How to Flatten a Multidimensional Array?

Question

Is it possible, in PHP, to flatten a (bi/multi)dimensional array without using recursion or references?

I'm only interested in the values so the keys can be ignored, I'm thinking in the lines of array_map() and array_values().

Answer 1

This solution is non-recursive. Note that the order of the elements will be somewhat mixed.

function flatten($array) {
    $return = array();
    while(count($array)) {
        $value = array_shift($array);
        if(is_array($value))
            foreach($value as $sub)
                $array[] = $sub;
        else
            $return[] = $value;
    }
    return $return;
}

Answer 2

Uses recursion. Hopefully upon seeing how not-complex it is, your fear of recursion will dissipate once you see how not-complex it is.

function flatten($array) {
    if (!is_array($array)) {
        // nothing to do if it's not an array
        return array($array);
    }

    $result = array();
    foreach ($array as $value) {
        // explode the sub-array, and add the parts
        $result = array_merge($result, flatten($value));
    }

    return $result;
}


$arr = array('foo', array('nobody', 'expects', array('another', 'level'), 'the', 'Spanish', 'Inquisition'), 'bar');
echo '<ul>';
foreach (flatten($arr) as $value) {
    echo '<li>', $value, '</li>';
}
echo '<ul>';

Output:

foo

nobody

expects

another

level

the

Spanish

Inquisition

bar

Answer 3

As of PHP 5.3 the shortest solution seems to be array_walk_recursive() with the new closures syntax:

function flatten(array $array) {
    $return = array();
    array_walk_recursive($array, function($a) use (&$return) { $return[] = $a; });
    return $return;
}

Answer 4

You can use the Standard PHP Library (SPL) to "hide" the recursion.

$a = array(1,2,array(3,4, array(5,6,7), 8), 9);
$it = new RecursiveIteratorIterator(new RecursiveArrayIterator($a));
foreach($it as $v) {
  echo $v, " ";
}

prints

1 2 3 4 5 6 7 8 9