From: Stefano Zacchiroli <zack@...>
Date: 2007-01-15T22:29:11+09:00
Subject: Re: newbie question about sets

On Mon, Jan 15, 2007 at 10:07:47PM +0900, Xavier Noria wrote:
> Yes, that implements sets as defined in math. What you wanted is  
> called "multiset"[*], though in my degree I never saw any of those  
> even mentioned.

I guess one can build them by herself on the following lines:

  class MultiSet < Set
    def initialize
      super
      @arity = {}
    end
    attr_reader :arity
    def add(o)
      super(o)
      @arity[o] = 0 unless @arity.member? o
      @arity[o] += 1
    end
    def delete(o)
      super(o)
      @arity[o] -= 1
      @arity.delete o if @arity[o] == 0
    end
  end

but beware that the above is newbie's code (and also utterly simplified
...) :-)

And by the way, why the above class does not work properly with <<
(which is an alias for add according to my set.rb) while it works
properly with add?

E.g.

irb(main):019:0> m=MultiSet.new
=> #<MultiSet: {}>
irb(main):020:0> m.add 1
=> 1
irb(main):021:0> m.add 1
=> 2
irb(main):022:0> m.arity[1]
=> 2                          # so far so good

irb(main):023:0> m << 2
=> #<MultiSet: {1, 2}>
irb(main):024:0> m << 2
=> #<MultiSet: {1, 2}>
irb(main):025:0> m.arity[2]
=> nil                        # aargh, do I need to alias again in
                              # derived classes? that's too bad ...

TIA,
Cheers.

-- 
Stefano Zacchiroli -*- Computer Science PhD student @ Uny Bologna, Italy
zack@{cs.unibo.it,debian.org,bononia.it} -%- http://www.bononia.it/zack/
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