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.

Practice

1.

What is the main purpose of including the Comparable module in a Ruby class?

easy

A. To allow the class to inherit from another class

B. To automatically generate getter and setter methods

C. To enable comparison operators like <, <=, ==, >, >= for objects of that class

D. To add string manipulation methods to the class

Solution

1. Step 1: Understand Comparable module purpose

   The Comparable module provides comparison operators such as <, <=, ==, >, >= for objects.

2. Step 2: Recognize what including Comparable does

   Including Comparable requires defining the <=> method and then gives the class comparison operators automatically.

3. Final Answer:

   To enable comparison operators like <, <=, ==, >, >= for objects of that class -> Option C

4. Quick Check:

   Comparable adds comparison operators = A [OK]

Hint: Comparable adds comparison operators if <=> is defined [OK]

Common Mistakes:

• Thinking Comparable adds getters/setters
• Confusing Comparable with inheritance
• Assuming Comparable adds unrelated methods

2.

Which of the following is the correct way to define the comparison method <=> inside a Ruby class to use with Comparable?

class Box
  include Comparable

  def initialize(volume)
    @volume = volume
  end

  def <=>(other)
    ?
  end
end

easy

A. @volume <=> other.instance_variable_get(:@volume)

B. self.volume <=> other.volume

C. @volume <=> other.volume

D. @volume <=> other.@volume

Solution

1. Step 1: Understand instance variable access

   The instance variable @volume is private, so to compare, we must access it properly.

2. Step 2: Check each option for syntax and correctness

   self.volume <=> other.volume uses self.volume but no getter method is defined, so it fails. @volume <=> other.volume tries other.volume but no method exists. @volume <=> other.instance_variable_get(:@volume) uses other.instance_variable_get(:@volume), which accesses the variable correctly. @volume <=> other.@volume uses invalid syntax other.@volume.

3. Final Answer:

   @volume <=> other.instance_variable_get(:@volume) -> Option A

4. Quick Check:

   Access private var with instance_variable_get = B [OK]

Hint: Use instance_variable_get to access private vars in <=> [OK]

Common Mistakes:

• Trying to access other.@volume directly (syntax error)
• Assuming getter methods exist without defining them
• Using self.volume without a method

3.

What will be the output of the following Ruby code?

class Person
  include Comparable

  attr_reader :age

  def initialize(age)
    @age = age
  end

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

p1 = Person.new(30)
p2 = Person.new(25)
puts p1 > p2

medium

A. false

B. true

C. nil

D. Error

Solution

1. Step 1: Understand the <=> method

   The <=> method compares the ages of two Person objects using age <=> other.age.

2. Step 2: Compare p1 and p2 using >

   p1.age is 30, p2.age is 25, so 30 > 25 is true.

3. Final Answer:

   true -> Option B

4. Quick Check:

   30 > 25 = true [OK]

Hint: Compare attributes inside <=> for correct operator results [OK]

Common Mistakes:

• Forgetting to include Comparable
• Not defining <=> method
• Confusing attribute names

4.

Identify the error in this Ruby class using Comparable and how to fix it:

class Item
  include Comparable

  def initialize(weight)
    @weight = weight
  end

  def <=>(other)
    @weight - other.weight
  end
end

medium

A. Method <=> returns a number, should return -1, 0, or 1

B. Syntax error in method definition spacing

C. Missing include Comparable line

D. Cannot call other.weight because no weight method is defined

Solution

1. Step 1: Check method calls inside <=>

   The code calls other.weight, but no weight method is defined in the class.

2. Step 2: Fix by adding an attr_reader for weight

   Adding attr_reader :weight allows other.weight to work correctly.

3. Final Answer:

   Cannot call other.weight because no weight method is defined -> Option D

4. Quick Check:

   Missing getter method = D [OK]

Hint: Define attr_reader for accessed attributes in <=> [OK]

Common Mistakes:

• Assuming instance variables are accessible without getters
• Returning subtraction result instead of using <=> operator
• Ignoring spacing in method definition (which is valid)

5.

You want to sort an array of Product objects by their price using Comparable. Which of the following is the best way to implement the <=> method?

class Product
  include Comparable

  attr_reader :price

  def initialize(price)
    @price = price
  end

  def <=>(other)
    ?
  end
end

hard

A. price <=> other.price

B. other.price <=> price

C. price - other.price

D. price > other.price ? 1 : -1

Solution

1. Step 1: Understand the <=> method contract

   The <=> method should return -1, 0, or 1 depending on comparison.

2. Step 2: Choose the correct comparison expression

   Using price <=> other.price correctly returns -1, 0, or 1. other.price <=> price reverses order, sorting descending. price - other.price returns a number but not guaranteed -1,0,1. price > other.price ? 1 : -1 returns only 1 or -1, missing 0 case.

3. Final Answer:

   price <=> other.price -> Option A

4. Quick Check:

   Use <=> operator for proper comparison = C [OK]

Hint: Use <=> operator directly for clean comparison [OK]

Common Mistakes:

• Reversing comparison order unintentionally
• Returning subtraction result instead of <=>
• Using ternary without zero equality case