From: Robert Klemme <bob.news@...>
Date: 2005-10-16T21:52:00+09:00
Subject: Re: How to get non-unique elements from an array?

para.hsu@gmail.com wrote:
> Hello!
>
> How about this ?
> (0...a.length-1).select{|i| a[i]==a[i+1]}.map{|i|a[i]}.uniq

a needs to be sorted for this to work.  As far as I remember that was not a 
guaranteed precondition so you would have to add that here.

IMHO most efficient variants are still those that use a hash for element 
counting.

Kind regards

    robert