can already do this. I know array_slice can do something like this if the values are next to each other, but is there another function that can slice based on an array of indexes?

Edited

This actually doesn't work unless my $reference array in constant. So, again, good solution if $reference isn't constant?

arrays php
2 Answers

You can use array_intersect_key() after using array_flip() to convert $indexArray to an array whose keys are the values.

$result = array_intersect_key(array_values($reference), array_flip($indexArray));
user_14918950

My approach can be written as a one-liner by nesting the functions inside of each other, but I will employ single-use variables to better express the functionality with intuitive variable names.

Code: (Demo)

$reference = [
    'a' => 'this is A',
    'b' => 'here is B',
    'c' => 'C here',
    'd' => 'finally, d',
];

$indexesToKeep = [1, 3];
$mapKeysToIndexes = array_flip(array_keys($reference));
$keysToKeep = array_intersect($mapKeysToIndexes, $indexesToKeep);
var_export(array_intersect_key($reference, $keysToKeep));

/* var_export(
       array_intersect_key(
           $reference,
           array_intersect(
               array_flip(
                   array_keys($reference)
               ),
               $indexesToKeep
           )
       )
   ); */

Output:

array (
  'b' => 'here is B',
  'd' => 'finally, d',
)

Or you can achieve the same result with more efficient time complexity by incrementing a index/counter variable as you iterate.

Code: (Demo)

$lookup = array_flip($indexesToKeep);
$index = 0;
$result = [];
foreach ($reference as $key => $value) {
    if (isset($lookup[$index])) {
        $result[$key] = $value;
    }
    ++$index;
}
var_export($result);
user_29434030
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