From: Toby DiPasquale <toby@...>
Date: 2006-03-10T00:40:08+09:00
Subject: Re: nuby question: sorting with nils

Farrel Lifson wrote:
> Actually you can shorten it a bit further if you use
> 
> @date ? @date <=> anItem.date : -1
> 
> That's due to the fact that in Ruby nil is equivalent to false in
> conditional expression.

Speaking of this very topic, I am having an issue where I want to sort 
objects by two of its attributes in a SortedSet, but its not working 
out:

I'd like it to be sorted first by its :a attribute, but if they are 
equal, by whatever sorting on the :b attributes comes up with. Anybody 
know what I'm doing wrong in this test code? The third and fourth 
elements are not in the proper order (excuse the overly verbose <=> 
method... I was trying to get it working).

<snip>
puma:~> cat settest
#!/usr/bin/env ruby
X = Struct.new :a, :b
class X
  def <=> other
    if not @a
      1
    elsif not @b
      -1
    else
      if @a == other.a
        @b <=> other.b
      else
        @a <=> other.a
      end
    end
  end
end

require 'set'
s = SortedSet.new
s << X.new( 1, 2)
s << X.new( 2, 1)
s << X.new( 3, 3)
s << X.new( 3, 4)
s << X.new( 1, 2)
p s

x = X.new 3, 3
y = X.new 3, 4
p x <=> y
p y <=> x
p x <=> x

puma:~> ./settest
#<SortedSet: {#<struct X a=1, b=2>, #<struct X a=2, b=1>, #<struct X 
a=3, b=4>, #<struct X a=3, b=3>}>
1
1
1
puma:~>
</snip>

--
Toby DiPasquale

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