From: Jia Pu <very.funny.j@...>
Date: 2006-09-01T13:07:13+09:00
Subject: Re: Q: How to use non-builtin class as hash key?

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Thanks, that works.
I am wondering the requirement of #eql? is necessary. If the programmer
wants to have non-equal objects to have same hash value, it is totally up to
the programmer.

On 8/31/06, Joel VanderWerf <vjoel@path.berkeley.edu> wrote:
>
> Jia Pu wrote:
> > I have a class defined as:
> >
> > ###########################
> > class MyClass
> >    def initialize(s1, s2, s3)
> >        @string1 = s1
> >        @string2 = s2
> >        @string3 = s3
> >    end
> >
> >    def hash
> >        return (@string1 + @string2 + @string3).hash
> >    end
> > end
> > ###########################
> >
> > Then I create two instances of this class:
> > ###########################
> > obj1 = MyClass.new('a', 'b', 'c')
> > obj2 = MyClass.new('a', 'b', 'c')
> > ###########################
> >
> > If verify obj1 and obj2 do have the same hash value, I check that
> > 'obj1.hash == obj2.hash'
> > returns true
> >
> > Now i try to use those objects as hash key:
> > h[obj1] = 'abc'
> > h[obj2] = 'def'
> >
> > I expected h contains only one item with value 'def', since obj1 and
> obj2
> > have the same hash value. But it turns out it has 2 items instead.
> >
> > Did I miss something?
> >
>
> You need to define #eql? in your class, since that is how Hash compares
> objects that hash the same:
>
> class MyClass
>     def contents
>         [@string1, @string2, @string3]
>     end
>
>     def eql?(other)
>         other.class == self.class and other.contents == self.contents
>     end
> end
>
> --
>        vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407
>
>

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