php - Intersecting multidimensional array of varying size

Question

I have a multidimensional array that looks like this:

Array
(
[0] => Array
    (
        [0] => Array
            (
                [id] => 3
            )

        [1] => Array
            (
                [id] => 1
            )

        [2] => Array
            (
                [id] => 2
            )

        [3] => Array
            (
                [id] => 5
            )

        [4] => Array
            (
                [id] => 4
            )

    )

[1] => Array
    (
        [0] => Array
            (
                [id] => 1
            )

        [1] => Array
            (
                [id] => 3
            )

        [2] => Array
            (
                [id] => 4
            )

        [3] => Array
            (
                [id] => 5
            )

    )

[2] => Array
    (
        [0] => Array
            (
                [id] => 3
            )

    )

)

I need to find a way to return the intersecting value(s). In this case that would be

[id] => 3

The length of the array could be different, so I can't just use array_intersect().

Answer 1

This would be simple if your arrays contained only integers, but as they contain an another array, it gets a bit more complicated. But this should do it:

function custom_intersect($arrays) {
    $comp = array_shift($arrays);
    $values = array();

    // The other arrays are compared to the first array:
    // Get all the values from the first array for comparison
    foreach($comp as $k => $v) {
        // Set amount of matches for value to 1.
        $values[$v['id']] = 1;
    }

    // Loop through the other arrays
    foreach($arrays as $array) {
        // Loop through every value in array
        foreach($array as $k => $v) {
            // If the current ID exists in the compare array
            if(isset($values[$v['id']])) {
                // Increase the amount of matches
                $values[$v['id']]++;
            }
        }
    }

    $result = array();

    // The amount of matches for certain value must be
    // equal to the number of arrays passed, that's how
    // we know the value is present in all arrays.
    $n = count($arrays) + 1;
    foreach($values as $k => $v) {
        if($v == $n) {
            // The value was found in all arrays,
            // thus it's in the intersection
            $result[] = $v;
        }
    }
    return $result;
}

Usage:

$arrays = array(
    array(array('id' => 3), array('id' => 1), array('id' => 2), array('id' => 5), array('id' => 4)),
    array(array('id' => 1), array('id' => 3), array('id' => 4), array('id' => 5)),
    array(array('id' => 3))
);

print_r(custom_intersect($arrays));

Result:

Array
(
    [0] => 3
)

This function isn't perfect: if you have duplicate ID's in one array, it will not work. That would require a bit more code to first make the array values unique, but this will probably work in your case.