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Ruby provides some very interesting set operations on arrays.

given the two arrays A and B wich look like this

A = ["A", "B", "C", "D"]
B = ["C", "D", "E"]

There are three set operations we can use that union, intersection and difference.

Union

A | B

contains all elements from both sets without doublicates. So this results in ["A", "B", "C", "D", "E"]

Difference

A - B

contains all elements from set A that are not in set B. So this results in ["A", "B"]

Intersection

A & B

contains all elements that are in set A and in set B. So this results in ["C", "D"]