From: Park Heesob <phasis@...>
Date: 2005-10-18T10:13:41+09:00
Subject: Re: How to get non-unique elements from an array?

Hi,
>>
>> On second thought, Mark's may have been the best overall. Either way,
>> both Mark's and Martin's are good algorithms. Many of the others were
>> good as well.
>>
>
> Mine isn't the shortest, but it's the fastest I've seen thus far! Twice
> as fast as the next closest! Why does everyone leave me out of benchmark, 
> ;)
>
> ---my code
>  t=[]; a.delete_if{ |e| r=(not t.include? e); t.push(e); r }
> ---end my code
>
> Zach
>
Here is my code

t = Hash.new(0); a.each{|e|t[e]+=1};t.delete_if{|k,v|v==1}.keys

It is reasonably fast for both small and large arrays
small array
zach          0.328000   0.000000   0.328000 (  0.328000)
markvh        0.390000   0.000000   0.390000 (  0.390000)
martin        0.422000   0.000000   0.422000 (  0.422000)
xxx           0.344000   0.000000   0.344000 (  0.344000)  <== MINE

large array
zach          2.204000   0.000000   2.204000 (  2.203000)
markvh        0.015000   0.000000   0.015000 (  0.016000)
martin        0.000000   0.000000   0.000000 (  0.000000)
xxx           0.000000   0.000000   0.000000 (  0.000000)  <== MINE

Park Heesob