ruby array detect non linear data

Is there a way to detect in an array of say 1..500, if some of the
numbers may be missing from the set.

If you have an array of 1 to 500 called 'a' and you wanted to know if
any were missing you could do:

a.uniq.size and if that doesn't equal 500 you are missing some.

If you need to know which ones are missing you could create a new
array of 1 to 500 and subtract the original

b = (1..500).to_a

b - a