Class methods with self prefix in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class method named greet.

Ruby

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

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

Aclass

Bself

Cthis

Ddef

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of 'self' to define class methods.

Forgetting to prefix the method name with 'self.'

In Ruby, class methods are defined by prefixing the method name with self. inside the class.

2fill in blank

medium

Complete the code to call the class method greet on the Person class.

Ruby

Person.[1]

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

Agreet

Bnew

Chello

Dsay

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call the method on an instance instead of the class.

Using a wrong method name.

Class methods are called on the class itself, so use Person.greet to call the method.

3fill in blank

hard

Fix the error in the class method definition by completing the blank.

Ruby

class Calculator
  def [1].add(a, b)
    a + b
  end
end

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

Athis

Bclass

Cself

Ddef

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'this' instead of 'self' in Ruby.

Omitting the prefix and defining an instance method instead.

Class methods must be defined with self. prefix, not 'this' or other keywords.

4fill in blank

hard

Fill both blanks to define a class method description that returns a string.

Ruby

class Book
  def [1].[2]
    "A great read"
  end
end

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

Aself

Bclass

Cdescription

Dinfo

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'class' instead of 'self' for the prefix.

Using wrong method names like 'info'.

Class methods use self prefix and the method name is description here.

5fill in blank

hard

Fill all three blanks to define and call a class method info that returns a string.

Ruby

class Animal
  def [1].[2]
    "Friendly animal"
  end
end

puts Animal.[3]

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

Aself

Binfo

Ddescription

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names in definition and call.

Forgetting the self prefix in the method definition.

The class method is defined with self.info and called as Animal.info.

Practice

1. What does adding self. before a method name inside a Ruby class do?

easy

A. Defines a method that can only be called inside other methods

B. Makes the method private and inaccessible outside the class

C. Makes the method an instance method, callable on objects

D. Makes the method a class method, callable on the class itself

Solution

1. Step 1: Understand the role of self. in method definition

   In Ruby, prefixing a method name with self. inside a class defines it as a class method.

2. Step 2: Identify how class methods are called

   Class methods are called on the class itself, not on instances of the class.

3. Final Answer:

   Makes the method a class method, callable on the class itself -> Option D

4. Quick Check:

   self.method_name defines class method [OK]

Hint: Remember: self.method means class method, no instance needed [OK]

Common Mistakes:

• Thinking self. makes method private
• Confusing class methods with instance methods
• Assuming self. methods are only for internal use

2. Which of the following is the correct syntax to define a class method named greet inside a Ruby class?

easy

A. def @greet; puts 'Hello'; end

B. def greet; puts 'Hello'; end

C. def self.greet; puts 'Hello'; end

D. def ::greet; puts 'Hello'; end

Solution

1. Step 1: Recall class method syntax

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

2. Step 2: Check each option

   def self.greet; puts 'Hello'; end uses def self.greet, which is the correct syntax for a class method.

3. Final Answer:

   def self.greet; puts 'Hello'; end -> Option C

4. Quick Check:

   Class method syntax = def self.method_name [OK]

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

Common Mistakes:

• Omitting self. for class methods
• Using instance variable syntax (@) for methods
• Trying to use :: inside method definition

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

class Calculator
  def self.add(a, b)
    a + b
  end
end

puts Calculator.add(3, 4)

medium

A. 7

B. 34

C. Error: undefined method 'add'

D. nil

Solution

1. Step 1: Identify the method type and call

   The method add is defined as a class method using self.add, so it can be called on the class Calculator.

2. Step 2: Evaluate the method call

   Calling Calculator.add(3, 4) returns the sum of 3 and 4, which is 7.

3. Final Answer:

   7 -> Option A

4. Quick Check:

   Class method call returns sum 7 [OK]

Hint: Class methods called on class, so Calculator.add works [OK]

Common Mistakes:

• Trying to call add on an instance
• Confusing string concatenation with addition
• Expecting error due to method definition

4. Find the error in this Ruby class method definition:

class Person
  def self.say_hello
    puts "Hello!"
  end

  def self.say_goodbye
    puts "Goodbye!"
  end

  def say_hello
    puts "Hi!"
  end
end

Person.say_hello
Person.say_goodbye
Person.new.say_hello
Person.say_hello()

medium

A. Calling Person.say_hello() with parentheses causes error

B. No error, all calls are valid

C. Person.say_hello is both class and instance method causing conflict

D. Person.new.say_hello calls instance method correctly, no error

Solution

1. Step 1: Analyze method definitions

   There are two class methods self.say_hello and self.say_goodbye, and one instance method say_hello.

2. Step 2: Check method calls

   Calling Person.say_hello calls the class method. Person.new.say_hello calls the instance method. Parentheses in Person.say_hello() are allowed in Ruby.

3. Final Answer:

   No error, all calls are valid -> Option B

4. Quick Check:

   Class and instance methods coexist without conflict [OK]

Hint: Class and instance methods can share names without error [OK]

Common Mistakes:

• Assuming parentheses cause errors in method calls
• Thinking class and instance methods with same name conflict
• Expecting error when calling class method with parentheses

5. You want to create a Ruby class Counter that keeps track of how many times its class method increment is called. Which code correctly implements this behavior?

hard

A. class Counter
def self.increment
@count ||= 0
@count += 1
end
def self.count
@count
end
end

B. class Counter
@count = 0
def increment
@count += 1
end
def count
@count
end
end

C. class Counter
def self.increment
@@count += 1
end
def self.count
@@count
end
end

D. class Counter
def increment
@@count ||= 0
@@count += 1
end
def count
@@count
end
end

Solution

1. Step 1: Understand class variable vs class instance variable

   Using @@count is a class variable shared across subclasses, but @count inside class methods is a class instance variable, safer for this use.

2. Step 2: Check method definitions

   class Counter def self.increment @count ||= 0 @count += 1 end def self.count @count end end uses @count ||= 0 inside the class method to initialize the class instance variable, then increments it. It also provides a class method count to read the value.

3. Final Answer:

   class Counter def self.increment @count ||= 0 @count += 1 end def self.count @count end end -> Option A

4. Quick Check:

   Use class instance variable @count inside self methods [OK]

Hint: Use @count with ||= inside self methods to track class state [OK]

Common Mistakes:

• Using instance methods to change class state
• Using @@count without initialization causing errors
• Confusing instance variables with class variables