Question

I have two list

a = [1, 2, 3, 4, 1, 2]
b = a.uniq # b = [1, 2, 3, 4]

c = a - b

the result gets c = []

I expects the result c = [1, 2]

How can I get this?

Answer 1

This is the expected result according to the Array documentation.

If you want to get the values which have multiple occurrences within the array you could something like:

a.uniq.keep_if {|v| a.count(v) > 1}

If Ruby 1.8 is used, keep_if is not available, so you need to use something like:

a.uniq.delete_if {|v| a.count(v) == 1}

(Both methods keep a intact)