Overriding Ruby's spaceship operator <=>

Question

I am trying to override Ruby's <=> (spaceship) operator to sort apples and oranges so that apples come first sorted by weight, and oranges second, sorted by sweetness. Like so:

module Fruity
  attr_accessor :weight, :sweetness

  def <=>(other)
    # use Array#<=> to compare the attributes
    [self.weight, self.sweetness] <=> [other.weight, other.sweetness]
  end
  include Comparable
end

class Apple
include Fruity

def initialize(w)
  self.weight = w
end

end

class Orange
include Fruity

def initialize(s)
  self.sweetness = s
end

end

fruits = [Apple.new(2),Orange.new(4),Apple.new(6),Orange.new(9),Apple.new(1),Orange.new(22)]

p fruits

#should work?
p fruits.sort

But this does not work, can someone tell what I am doing wrong here, or a better way to do this?

Redeploy

UPDATE: It turns out I was going about this the wrong way. Hint: the answer lies here -- blog.hasmanythrough.com/2008/8/17/sorting-things-out – Redeploy

Answer 1

Your problem is you are only initializing one of the properties on either side, the other one will still be nil. nil isn't handled in the Array#<=> method, which ends up killing the sort.

There are a few ways to handle the problem first would be something like this

[self.weight.to_i, self.sweetness.to_i] <=> [other.weight.to_i, other.sweetness.to_i]

nil.to_i gives you 0, which will let this work.

Labe

thanks, it worked. They are ordered by weight then sweetness, but in descending order. Is there a 'best' way to order them ascending? – Redeploy

let me clarify my above comment-- if i make the following change: [self.weight.to_i, self.sweetness.to_i] <=> [other.weight.to_i, other.sweetness.to_i] I get oranges first ascending, then apples ascending, but I am not able to get apples first ascending, then oranges ascending. I have tried permuting the arrays, and even toying with Array#reverse with no luck. Also, does it matter if the Comparable mixin is included before or after the <=> definition? I am getting similar results either way. – Redeploy

it doesn't really matter where you put the include. maybe try result.sort.reverse ? not the best way, but probably the most straightforward – Labe

that gives me oranges first then apples, but i'm trying to get apples first sorted ascending, followed by oranges ascending. Still no luck. Anyone have any ideas? – Redeploy

okay, so why is it that Array#sort does not use its own <=> method to perform the comparisons, but instead uses my overridden one?? Doesn't this violate ruby's dynamic method lookup? – Redeploy

You are overriding <=> on fruity, not array. – Labe

UPDATE: It turns out I was going about this the wrong way. Hint: the answer lies here -- blog.hasmanythrough.com/2008/8/17/sorting-things-out – Redeploy

Answer 2

Probably late, nevertheless...

add the following monkeypatch

class Array
  def to_i(default=Float::INFINITY)
    self.map do |element|
      element.nil? ? default : element.to_i
    end
  end
end

And change the body of Fruity::<=> to

[self.weight, self.sweetness].to_i <=> [other.weight, other.sweetness].to_i

Stirpiculture

sorry, but this is bad advice. (monkey patching always is, but this problem can be solved a lot more elegantly). monkey patches like this can blow up a lot of other things. – Spermatid