From: Benoit Daloze <eregontp@...>
Date: 2010-05-17T00:24:39+09:00
Subject: Re: Comparing Arrays

Hi,

> An answer:
>
> # array_cmp.rb
> def share_elements?(a1,a2)
>   not (a1 & a2).empty?
> end
>
> a = %w(m w f)
> b = %w(m w)
> c = %w(t r)
>

Watch it, though, the comparison is based on Object#hash:

irb for ruby-1.9.3-r27796
> class N
>   attr_reader :n
>   def initiaize(n)
>     @n = n
>     end
>   end
> [N.new(1), N.new(2)] & [N.new(2), N.new(3)]
=> []
> [N.new(2), N.new(2)].map(&:hash)
=> [2562895216144860017, -4480771430923711611]

So, to make it works as expected:

> class N
>   def hash
>     @n.hash
>     end
>   def eql?(n)
>     @n == n.n
>     end
>   end
> [N.new(2), N.new(2)].map(&:hash)
=> [-1851237208519162443, -1851237208519162443]
> [N.new(1), N.new(2)] & [N.new(2), N.new(3)]
=> [#<N:0x00000101161b40>] # 2

It sounds like a bug to me this ...
(or at least documentation not clear:
"Set Intersection---Returns a new array containing elements common to the two
arrays, with no duplicates.")

Programming Ruby is more complete:
"Set Intersection—Returns a new array containing elements common to
the two arrays, with no duplicates.(identical)
The rules for comparing elements are the same as for hash keys. If you
need setlike behavior, see the library class Set"

then why is it called Set Intersection?
(anyway, Set#& behaves the same)