From: Rob Biedenharn <Rob@...>
Date: 2007-05-11T12:10:33+09:00
Subject: Re: how to remove dups from 2 lists?


On May 10, 2007, at 10:18 PM, Mike Steiner wrote:

> I'm trying to write some code that removes all elements from 2  
> lists that
> are in both lists. However, I don't want any duplicates from each list
> deleted also (which is what the array "-" operator does). The code  
> I have
> now doesn't handle restarting the current iteration for both loops  
> when a
> match is found and deleted in both loops. Here's the code:
>
> def RemoveDupsFromLists ( list1 , list2 )
>    list1.each_index do | i |
>        list2.each_index do | j |
>            if list1[i] == list2[j]
>                list1.delete_at ( i )
>                list2.delete_at ( j )
>            end
>        end
>    end
>    return [ list1 , list2 ]
> end
>
> What's weird is that doing this is easy in C (my first language), but
> difficult in Ruby. Everything else I've seen has been MUCH easier  
> in Ruby.
>
> Mike Steiner

Having some examples might help, but is something like this what  
you're after?

 >> a = %w[ a a a b b c ]
=> ["a", "a", "a", "b", "b", "c"]
 >> b = %w[ b c c c d ]
=> ["b", "c", "c", "c", "d"]
 >> a + b
=> ["a", "a", "a", "b", "b", "c", "b", "c", "c", "c", "d"]
 >> a | b
=> ["a", "b", "c", "d"]
 >> a - b
=> ["a", "a", "a"]
 >> a & b
=> ["b", "c"]
 >> a - (a & b)
=> ["a", "a", "a"]
 >> b - (a & b)
=> ["d"]

In particular, which do you expect from RemoveDupsFromLists( a, b )?

What I have for the last two expressions above:
=> [ ["a", "a", "a"], ["d"] ]

Or:
=> [ ["a", "a", "a", "b"], ["c", "c", "d"] ]
because it works like canceling terms in a fraction:

  ["a", "a", "a", "b", "b", "c"]
-----------------------------------------------
                      ["b", "c", "c", "c", "d"]


Basically, write a test:

require 'test/unit'
class RemoveDupsTest < Test::Unit::TestCase
   def test_simple
     list1 =  ["a", "a", "a", "b", "b", "c"]
     list2 =  ["b", "c", "c", "c", "d"]
     expects = [ [__,__], [__,__] ] #<=== fill me it!
     assert_equals expects, RemoveDupsFromLists(list1, list2)
   end
end

Then you'll know what you want (and so will we!) and you'll be sure  
when you get it working.

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com