Comparable module usage in Ruby - Mini Project: Build & Apply

Using the Comparable Module in Ruby

📖 Scenario: You are creating a simple program to compare different books by their number of pages. This helps readers decide which book is longer or shorter.

🎯 Goal: Build a Ruby class Book that uses the Comparable module to compare books by their page count.

📋 What You'll Learn

Create a Book class with title and pages attributes

Include the Comparable module in the Book class

Define the <=> method to compare books by their pages

Create at least two Book objects with exact titles and page counts

Use comparison operators like < and == to compare the books

Print the comparison results exactly as instructed

💡 Why This Matters

🌍 Real World

Comparing objects like books, products, or events by certain attributes is common in apps and websites to help users make choices.

💼 Career

Understanding how to use modules like Comparable helps you write cleaner, reusable code and is a valuable skill for Ruby developers.

1

Create the Book class with attributes

Create a Ruby class called Book with an initialize method that takes title and pages as parameters and sets them as instance variables.

Ruby

class Book
  # Your code here
end

Hint

Use def initialize(title, pages) and set @title and @pages inside.

2

Include Comparable and define spaceship operator

Inside the Book class, include the Comparable module and define the <=> method to compare Book objects by their pages attribute.

Ruby

class Book
  attr_reader :title, :pages

  def initialize(title, pages)
    @title = title
    @pages = pages
  end

  # Your code here
end

Hint

Use include Comparable and define def <=>(other) to compare pages.

3

Create Book objects and compare them

Create two Book objects named book1 with title 'Ruby Basics' and pages 150, and book2 with title 'Advanced Ruby' and pages 200. Then, use book1 < book2 and book1 == book2 to compare them and store the results in variables is_shorter and is_equal respectively.

Ruby

class Book
  attr_reader :title, :pages

  include Comparable

  def initialize(title, pages)
    @title = title
    @pages = pages
  end

  def (other)
    pages  other.pages
  end
end

# Your code here

Hint

Create book1 and book2 with exact titles and pages, then compare using < and ==.

4

Print the comparison results

Print the values of is_shorter and is_equal exactly as shown: first print is_shorter, then print is_equal on the next line.

Ruby

class Book
  attr_reader :title, :pages

  include Comparable

  def initialize(title, pages)
    @title = title
    @pages = pages
  end

  def (other)
    pages  other.pages
  end
end

book1 = Book.new('Ruby Basics', 150)
book2 = Book.new('Advanced Ruby', 200)
is_shorter = book1 < book2
is_equal = book1 == book2

# Your code here

Hint

Use puts is_shorter and puts is_equal to print the results.

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