Class methods with self prefix in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class method call

What is the output of this Ruby code?

Ruby

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

puts Calculator.add(3, 4)

A Error: undefined method 'add' for Calculator

B 34

C nil

D 7

💡 Hint

Class methods are defined with self. and called on the class itself.

The method add is a class method because it is defined with self.add. Calling Calculator.add(3, 4) returns the sum 7.

❓ Predict Output

intermediate

Calling instance method as class method

What happens when you run this Ruby code?

Ruby

class Person
  def greet
    "Hello!"
  end
end

puts Person.greet

A Hello!

B nil

C Error: undefined method 'greet' for Person

D Error: wrong number of arguments (given 0, expected 1)

💡 Hint

Instance methods need an object to be called on.

The method greet is an instance method. Calling Person.greet tries to call it on the class, which causes an error.

🔧 Debug

advanced

Why does this class method fail?

This Ruby code raises an error. Which option explains the cause?

Ruby

class Box
  def self.volume
    @length * @width * @height
  end

  def initialize(length, width, height)
    @length = length
    @width = width
    @height = height
  end
end

puts Box.volume

A Class method cannot access instance variables like @length

B Missing return statement in volume method

C Initialize method is not called automatically for class methods

D Class method volume requires arguments but none given

💡 Hint

Class methods do not have access to instance variables because they belong to the class, not an object.

Instance variables like @length belong to objects. Class methods run on the class itself and cannot access these instance variables.

📝 Syntax

advanced

Identify the correct class method syntax

Which option correctly defines a class method named info in Ruby?

A

def info(self)
  "Class info"
end

B

def self.info
  "Class info"
end

C

def info:
  "Class info"
end

D

def info()
  "Class info"
end

💡 Hint

Class methods use self.method_name syntax.

Option B correctly uses def self.info to define a class method. Option B has wrong parameter syntax. Option B has invalid colon syntax. Option B defines an instance method.

🚀 Application

expert

How many times is the class method called?

Consider this Ruby code. How many times is the class method count_calls executed?

Ruby

class Counter
  @calls = 0

  def self.count_calls
    @calls += 1
  end

  def self.calls
    @calls
  end
end

3.times { Counter.count_calls }

puts Counter.calls

A 3

B 0

C 1

D Error: undefined method '+' for nil

💡 Hint

Class instance variables (@variables at class level) are accessible in class methods.

The class instance variable @calls is initialized to 0 at class level. Class methods can access and modify it. count_calls is executed 3 times, incrementing @calls to 3, which Counter.calls returns.