how to get repeated elements from ruby array? [duplicate]

Answer 1

arr = [1,2,3,1,5,2]
arr.select { |x| arr.count(x) > 1 } .uniq

A longer solution using reduce should be quicker.

arr.reduce [{}, []] do |(seen, out), cur|
  case seen[cur]
    when :seen then
      [seen.merge({cur => :added}), out << cur]
    when :added then
      [seen, out]
    else
     [seen.merge({cur => :seen}), out]
  end
end.last

Answer 2

arr = [1,2,3,1,5,2]
arr.group_by {|e| e}.map { |e| e[0] if e[1][1]}.compact

Pretty ugly... but does the job without an n+1 problem.