Nth element

Sometimes we want to keep only every other element in an array, and place those elements in another array. Other distances, like every 3 elements, can also be used.

With PHP it is possible to develop a function that performs this task. We can use modulo division to determine whether an element at index is supposed to be retained.

Example

Here we introduce the every_nth_element() function. It creates a new array that is populated in the foreach loop, and then returned at the end.

Step 1 We use a foreach-loop over the keys in the array by calling array_keys. This gives us the indexes of each array element.

Step 2 We use modulo division based on the "n" argument to determine if the current element is in an index where we keep the element.

Step 3 We return the array we built up in the foreach-loop. This array then can be used when every_nth_element is called.

function every_nth_element($array, $n) {
    $result = [];
    // Step 1: Loop over array keys (indexes of the array).
    foreach (array_keys($array) as $i) {
        // Step 2: If the index is evenly divisible by the argument, add it to the array.
        if ($i % $n == 0) {
            $result[] = $array[$i];
        }
    }
    // Step 3: Return the array we built up.
    return $result;
}

// Use the function on this array.
$letters = ["a", "b", "c", "d", "e", "f", "g", "h", "i"];

$result = every_nth_element($letters, 2);
print_r($result);

$result = every_nth_element($letters, 3);
print_r($result);Array
(
    [0] => a
    [1] => c
    [2] => e
    [3] => g
    [4] => i
)
Array
(
    [0] => a
    [1] => d
    [2] => g
)

Results

It is easy to see that when the argument to every_nth_element is 2, we keep every other element in the letters array. With an argument of 3, we skip over 2 letters after each letter.

With a foreach-loop, we can avoid using an error-prone for-loop for our PHP functions. By appending to a new array, we can build up the result array.