Remove all elements of an array with non-numeric keys

Question

I have an array that looks something like this:

Array
(
    [0] => apple
    ["b"] => banana
    [3] => cow
    ["wrench"] => duck
)

I want to take that array and use array_filter or something similar to remove elements with non-numeric keys and receive the follwoing array:

Array
(
    [0] => apple
    [3] => cow
)

I was thinking about this, and I could not think of a way to do this because array_filter does not provide my function with the key, and array_walk cannot modify array structure (talked about in the PHP manual).

Answer 1

Using a foreach loop would be appropriate in this case:

foreach ($arr as $key => $value) {
    if (!is_int($key)) {
        unset($arr[$key]);
    }
}

Answer 2

It can be done without writing a loop in one (long) line:

$a = array_intersect_key($a, array_flip(array_filter(array_keys($a), 'is_numeric')));

What it does:

• Since array_filter works with values, array_keys first creates a new array with the keys as values (ignoring the original values).
• These are then filtered by the is_numeric function.
• The result is then flipped back so the keys are keys once again.
• Finally, array_intersect_key only takes the items from the original array having a key in the result of the above (the numeric keys).

Don't ask me about performance though.

Answer 3

As of PHP 5.6, it's now possible to use array_filter in a compact form:

array_filter($array, function ($k) { return is_numeric($k); }, ARRAY_FILTER_USE_KEY);

Demo.

This approach is about 20% slower than a for loop on my box (1.61s vs. 1.31s for 1M iterations).

---

As of PHP 7.4, it's possible to also use short closures::

array_filter($array, fn($k) => is_numeric($k), ARRAY_FILTER_USE_KEY);

Answer 4

Here's a loop:

foreach($arr as $key => $value) {
    if($key !== 0 and !intval($key)) {
         unset($arr[$key]);
    }
}