The Comparable Module

Part of the Object Oriented Programming section of Coddy's Ruby journey. Lesson 29 of 57.

Ruby's built-in module Comparable gives a class the operators <, <=, ==, >, >= and the methods between? and clamp. It needs one method from the class: <=>, which returns a negative number, 0 or a positive number when the object is smaller, equal or larger:

class Weight
  include Comparable
  attr_reader :grams

  def initialize(grams)
    @grams = grams
  end

  def <=>(other)
    grams <=> other.grams
  end
end

a = Weight.new(500)
b = Weight.new(1200)
puts a < b
puts a == Weight.new(500)
puts b.between?(a, Weight.new(2000))

Output:

true
true
true

Numbers and strings already have <=>, so a class usually delegates to one of its values. Comparing arrays works element by element, which sorts by several fields at once:

class Person
  include Comparable
  attr_reader :last, :first

  def initialize(first, last)
    @first = first
    @last = last
  end

  def <=>(other)
    [last, first] <=> [other.last, other.first]
  end
end

a = Person.new("Ada", "Lovelace")
b = Person.new("Alan", "Turing")
c = Person.new("Byron", "Lovelace")
puts a < c
puts b > c

Output:

true
true

sort, min and max use <=> too, so objects of the class can be sorted without a block. clamp keeps a value between two others:

class Weight
  include Comparable
  attr_reader :grams

  def initialize(grams)
    @grams = grams
  end

  def <=>(other)
    grams <=> other.grams
  end

  def to_s
    "#{grams}g"
  end
end

list = [Weight.new(900), Weight.new(150), Weight.new(400)]
puts list.sort.join(", ")
puts list.max
puts Weight.new(5000).clamp(Weight.new(0), Weight.new(1000))

Output:

150g, 400g, 900g
900g
1000g

<=> should return nil when the other object cannot be compared. Comparable then raises ArgumentError for < and returns false for ==, instead of failing with a confusing NoMethodError:

def <=>(other)
  return nil unless other.is_a?(Weight)

  grams <=> other.grams
end

Weight.new(1) == 5     # false
Weight.new(1) < 5      # ArgumentError: comparison of Weight with 5 failed

Challenge

Easy

Complete the class Weight. Weight.parse(text) reads 500g, 1.2kg or 2lb (1 lb is 453.6 g) and stores the grams. Include Comparable and define <=> on the grams. to_s returns the grams rounded to a whole number, like 1200g.

The supplied code parses every input line, then prints the weights sorted, the heaviest, the lightest, whether the first weight is between 1 kg and 2 kg, and the first weight clamped to at most 1 kg.

Your code goes in weight.rb. main.rb holds the supplied input/output code and cannot be edited.

Try it yourself

require_relative 'weight'

# Supplied input/output code: keep it as it is
input = []
while (line = gets)
  input << line.chomp
end
weights = input.map { |line| Weight.parse(line) }
puts "sorted: #{weights.sort.join(", ")}"
puts "heaviest: #{weights.max}"
puts "lightest: #{weights.min}"
puts "first between 1kg and 2kg: #{weights[0].between?(Weight.parse("1kg"), Weight.parse("2kg"))}"
puts "first clamped: #{weights[0].clamp(Weight.parse("0g"), Weight.parse("1kg"))}"