From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2009-01-15T21:21:58+09:00
Subject: Re: Returning a duplicate from an Array

On Thu, Jan 15, 2009 at 12:18 PM, Jimmy Kofler <koflerjim@mailinator.com> wrote:
>
> class Array
>   def dups
>      uniq.select{ |e| (self-[e]).size < self.size - 1 }
>   end
> end
>
> p [ 1, 1, 2, 2, 3, 3, 4, 5 ].dups   #=> [1, 2, 3]

Regarding this and other solutions: the OP accepted a solution from
Andy Cooper which return *all* duplicates.
I mean, if a number appears three times, it will appear two in the
solution, and not one:

irb(main):001:0> a = [1,1,1,1,2,2,3,4,5,5,5]
=> [1, 1, 1, 1, 2, 2, 3, 4, 5, 5, 5]
irb(main):008:0> class Array
irb(main):009:1>   def dups
irb(main):010:2>      uniq.select{ |e| (self-[e]).size < self.size - 1 }
irb(main):011:2>   end
irb(main):012:1> end
=> nil
irb(main):013:0> a.dups
=> [1, 2, 5]

Should be [1,1,1,2,3,4,5,5]. If order is not important:

irb(main):014:0> class Array
irb(main):015:1> def dups
irb(main):016:2> h = Hash.new 0
irb(main):017:2> each {|x| h[x] += 1}
irb(main):018:2> h.inject([]) {|res, (k,v)| res << ([k] * (v-1)) if v
> 1; res.flatten}
irb(main):019:2> end
irb(main):020:1> end
=> nil
irb(main):021:0> a.dups
=> [5, 5, 1, 1, 1, 2]

Jesus.