From: Ryan Leavengood <leavengood@...>
Date: 2005-10-18T05:11:41+09:00
Subject: Re: How to get non-unique elements from an array?

On 10/17/05, Sam Kong <sam.s.kong@gmail.com> wrote:
>
> I wonder if my slightly modified version works.

It works, but is considerably slower (I've included a few other recent
submissions as well):

require 'benchmark'
include Benchmark

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

BMN = 50000
bm(5) do |bx|
  bx.report('index ') { BMN.times { a.select{|e| a.index(e) !=
a.rindex(e)}.uniq } }
  bx.report('grep  ') { BMN.times { a.select{|e| a.grep(e).length > 1}.uniq } }
  bx.report('scan  ') { BMN.times { a.sort.join('
').to_s.scan(/(\d+)\s(\1)+/).flatten.uniq } }
  bx.report('inject') { BMN.times { a.uniq.inject(a.dup) { |b,i|
b.delete_at(b.index(i)) ; b } } }
  bx.report('gsub  ') { BMN.times {
a.to_s.gsub(/(.)(?!.*\1)/,'').split('').map{|i|i.to_i} } }
end

Results on my laptop:

           user     system      total        real
index   1.281000   0.000000   1.281000 (  1.281000)
grep    3.813000   0.015000   3.828000 (  3.827000)
scan    3.515000   0.000000   3.515000 (  3.531000)
inject  1.594000   0.000000   1.594000 (  1.593000)
gsub    3.063000   0.016000   3.079000 (  3.078000)

Whoever created that index and rindex based solution created a very
fast one. I'll leave it as an exercise for the reading to benchmark
all the other solutions.

Ryan