The problem is that you are stuck with lookups into the larger array and that's O(n). Your best choice if you have to do this a lot and the larger array isn't so large that memory usage would make it unfeasible, would be to make a hash keyed by the elements in the larger array where the values are those elements' indexes. Otherwise, what you are doing is pretty reasonable.

-sauoq
"My two cents aren't worth a dime.";