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?

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.

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