From: jogloran <jogloran@...>
Date: 2006-04-14T11:35:24+09:00
Subject: Re: Array#-

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Ah, thank you. But could this change in future versions? The fact that the
operation is backed by a hash operation is not mentioned in the
documentation.

Thanks

On 4/14/06, Mauricio Fernandez <mfp@acm.org> wrote:
>
> On Thu, Apr 13, 2006 at 11:21:18PM +0900, jogloran wrote:
> > I need items to be compared based on their values, and not by object
> > equality, so I provide #=3D=3D for the array elements. The problem is t=
hat
> I've
> > been trying to use Array#- as a set difference-like operation, and it
> > doesn't seem to respect any kind of user-provided means of defining
> > equality. A look into the Ruby source seems to indicate that it works b=
y
> > populating a hash with the elements of the second array, then appending
> to
> > the result array if an element of the first array is in the hash. Does
> this
> > mean that it is impossible to get the behaviour I want from Array#- as
> it is
> > implemented?
>
> You have to define #hash and #eql?:
>
> RUBY_VERSION                                       # =3D> "1.8.4"
> class X
>   attr_reader :foo
>   def initialize(foo); @foo =3D foo end
> end
>
> [X.new(1), X.new(2)] - [X.new(1)]                  # =3D> [#<X:0xb7deaf94
> @foo=3D1>, #<X:0xb7deaf80 @foo=3D2>]
>
> X.new(1).hash                                      # =3D> -605071554
> X.new(1).hash                                      # =3D> -605071594
> class X
>   def hash
>     @foo.hash
>   end
>   def eql?(x)
>     foo =3D=3D x.foo
>   end
> end
>
> [X.new(1), X.new(2)] - [X.new(1)]                  # =3D> [#<X:0xb7dead50
> @foo=3D2>]
>
> --
> Mauricio Fernandez  -   http://eigenclass.org   -  singular Ruby
>
>

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