Module methods in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of calling a module method directly

What is the output of this Ruby code?

Ruby

module Greetings
  def self.say_hello
    "Hello from module!"
  end
end

puts Greetings.say_hello

A Hello from module!

B NoMethodError

C nil

D SyntaxError

💡 Hint

Look at how the method is defined with self. It means it belongs to the module itself.

The method say_hello is defined as a module method using self. It can be called directly on the module name. So it prints the string.

❓ Predict Output

intermediate

Output of including a module with a module method

What happens when you include a module with a module method and call that method on the including class?

Ruby

module Tools
  def self.info
    "Tools module info"
  end
end

class Machine
  include Tools
end

puts Machine.info

A Tools module info

B NoMethodError

C nil

D SyntaxError

💡 Hint

Including a module adds instance methods, but module methods are not included as class methods.

Including a module adds its instance methods to the class. But module methods defined with self are not included. So Machine.info is undefined and raises NoMethodError.

🔧 Debug

advanced

Fix the code to call a module method from a class

This code tries to call a module method from a class method but raises an error. Which option fixes it?

Ruby

module MathHelpers
  def self.square(x)
    x * x
  end
end

class Calculator
  def self.square(x)
    square(x)
  end
end

puts Calculator.square(4)

A Change MathHelpers.square to def square(x) without self

B Add include MathHelpers inside Calculator

C Change Calculator.square to call MathHelpers.square(x)

D Change Calculator.square to an instance method

💡 Hint

Module methods with self need to be called with the module name.

The Calculator class method tries to call square(x) but no such method exists in Calculator. Calling MathHelpers.square(x) fixes the error.

📝 Syntax

advanced

Identify the syntax error in module method definition

Which option shows the correct way to define a module method in Ruby?

A

module M
  def method:
    puts 'Hi'
  end
end

B

module M
  def method(self)
    puts 'Hi'
  end
end

C

module M
  def method.self
    puts 'Hi'
  end
end

D

module M
  def self.method
    puts 'Hi'
  end
end

💡 Hint

Module methods use self. before the method name.

Option D correctly defines a module method using def self.method. Other options have invalid syntax.

🚀 Application

expert

How many methods does this module add to a class?

Given this module and class, how many methods does the class have after including the module?

Ruby

module Features
  def self.module_method
    'module method'
  end

  def instance_method
    'instance method'
  end
end

class Product
  include Features
end

methods = Product.methods - Object.methods
instance_methods = Product.instance_methods(false)

A methods: 0, instance_methods: 1

B methods: 1, instance_methods: 1

C methods: 1, instance_methods: 0

D methods: 0, instance_methods: 0

💡 Hint

Including a module adds instance methods, but not module methods as class methods.

The module method defined with self is not added to the class methods. The instance_method is added as an instance method. So Product.methods - Object.methods is empty (0), and Product.instance_methods(false) includes :instance_method (1).

Practice

1. What is the correct way to define a method inside a Ruby module so it can be called directly on the module?

easy

A. Define the method outside the module

B. Define the method without self inside the module

C. Define the method with self.method_name inside the module

D. Define the method with def module.method_name inside the module

Solution

1. Step 1: Understand module method definition

   In Ruby, to make a method callable directly on a module, you define it with self.method_name inside the module.

2. Step 2: Compare options

   Define the method with self.method_name inside the module uses self.method_name, which is correct. Other options either omit self or use invalid syntax.

3. Final Answer:

   Define the method with self.method_name inside the module -> Option C

4. Quick Check:

   Module methods use self.method_name = A [OK]

Hint: Use self.method_name to define module methods [OK]

Common Mistakes:

• Forgetting to add self before method name
• Defining methods outside the module
• Using invalid syntax like def module.method_name

2. Which of the following is the correct syntax to call a module method named greet defined in module Welcome?

easy

A. Welcome#greet

B. greet.Welcome

C. Welcome.new.greet

D. Welcome.greet

Solution

1. Step 1: Recall how to call module methods

   Module methods are called using dot notation: ModuleName.method_name.

2. Step 2: Check options

   Welcome.greet is the correct syntax using dot notation. Welcome#greet incorrectly uses #. greet.Welcome reverses the order. Welcome.new.greet confuses it with an instance method call.

3. Final Answer:

   Welcome.greet -> Option D

4. Quick Check:

   Call module methods with dot = D [OK]

Hint: Call module methods with ModuleName.method_name [OK]

Common Mistakes:

• Using # instead of . to call module methods
• Trying to call method with invalid syntax
• Confusing instance and module method calls

3. What will be the output of the following Ruby code?

module MathTools
  def self.square(x)
    x * x
  end
end

puts MathTools.square(4)

medium

A. 16

B. square

C. Error: undefined method

D. 4

Solution

1. Step 1: Understand the method definition

   The method square is defined as a module method using self.square. It returns the square of the input.

2. Step 2: Calculate the output

   Calling MathTools.square(4) returns 4 * 4 = 16, which is printed.

3. Final Answer:

   16 -> Option A

4. Quick Check:

   4 squared = 16 [OK]

Hint: Multiply input by itself in module method [OK]

Common Mistakes:

• Forgetting self and causing undefined method error
• Confusing output with input
• Expecting method name as output

4. Identify the error in this Ruby module method code:

module Greetings
  def greet
    "Hello!"
  end
end

puts Greetings.greet

medium

A. Method greet is not defined as a module method

B. Module name is incorrect

C. Missing parentheses in method call

D. Syntax error in method body

Solution

1. Step 1: Check method definition inside module

   The method greet is defined without self., so it is an instance method, not a module method.

2. Step 2: Understand method call

   Calling Greetings.greet tries to call a module method, but none exists, causing an error.

3. Final Answer:

   Method greet is not defined as a module method -> Option A

4. Quick Check:

   Missing self. means no module method [OK]

Hint: Add self. to define module methods [OK]

Common Mistakes:

• Defining instance methods instead of module methods
• Calling module method without defining it
• Assuming parentheses are required in method call

5. Given the module below, how can you add a module method double that returns twice the input number?

module Calculator
  # Add your method here
end

hard

A. def double(x)
x + x
end

B. def self.double(x)
x * 2
end

C. def Calculator.double(x)
x * 2
end

D. def self.double
x * 2
end

Solution

1. Step 1: Define a module method with self

   To create a module method, define it with self.method_name inside the module.

2. Step 2: Write method to return twice the input

   The method should take one argument x and return x * 2.

3. Final Answer:

   def self.double(x) x * 2 end -> Option B

4. Quick Check:

   Module method with self and correct logic = A [OK]

Hint: Use def self.method(args) for module methods [OK]

Common Mistakes:

• Omitting self and creating instance method
• Missing method argument
• Defining method outside module