From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-10-16T23:11:56+09:00
Subject: Re: How to get non-unique elements from an array?

On Sun, 16 Oct 2005, Robert Klemme wrote:

> 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.

and you needed even count i think:

   harp:~ > cat a.rb
   class Array
     def dups
       h, d = {}, []; each{|e| h[e] ? (d << h.delete(e)) : (h[e] = e) }; d
     end
   end

   a = 0, 1, 2, 3, 4, 5, 2, 3, 2

   p a.dups


   harp:~ > ruby a.rb
   [2, 3]

regards.

-a
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