How to count duplicates in Ruby Arrays

Question

How do you count duplicates in a ruby array?

For example, if my array had three a\'s, how could I count that

Answer 1

requires 1.8.7+ for group_by

ary = %w{a b c d a e f g a h i b}
ary.group_by{|elem| elem}.select{|key,val| val.length > 1}.map{|key,val| key}
# => ["a", "b"]

with 1.9+ this can be slightly simplified because Hash#select will return a hash.

ary.group_by{|elem| elem}.select{|key,val| val.length > 1}.keys
# => ["a", "b"]