Demo (I expect result [3]):
[1,2] - [1,2,3] => [] # Hmm
[1,2,3] - [1,2] => [3] # I see
a = [1,2].to_set => #<Set: {1, 2}>
b = [1,2,3].to_set => #<Set: {1, 2, 3}>
a - b => #<Set: {}> WTF!
And:
[1,2,9] - [1,2,3] => [9] # Hmm. Would like [[9],[3]]
How is one to perform a real set difference regardless of order of the inputs?
Ps. As an aside, I need to do this for two 2000-element arrays. Usually, array #1 will have fewer elements than array #2, but this is not guaranteed.
The - operator applied to two arrays a and b gives the relative complement of b in a (items that are in a but not in b).
What you are looking for is the symmetric difference of two sets (the union of both relative complements between the two). This will do the trick:
a = [1, 2, 9]
b = [1, 2, 3]
a - b | b - a # => [3, 9]
If you are operating on Set objects, you may use the overloaded ^ operator:
c = Set[1, 2, 9]
d = Set[1, 2, 3]
c ^ d # => #<Set: {3, 9}>
For extra fun, you could also find the relative complement of the intersection in the union of the two sets:
( a | b ) - ( a & b ) # => #<Set: {3, 9}>
来源：https://stackoverflow.com/questions/11238256/how-to-do-sane-set-difference-in-ruby