From: Patrick Hurley <phurley@...>
Date: 2006-04-14T06:22:59+09:00
Subject: Re: Rubyesque way to do multiple <=>

On 4/13/06, Victor Shepelev <vshepelev@imho.com.ua> wrote:
> > > I have class with some fields
> > >
> > > class Human
> > >   attr_accessor :sex, :age, :second_name, :first_name
> > > end
> > >
> > > now I want to sort objects of this class. I've defined operator <=>
> > >
> > > class Human
> > >   def <=>(other)
> > >     [sex, age, second_name, first_name] <=> [sex, age, second_name,
> > > first_name]
> > >   end
> > > end
> > >
> > > Looks good, yeah? But! If I use third-party complex comparator for some
> > of
> > > the fields, I receive something like:
> > >
> > > def <=> (other)
> > >   res = ComplexCustomComparator.compare(sex, other.sex)
> > >   return res if res != 0
> > >
> > >   #if they have the same sex, do other comparisons
> > > end
> > >
> > > Foooooo!
> > >
> > > How can I do the former in more elegant way?
> > >
> > > Thanks.
> > >
> > > Victor.
>
>
> > Actually you just use your first version of method <=> without worrying
> > about the second.
> > Run the following code and you will clearly see why ;)
> > ------------------8<-------------------------------
> > class A
> >     def <=>(other)
> >         puts "<=> in A"
> >         0
> >     end
> > end
> >
> > puts "If at first you do not succeed"
> > [1, A.new] <=> [2, A.new]
> > puts "..."
> > [A.new ] <=> [A.new]
> > ----------------------->8----------------------------
> >
> > Ruby takes care of everything for you
>
> You haven't got it. I already USED first version, but I CAN'T if comparison
> is done through stand-alone function instead of <=>
>
> > Cheers
> > Robert
>
> Victor.
>
>
>

Can you provide a <=> for the special cases? Otherwise I guess
something like this:

class Array
  def fancy_cmp(other, sym = :<=>)
    each_with_index do |val,i|
      result = val.send(sym, other[i])
      return result unless result.zero?
    end
    self.size <=> other.size
  end
end

Might help
pth