From: Robert Klemme <bob.news@...>
Date: 2005-10-16T04:31:55+09:00
Subject: Re: How to get non-unique elements from an array?

Sam Kong <sam.s.kong@gmail.com> wrote:
>> You don't need the exact number of repetitions. Try this:
>> a = [0,1,2,3,4,5,2,3]
>> h = {}
>> u = a.inject([]) {|res, x| h[x] ? res + [x] : h[x] = res}
> 
> Thank you for the insight.
> 
> However, your code doesn't work if an element repeats more than twice.
> 
> a = [0,1,2,3,4,5,2,3,2] #there are three 2's.
> Your code's result is [2, 3, 2] instead of [2, 3]
> It should be modified like
> u = a.inject([]) {|res, x| h[x] ? res + [x] : h[x] = res}.uniq
> 
> Other than that, I like your way.
> Thank you again.
> 
> Sam

>> a = [0,1,2,3,4,5,2,3]
=> [0, 1, 2, 3, 4, 5, 2, 3]
>> a.inject(Hash.new(0)) {|h,e| h[e]+=1;h}.select {|k,v|v>1}.map {|a,b|a}
=> [2, 3]

Quite complicated...

    robert