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NEVER USE ARRAY_MERGE IN A LOOP
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Date: 2020-11-10
Tags: php, clean-code, refactoring
URL: https://chemaclass.com/blog/array-merge-in-loop/
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Using array_merge inside a loop is a performance killer. The spread operator
will help you to improve this by flatting the array.

Using arraymerge inside a loop is a performance killer. The spread operator will
help you to improve this by flatting the array.

FLATTENING A ONE-LEVEL ARRAY
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I have seen people using the arraymerge function in a loop like:

    <?php
$lists = [ [1, 2], [3, 4], [5, 6], ]; $merged = []; foreach($lists as $list) {
    $merged = arraymerge($merged, $list);
} // $merged === [1, 2, 3, 4, 5, 6];

This is a very bad practice because it's a (memory) performance killer! Instead,
you should use the spread operator (in PHP since 5.6!):

    <?php
$lists = [ [1, 2], [3, 4], [5, 6], ]; $merged = arraymerge(...$lists); // ===
[1, 2, 3, 4, 5, 6];

UNPACKING AN ASSOC-ARRAY
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What if you had an assoc-array instead like this one?

    <?php
$lists = [ 'key-1' => [1, 2], 'key-2' => [3, 4], 'key-3' => [5, 6], ];

In that case, you will need to unpack its values:

    <?php
$merged = arraymerge(...arrayvalues($lists)); // === [1, 2, 3, 4, 5, 6];

In Functional Programming, this is known as flatting a list. No loops & no more
performance problem.

FLATTING A MULTILEVEL ARRAY
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What if you wanted to flat a multilevel array like this one?

    <?php
$lists = [[1], 2, [[3, 4], 5], [[[]]], [[[6]]], 7, 8, []];

Or like this one, even with key-values?

    <?php
$lists = [
    'key-1' => [
        1,
        [2],
        'key-2' => [
            3,
            [
                'key-3' => [4, 5],
            ],
        ],
    ],
    6,
    'key-4' => [7, 8],
];

In these cases, you might want to use the internal standard library:

    <?php
$merged = iteratortoarray(
    new RecursiveIteratorIterator(
        new RecursiveArrayIterator($lists)
    ),
    $usekeys = false
); // $merged === [1, 2, 3, 4, 5, 6, 7, 8];

WHICH FLATTENING APPROACH TO REACH FOR
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Usually, being aware of how to flat a "2 level" array might be sufficient:

    <?php
$flattenList = arraymerge(...arrayvalues($lists));

Otherwise, the internal standard library will help you deal with it.

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References

- Spreadoperatorforarray - Wikipedia
(https://wiki.php.net/rfc/spreadoperatorforarray) - Flattenalist - Rosettacode
(https://rosettacode.org/wiki/Flattena_list)

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