I have two relatively large 1D arrays (I could make them into a two D
with a dummy column if that would help, but I can't imagine how). Both
arrays contain a series of strings, with each string being exactly the
same length (in this case 18 characters). The characters are numbers
as strings. Each array contains 20,000-30,000 elements.

The challenge is finding a fast way to compare each element in one
array to see if it exists in the second array. I find that if I do
this through any simple stepwise looping it takes forever. Any
suggestions?