From: Robert Klemme <bob.news@...>
Date: 2005-10-17T02:11:56+09:00
Subject: Re: How to get non-unique elements from an array?

Ara.T.Howard <Ara.T.Howard@noaa.gov> wrote:
> 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]

Did you mean to provide this as an example that you actually need to count? 
Because that's what it is:

?>    a = 0, 1, 2, 3, 4, 5, 2, 3, 2, 2
=> [0, 1, 2, 3, 4, 5, 2, 3, 2, 2]
>>
?>    p a.dups
[2, 3, 2]
=> nil
>>

Here's another alternative with limited counting:

>> a = 0, 1, 2, 3, 4, 5, 2, 3, 2, 2
=> [0, 1, 2, 3, 4, 5, 2, 3, 2, 2]
>> a.inject({}) do |h,i|
?>   case h[i]
>>     when nil
>>       h[i] = 1
>>     when 1
>>       h[i] = 2
>>     when 2
>>       # ok
?>     else
?>       raise "Error"
>>   end
>>   h
>> end.inject([]) {|ar,(k,v)| ar << k if v == 2; ar}
=> [2, 3]

I doubt though that performance is better than that with counting.

Kind regards

    robert