Question

In PHP, one can use the following function to determine if one list (the child) is a subset of another (the parent):

function issubset($child, $parent)
{
        $c = count($child);
        $valid = 1;
        for($i=0;$i<$c;$i++) {
            if(!in_array($child[$i], $parent)) {
                $valid = 0;
                return $valid;
            }
        }
    return $valid;
}

A similar but opposite concept is the idea of two lists being disjoint, whereby they have no elements in common whatsoever.

For example, the lists 1,2,3,4 and 4,5,6,7 are not disjoint because they have the common element 4, but the lists 1,2,3 and 4,5,6 are disjoint as they have no elements in common.

How might a function to check disjointness be designed?

Answer 1

if (count(array_intersect($a, $b)) == 0) { /* do something */ }