Well, until someone catches the horrible gaff on my last try by solving
the EXACT OPPOSITE PROBLEM WHILE BEING CONDESCENDING (my wife is used
to this), I'll keep trying:

Of course, the following is what works, finding duplicates ELEMENTS:

a.sort.to_s.scan(/(.)(\1)+/).flatten.uniq

Yeah... I guess it's clear this finds individual duplicate ELEMENTS,
not numbers.  Works on single digits/letters/etc, blows up on all else.
 How's that for useless?

What would do the trick is a 'non-greedy' array subtraction:

Normal(greedy): [0,1,2,3,4,5,2,3] - [0,1,2,3,4,5] = []
Non-greedy: [0,1,2,3,4,5,2,3] - [0,1,2,3,4,5] = [2,3]

Seems like there has got to be a way of solving this w/o counters or
blocks...

Sam Kong wrote:
> Hello!
>
> I need to get non-unique elements from an array.
> The best I came up with was using a hash as a counter for each unique
> elements.
>
> a = [0,1,2,3,4,5,2,3]
>
> #What I want to get is [2,3] as 2,3 are non-unique elements.
>
> h = {}
> a.each do |i|
>   if h[i]
>     h[i] += 1
>   else
>     h[i] = 1
>   end
> end
>
> u = []
> h.each do |k, v|
>   if v > 1
>     u << k
>   end
> end
>
> #now u == [2,3]
>
> This works fine.
> But I think there's a better way.
> How do you handle such a case?
> 
> Thanks in advance.
> 
> Sam