Self keyword behavior in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of self inside instance method

What is the output of this Ruby code?

Ruby

class Person
  def initialize(name)
    @name = name
  end

  def show_self
    puts self
  end
end

p = Person.new("Alice")
p.show_self

A Alice

B #<Person:0x00007fffe30b8c10>

C Person

D self

💡 Hint

Inside an instance method, self refers to the current object instance.

In Ruby, inside an instance method, self refers to the current object instance. Printing self shows the object's default string representation, which includes the class name and an object id.

❓ Predict Output

intermediate

Output of self inside class method

What will this Ruby code print?

Ruby

class Car
  def self.show_self
    puts self
  end
end

Car.show_self

A Car

B #<Car:0x00007fffe30b8c10>

C Object

D self

💡 Hint

Inside a class method, self refers to the class itself.

In Ruby, inside a class method, self refers to the class object. So printing self prints the class name.

🔧 Debug

advanced

Why does this code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

class Animal
  def self.describe
    puts "This is an animal"
    puts self.name
  end

  def name
    "Generic Animal"
  end
end

Animal.describe

A self.name calls a class method 'name' which is not defined, causing NoMethodError

B Instance method 'name' is called correctly, no error

C Syntax error due to missing 'end' keyword

D self refers to an instance, but no instance exists

💡 Hint

Check if 'name' is defined as a class method or instance method.

The method describe is a class method, so self inside it refers to the class Animal. The call self.name tries to call a class method name, but only an instance method name is defined. This causes a NoMethodError.

🧠 Conceptual

advanced

Meaning of self in module method

In Ruby, inside a module method defined with def self.method_name, what does self refer to?

A The class that includes the module

B An instance of the module

C The module object itself

D The main Object

💡 Hint

Modules are objects too in Ruby.

In Ruby, modules are objects of class Module. Inside a module method defined as def self.method_name, self refers to the module object itself.

❓ Predict Output

expert

Output of self in nested context

What does this Ruby code print?

Ruby

class Outer
  def self.show
    puts self
    class Inner
      def self.show_inner
        puts self
      end
    end
    Inner.show_inner
  end
end

Outer.show

A

Outer::Inner
Outer

B

Outer
Inner

C

Outer
self

D

Outer
Outer::Inner

💡 Hint

Check what self refers to in each class method.

In Outer.show, self is Outer. Inside Inner.show_inner, self is Outer::Inner, the nested class's name.

Practice

1. What does the self keyword represent inside an instance method in Ruby?

easy

A. The superclass of the current class

B. The class itself

C. A global variable

D. The current instance (object) of the class

Solution

1. Step 1: Understand instance methods context

   Inside an instance method, the code runs for a specific object created from the class.

2. Step 2: Identify what self points to

   In this context, self refers to that specific object (instance) running the method.

3. Final Answer:

   The current instance (object) of the class -> Option D

4. Quick Check:

   self in instance method = instance [OK]

Hint: Inside instance methods, think "self" means the object itself [OK]

Common Mistakes:

• Confusing self with the class inside instance methods
• Thinking self is a global or class variable
• Assuming self is always the class

2. Which of the following is the correct way to define a class method using self in Ruby?

easy

A. def self.my_method; 'hello'; end

B. def my_method.self; 'hello'; end

C. def my_method(self); 'hello'; end

D. def self:my_method; 'hello'; end

Solution

1. Step 1: Recall class method syntax

   Class methods are defined by prefixing the method name with self. inside the class.

2. Step 2: Check each option's syntax

   def self.my_method; 'hello'; end uses def self.my_method which is correct syntax. Others are invalid Ruby syntax.

3. Final Answer:

   def self.my_method; 'hello'; end -> Option A

4. Quick Check:

   Class method syntax = def self.method_name [OK]

Hint: Class methods start with def self.method_name [OK]

Common Mistakes:

• Placing self after method name
• Using colon instead of dot
• Passing self as a parameter

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

class Dog
  def speak
    self.class
  end
end

fido = Dog.new
puts fido.speak

medium

A. Dog

B. #

C. fido

D. Object

Solution

1. Step 1: Understand what self is inside speak

   Inside the instance method speak, self is the instance fido.

2. Step 2: Evaluate self.class

   Calling class on the instance returns its class, which is Dog.

3. Final Answer:

   Dog -> Option A

4. Quick Check:

   self.class in instance method = class name [OK]

Hint: self.class inside instance method returns the object's class [OK]

Common Mistakes:

• Thinking self.class returns the instance
• Confusing class with object name
• Expecting output to be an object instead of class name

4. Identify the error in this Ruby code snippet:

class Cat
  def self.meow
    puts "Meow!"
  end

  def meow
    puts "Meow from instance"
  end

  def self.instance_meow
    meow
  end
end

Cat.instance_meow

medium

A. Missing self in instance method definition

B. Calling instance method meow inside a class method without an instance

C. Incorrect syntax for class method definition

D. Using puts inside methods is not allowed

Solution

1. Step 1: Analyze method calls inside self.instance_meow

   This is a class method trying to call meow without specifying an instance.

2. Step 2: Understand method types

   meow without self. is an instance method, so it needs an object to call it on.

3. Step 3: Identify the error

   Calling an instance method directly inside a class method causes a NoMethodError because no instance is given.

4. Final Answer:

   Calling instance method meow inside a class method without an instance -> Option B

5. Quick Check:

   Class methods cannot call instance methods without an object [OK]

Hint: Class methods need an instance to call instance methods [OK]

Common Mistakes:

• Trying to call instance methods directly inside class methods
• Confusing self in class vs instance methods
• Ignoring need for object to call instance methods

5. Given the following Ruby code, what will be the output?

class Calculator
  def self.description
    "I am a Calculator"
  end

  def initialize(value)
    @value = value
  end

  def show_description
    self.class.description
  end
end

calc = Calculator.new(10)
puts calc.show_description

hard

A. NoMethodError

B. "Calculator"

C. "I am a Calculator"

D. "10"

Solution

1. Step 1: Understand self.class.description inside instance method

   Inside show_description, self is the instance calc. Calling class returns the class Calculator.

2. Step 2: Call class method description

   Calling description on the class returns the string "I am a Calculator".

3. Final Answer:

   "I am a Calculator" -> Option C

4. Quick Check:

   Instance calls class method via self.class = output string [OK]

Hint: Use self.class to call class methods from instances [OK]

Common Mistakes:

• Expecting instance variable value instead of class method output
• Confusing self with class inside instance methods
• Thinking NoMethodError will occur