Arrayから重複する要素の配列を抜き出す

やり過ぎはダメだと言われても、やっぱり基底クラスの拡張は楽しい！

class Array
  def duplicated
    select{ |v| count(v) > 1}.uniq
  end
end

使い方

a = [3, 5, 3, 4]
=> [3, 5, 3, 4]
[15] pry(main)> a.duplicated
=> [3]
[16] pry(main)> a = [3, 5, 3, 4, 5]
=> [3, 5, 3, 4, 5]
[17] pry(main)> a.duplicated
=> [3, 5]

参考

[Ruby: How to find and return a duplicate value in array? – Stack Overflow][1]