From: Gavin Kistner <gavin.kistner@...>
Date: 2006-10-28T02:54:36+09:00
Subject: Re: how to get only the duplicatet items from an array

From: Nico Landgraf [mailto:nico@nico-landgraf.de] 
> is there a oneline to get only the duplicatet item from an array?
> 
> i have an array like this
> aTemp =  [1, 1, 2, 2, 3, 4]
> and i want to know, which items are duplicatet.

There are faster ways to do this, but since you asked for a one-line:
a = [1, 1, 2, 2, 3, 4]
duplicates = a.delete_if{ |x| a.grep(x).length == 1 }.uniq
#=> [1, 2]