How to return two arrays using one WHERE statement

Question

Is it possible to combine these two statements into one which returns "contains" and "not contains" results?

string[] words = { "one", "two", "three", "four" };
int[] numbers = { 4, 5, 6 };

string[] contains =
    (from w in words
     where numbers.Contains(w.Length)
     select w).ToArray();

string[] notContains =
    (from w in words
     where !numbers.Contains(w.Length)
     select w).ToArray();

Answer 1

You can do:

var groups = words.GroupBy(w => numbers.Contains(w.Length));

This will return at most two groups: one with the key false and one with the key true.

Edit

As vc 74 pointed out in the comments, numbers.Contains is an O(n) operation when numbers is an array. Converting it to a HashSet instead will make this a constant time operation, which is asymptotically much faster.

So, here's the updated code:

var numberHS = new HashSet<int>(numbers);
var groups = words.GroupBy(w => numberHS.Contains(w.Length));