From: Robert Klemme <shortcutter@...>
Date: 2008-03-31T18:26:03+09:00
Subject: Re: How does Array#- do comparisons?

2008/3/31, Adam Bender <abender@gmail.com>:
> How does Array#- (as in, [1, 2, 3, 1, 1] - [1]) compare two objects to
>  see if they are equal?  I would think the .eql? method, but I've
>  written a class that implements that method so that two different
>  objects with the same values are considered equal and '-' isn't
>  functioning as I would expect.  A contrived example:
>
>  irb(main):004:0> class Hash
>  irb(main):005:1> def eql?(h)
>  irb(main):006:2> true
>  irb(main):007:2> end
>  irb(main):008:1> end
>  => nil
>  irb(main):010:0> h1 = {'a' => 1}
>  => {"a"=>1}
>  irb(main):011:0> h2 = {'a' => 1}
>  => {"a"=>1}
>  irb(main):012:0> h1 == h2
>  => true
>  irb(main):013:0> h1.eql? h2
>  => true
>  irb(main):014:0> [h1] - [h2]
>  => [{"a"=>1}]
>
>  I would expect [] as a result, just like the following:
>  irb(main):015:0> ["asdf"] - ["asdf"]
>  => []

Another illustration of Stefano's point:

irb(main):001:0> class Foo; end
=> nil
irb(main):002:0> [Foo.new] - [Foo.new]
=> [#<Foo:0x7ff994b0>]
irb(main):003:0> class Foo
irb(main):004:1> def eql?(o) true end
irb(main):005:1> end
=> nil
irb(main):006:0> [Foo.new] - [Foo.new]
=> [#<Foo:0x7ff80988>]
irb(main):007:0> class Foo
irb(main):008:1> def hash;0 end
irb(main):009:1> end
=> nil
irb(main):010:0> [Foo.new] - [Foo.new]
=> []

Kind regards

robert

-- 
use.inject do |as, often| as.you_can - without end