Module methods in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a module method in Ruby?

A module method is a method defined inside a module that can be called on the module itself or included in classes. It helps group related methods without creating objects.

beginner

How do you define a module method that can be called directly on the module?

You define it using def self.method_name inside the module. This makes the method callable on the module itself, like ModuleName.method_name.

intermediate

What is the difference between def method_name and def self.method_name inside a module?

def method_name defines an instance method for classes that include the module.
def self.method_name defines a module method callable directly on the module.

beginner

How can you call a module method after defining it with def self.method_name?

Call it by using the module name followed by the method name, like ModuleName.method_name.

beginner

Can module methods be used without including the module in a class?

Yes, module methods defined with def self.method_name can be called directly on the module without including it in any class.

How do you define a method inside a module that can be called directly on the module?

A def method_name

B def @method_name

C def module.method_name

D def self.method_name

Using def self.method_name defines a module method callable on the module itself.

What happens if you define def method_name inside a module and include that module in a class?

A The method becomes an instance method of the class

B The method becomes a class method of the class

C The method can only be called on the module

D The method is private and cannot be called

Instance methods defined in modules become instance methods of classes that include the module.

Which of these calls a module method correctly?

A ModuleName.method_name

B ModuleName#method_name

C method_name()

D include ModuleName.method_name

Module methods are called on the module using dot notation: ModuleName.method_name.

Can you call a module method without including the module in a class?

A No, you must include the module first.

B Yes, if it is defined with self.

C Only if the module is a subclass.

D Only if the method is public.

Module methods defined with self. can be called directly on the module without including it.

What keyword is used to define a module method inside a module?

A class

B module

C self

D defmodule

The keyword self is used to define methods on the module itself.

Explain how to define and call a module method in Ruby.

Think about how to make a method belong to the module itself.

Describe the difference between instance methods and module methods inside a Ruby module.

Focus on where and how each method is called.

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