Php - how to filter an associative array by values and get all keys?

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I have the following associative array:

PHP

$array = [
...
3 => '2023-01-01',
17 => '2023-08-26',
19 => '2023-08-15',
21 => '2023-09-01'
...
];

Values are dates in YYYY-MM-DD format.

I need to get all keys whose value is > current date.

I've found two solutions:

#1:

PHP

$filtered = [];
foreach ($array as $key => $val) {
	if ($val > date('Y-m-d')) array_push($filtered, $key);
}

#2:

PHP

$filtered = array_keys(array_filter($array, fn($v) => $v > date('Y-m-d')));

Which one is the best in terms of performance? And are there other (better) ways?

What I have tried:

#1:

PHP

$filtered = [];
foreach ($array as $key => $val) {
	if ($val > date('Y-m-d')) array_push($filtered, $key);
}

#2:

PHP

$filtered = array_keys(array_filter($array, fn($v) => $v > date('Y-m-d')));

Posted 14-Aug-23 0:12am

LB2371

Updated 23-Aug-23 1:14am

Deeksha Shenoy

v3

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Andre Oosthuizen · Accepted Answer · 2023-08-14T00:46:00

The performance difference between your two solutions will be very similar for small arrays.

Your foreach solution might have a slight edge in terms of readability as it uses a traditional foreach loop, and it explicitly pushes keys to the result array. This can be easier to understand but we are nitpicking here.

Your array_keys solution that uses array_filter combined with array_keys, which is a concise and functional approach can be appealing to developers who are comfortable with functional programming, but again, we are nitpicking.

In a case where you have to handle very large arrays (50 000+), the performance impact may become more noticeable, the only limitation will be your Memory_limit. If you have a too big array, it will give you an error like: Out Of Memory, but it really has to be very large! An alternative, which might be slightly more efficient for very large arrays, is to use a combination of array_reduce and array_filter as in:

PHP

$filtered = array_reduce(
    array_keys($array),
    function ($result, $key) use ($array) {
        if ($array[$key] > date('Y-m-d')) {
            $result[] = $key;
        }
        return $result;
    },
    []
);

I doubt, however, that you will ever have to revert to this in your case.