From: "Simon Kröger" <SimonKroeger@...>
Date: 2005-10-16T05:31:27+09:00
Subject: Re: How to get non-unique elements from an array?

Robert Klemme wrote:
> 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...

Yeah,

some thoughts of mine:

---------------------------------------------------------------------
require 'enumerator'

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

p a.sort.to_enum(:each_cons, 2).select{|i, j|i==j}.flatten.uniq

p a.select{|x|a.index(x) != a.rindex(x)}.uniq

p a.dup.reject{|x|a.delete(x)}.uniq
---------------------------------------------------------------------

all have downsides (performance wise and the last one is destructive)

cheers

Simon