Include for instance methods in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the include keyword do in Ruby?

It adds the methods from a module as instance methods to a class, so objects of that class can use those methods.

beginner

How do instance methods from a module become available in a Ruby class?

By using include ModuleName inside the class, the module's instance methods become instance methods of the class.

beginner

Given this module:

module Greet
  def hello
    "Hi!"
  end
end

How do you make hello available to instances of class Person?

Inside the Person class, write include Greet. Then any Person object can call hello.

intermediate

What is the difference between include and extend in Ruby?

<code>include</code> adds module methods as instance methods. <code>extend</code> adds them as class methods.

beginner

Can a Ruby class include multiple modules for instance methods?

Yes, a class can include many modules. All their instance methods become available to the class's objects.

What happens when you use include with a module in a Ruby class?

A The module is copied as a subclass.

B The module's methods become class methods of the class.

C Nothing happens automatically.

D The module's methods become instance methods of the class.

Using include adds the module's methods as instance methods to the class.

Which keyword adds module methods as class methods instead of instance methods?

A extend

B include

C require

D import

extend adds module methods as class methods.

If a module has a method greet, how do you call it on an object of a class that included the module?

A object.greet

B ClassName.greet

C ModuleName.greet

D You cannot call it

Instance methods from included modules are called on objects.

Can a Ruby class include more than one module to get instance methods?

A No, only one module can be included.

B Yes, multiple modules can be included.

C Only if modules are nested.

D Only if modules have different names.

Ruby classes can include many modules to get their instance methods.

What is the effect of including a module inside a Ruby class?

A It creates a new subclass.

B It copies the module's code into the class.

C It adds the module's instance methods to the class.

D It deletes existing methods in the class.

Including a module adds its instance methods to the class.

Explain how the include keyword works to add instance methods from a module to a Ruby class.

Think about how a class gets new methods from a module.

Describe the difference between include and extend when using modules in Ruby.

One affects objects, the other affects the class itself.

Practice

1. What does the include keyword do in Ruby when used inside a class?

easy

A. It deletes all methods from the class.

B. It adds the module's methods as class methods to the class.

C. It creates a new module inside the class.

D. It adds the module's methods as instance methods to the class.

Solution

1. Step 1: Understand the role of include

   The include keyword is used to add methods from a module into a class.

2. Step 2: Identify how methods are added

   When a module is included, its methods become instance methods of the class, meaning objects of the class can call them.

3. Final Answer:

   It adds the module's methods as instance methods to the class. -> Option D

4. Quick Check:

   Include = instance methods added [OK]

Hint: Include adds module methods as instance methods [OK]

Common Mistakes:

• Confusing include with extend (which adds class methods)
• Thinking include creates a new module
• Assuming include removes existing methods

2. Which of the following is the correct syntax to include a module named Greetings inside a class Person?

easy

A. class Person; include Greetings; end

B. class Person; extend Greetings; end

C. class Person; include(Greetings); end

D. class Person; use Greetings; end

Solution

1. Step 1: Recall correct syntax for including modules

   In Ruby, to add instance methods from a module, use include ModuleName inside the class body without parentheses.

2. Step 2: Check each option

   class Person; include Greetings; end uses include Greetings correctly. class Person; extend Greetings; end uses extend which adds class methods, not instance methods. class Person; include(Greetings); end uses parentheses which is not standard syntax. class Person; use Greetings; end uses use which is invalid in Ruby.

3. Final Answer:

   class Person; include Greetings; end -> Option A

4. Quick Check:

   Include syntax = include ModuleName [OK]

Hint: Use 'include ModuleName' without parentheses [OK]

Common Mistakes:

• Using extend instead of include
• Adding parentheses after include
• Using invalid keywords like 'use'

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

module Talk
  def speak
    "Hello!"
  end
end

class Dog
  include Talk
end

puts Dog.new.speak

medium

A. NoMethodError

B. Hello!

C. speak

D. nil

Solution

1. Step 1: Understand module inclusion

   The module Talk defines an instance method speak. Including it in Dog adds speak as an instance method of Dog.

2. Step 2: Trace the method call

   Creating a new Dog object and calling speak will execute the method from Talk, returning "Hello!".

3. Final Answer:

   Hello! -> Option B

4. Quick Check:

   Included method called = "Hello!" [OK]

Hint: Included module methods become instance methods [OK]

Common Mistakes:

• Expecting NoMethodError because of missing method
• Confusing instance and class methods
• Thinking output is method name or nil

4. Identify the error in the following Ruby code:

module Walk
  def walk
    "Walking"
  end
end

class Person
  extend Walk
end

puts Person.new.walk

medium

A. NoMethodError because walk is not an instance method

B. SyntaxError due to extend usage

C. No error, outputs "Walking"

D. TypeError because module cannot be extended

Solution

1. Step 1: Understand extend vs include

   Using extend Walk adds module methods as class methods, not instance methods.

2. Step 2: Analyze method call

   Person.new.walk calls walk on an instance, but walk is only a class method here, so it raises NoMethodError.

3. Final Answer:

   NoMethodError because walk is not an instance method -> Option A

4. Quick Check:

   Extend adds class methods, instance call fails [OK]

Hint: Extend adds class methods; instance calls fail [OK]

Common Mistakes:

• Confusing extend with include
• Expecting no error on instance method call
• Thinking extend causes syntax or type errors

5. Given the module and classes below, what will puts Cat.new.sound output?

module AnimalSounds
  def sound
    "Generic sound"
  end
end

class Dog
  include AnimalSounds
  def sound
    "Bark"
  end
end

class Cat
  include AnimalSounds
end

hard

A. Bark

B. NoMethodError

C. Generic sound

D. nil

Solution

1. Step 1: Check method definitions in classes

   Class Dog overrides sound method with "Bark". Class Cat does not override it.

2. Step 2: Determine method lookup for Cat instance

   Since Cat includes AnimalSounds and does not override sound, calling sound on Cat.new uses the module's method returning "Generic sound".

3. Final Answer:

   Generic sound -> Option C

4. Quick Check:

   Included method used if no override [OK]

Hint: Class method overrides module method if defined [OK]

Common Mistakes:

• Assuming Cat uses Dog's sound method
• Expecting NoMethodError for missing method
• Confusing instance and class methods