Comparable module usage in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of comparing two objects with Comparable

What is the output of this Ruby code using the Comparable module?

Ruby

class Box
  include Comparable
  attr_reader :volume
  def initialize(w, h, d)
    @volume = w * h * d
  end
  def <=>(other)
    volume <=> other.volume
  end
end

box1 = Box.new(2, 3, 4)
box2 = Box.new(3, 2, 4)
puts box1 == box2
puts box1 < box2
puts box1 > box2

A

true
true
false

B

false
true
false

C

false
false
true

D

true
false
false

💡 Hint

Remember that Comparable uses the <=> method to compare objects.

Both boxes have volume 24 (2*3*4 and 3*2*4). So box1 == box2 is true. Neither is less or greater than the other, so false for < and >.

❓ Predict Output

intermediate

Sorting objects with Comparable

What will be the output of this Ruby code that sorts objects using Comparable?

Ruby

class Person
  include Comparable
  attr_reader :age
  def initialize(age)
    @age = age
  end
  def <=>(other)
    age <=> other.age
  end
  def to_s
    "Age: #{age}"
  end
end

people = [Person.new(30), Person.new(20), Person.new(25)]
sorted = people.sort
sorted.each { |p| puts p }

A

Age: 30
Age: 25
Age: 20

B

Age: 20
Age: 25
Age: 30

C

Age: 25
Age: 20
Age: 30

D

Age: 20
Age: 30
Age: 25

💡 Hint

The sort method uses the <=> operator defined in Comparable.

The <=> compares ages, so sorting arranges people by increasing age: 20, 25, 30.

❓ Predict Output

advanced

Custom comparison with multiple attributes

What is the output of this Ruby code that compares objects by two attributes using Comparable?

Ruby

class Product
  include Comparable
  attr_reader :price, :rating
  def initialize(price, rating)
    @price = price
    @rating = rating
  end
  def <=>(other)
    comp = price <=> other.price
    comp.zero? ? rating <=> other.rating : comp
  end
end

p1 = Product.new(100, 4.5)
p2 = Product.new(100, 4.7)
p3 = Product.new(90, 5.0)

puts p1 > p2
puts p3 < p1
puts p2 == p2

A

false
true
true

B

true
false
true

C

false
false
false

D

true
true
false

💡 Hint

First compare price, then rating if prices are equal.

p1 and p2 have same price, but p1 rating 4.5 < 4.7 so p1 > p2 is false. p3 price 90 < 100 so p3 < p1 true. p2 == p2 true.

❓ Predict Output

advanced

Effect of missing <=> method in Comparable

What error or output does this Ruby code produce when Comparable is included but <=> is missing?

Ruby

class Item
  include Comparable
  attr_reader :value
  def initialize(value)
    @value = value
  end
end

item1 = Item.new(10)
item2 = Item.new(20)
puts item1 < item2

A NoMethodError: undefined method `<=>` for #<Item:...>

B false

C true

D SyntaxError: unexpected end-of-input

💡 Hint

Comparable requires <=> method to work.

Including Comparable without defining <=> causes NoMethodError when comparison operators are used.

🧠 Conceptual

expert

Determining number of items after sorting with Comparable

Given this Ruby code, how many items will be in the sorted array?

Ruby

class Score
  include Comparable
  attr_reader :points
  def initialize(points)
    @points = points
  end
  def <=>(other)
    points <=> other.points
  end
end

scores = [Score.new(10), Score.new(20), Score.new(10), Score.new(30)]
sorted_scores = scores.sort.uniq { |s| s.points }
puts sorted_scores.size

A 2

B 4

C 3

D 1

💡 Hint

uniq with a block removes duplicates based on the block's return value.

There are 4 scores but two have points 10. uniq { |s| s.points } removes duplicates by points, leaving 3 unique points: 10, 20, 30.