Self keyword behavior in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print the current object using self.

Ruby

puts [1]

Drag options to blanks, or click blank then click option'

Aself

Bthis

Cme

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' or 'me' which are not Ruby keywords.

Trying to print an undefined variable instead of self.

In Ruby, self refers to the current object. Using puts self prints that object.

2fill in blank

medium

Complete the method to return the class of the current object using self.

Ruby

def get_class
  [1].class
end

Drag options to blanks, or click blank then click option'

Athis

Bself

Cobject

Dme

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' or 'me' which are not valid in Ruby.

Calling class on an undefined variable.

The keyword self refers to the current object, so self.class returns its class.

3fill in blank

hard

Fix the error by replacing the blank with the correct keyword to call a method on the current object.

Ruby

def greet
  [1].say_hello
end

Drag options to blanks, or click blank then click option'

Aobject

Bthis

Cme

Dself

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' or 'me' which cause errors.

Omitting the receiver when explicit self is needed.

To call a method on the current object, use self. Other options are not valid Ruby keywords.

4fill in blank

hard

Fill the blank to define a class method using self.

Ruby

class Person
  def [1].greet
    puts "Hello!"
  end
end

Drag options to blanks, or click blank then click option'

Athis

Bme

Cself

Dobject

Attempts:

3 left

💡 Hint

Common Mistakes

Using instance method syntax instead of class method syntax.

Using invalid keywords like 'this' or 'me'.

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

5fill in blank

hard

Fill all three blanks to complete the code that prints the object id using self inside an instance method.

Ruby

class Item
  def print_id
    id = [1].[2]
    puts "ID: [3]"
  end
end

Drag options to blanks, or click blank then click option'

Aself

Bobject_id

C#{id}

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of 'self'.

Forgetting to use string interpolation to print the variable.

Inside an instance method, self.object_id gets the object's id. To print it inside a string, use string interpolation #{id}.

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