From: Rick DeNatale <rick.denatale@...>
Date: 2007-10-13T03:32:29+09:00
Subject: Re: Finding shared elements between to arrays.

On 10/11/07, Eric I. <rubytraining@gmail.com> wrote:

> Well a1 - (a1 - a2) is not necessarily the same as a2 - (a2 - a1).
> The difference is whether the duplicates in a1 are maintained (the
> former) or the duplicates is a2 are maintained.
>
> For example, suppose a1 is [1, 1, 2, 3, 4] and a2 is [1, 2, 2, 3, 5].
> The result would be either [1, 1, 2, 3] or [1, 2, 2, 3].

Yep, you're right, not enough test cases! <G>

I realize now that the OPs request for "an array which has the
elements shared by two other arrays" is a little ambiguous as to
duplications.
Note that the OPs solutions and the solutions using select produce the
same different results if you reverse the two arrays.

 a1 = [1,1,2,3,4]
 a2 = [1,2,2,3,5]
 a1 - (a1-a2) => [1, 1, 2, 3]
 a2 - (a2-a1)=> [1, 2, 2, 3]
 a1.select {|e| a2.include? e} => [1, 1, 2, 3]
 a2.select {|e| a1.include? e}=> [1, 2, 2, 3]

Should the elements shared between [1,1,2,3,4] and [1,2,2,3,5] be
different than the elements shared between  [1,2,2,3,5] and
[1,1,2,3,4]?

-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/