Constants in classes in Ruby - Mini Project: Build & Apply

Constants in classes

📖 Scenario: Imagine you are creating a simple program to manage information about cars. Each car has a brand and a fixed number of wheels. The number of wheels is always the same for all cars, so it makes sense to store it as a constant inside the class.

🎯 Goal: You will create a class called Car with a constant WHEELS set to 4. Then you will create a method to display the number of wheels and the brand of the car.

📋 What You'll Learn

Create a class called Car

Inside the class, define a constant WHEELS with the value 4

Add an initialize method that takes a brand parameter and saves it

Add a method display_info that prints the brand and the number of wheels

Create an instance of Car with brand "Toyota"

Call the display_info method on the instance

💡 Why This Matters

🌍 Real World

Constants in classes are useful to store values that do not change, like fixed settings or properties shared by all objects of that class.

💼 Career

Understanding constants in classes helps you write clear and maintainable code, which is important in software development jobs.

1

Create the Car class with a brand attribute

Create a class called Car with an initialize method that takes a parameter brand and saves it in an instance variable @brand.

Ruby

class Car
  # Your code here
end

Hint

Remember to use def initialize(brand) and assign @brand = brand.

2

Add a constant WHEELS with value 4

Inside the Car class, add a constant called WHEELS and set it to 4.

Ruby

class Car
  WHEELS = 4

  def initialize(brand)
    @brand = brand
  end
end
# Your code here

Hint

Constants in Ruby start with a capital letter. Write WHEELS = 4 inside the class.

3

Add a method to display brand and wheels

Add a method called display_info inside the Car class that prints the brand and the number of wheels using puts. Use @brand for the brand and Car::WHEELS to access the constant.

Ruby

class Car
  WHEELS = 4

  def initialize(brand)
    @brand = brand
  end

  # Your code here
end

Hint

Use def display_info and inside it write puts "Brand: #{@brand}, Wheels: #{Car::WHEELS}".

4

Create a Car instance and call display_info

Create a variable my_car and assign it a new Car object with brand "Toyota". Then call my_car.display_info to print the information.

Ruby

class Car
  WHEELS = 4

  def initialize(brand)
    @brand = brand
  end

  def display_info
    puts "Brand: #{@brand}, Wheels: #{Car::WHEELS}"
  end
end

# Your code here

Hint

Create my_car = Car.new("Toyota") and then call my_car.display_info.

Practice

1. What is true about constants in Ruby classes?

easy

A. Constants start with a lowercase letter.

B. Constants can be changed anytime like variables.

C. Constants start with a capital letter and are defined outside methods.

D. Constants must be defined inside methods only.

Solution

1. Step 1: Understand constant naming rules

   In Ruby, constants always start with a capital letter and are usually defined outside methods within classes or modules.

2. Step 2: Check where constants are defined

   Constants are not meant to be changed and are not defined inside methods but at the class/module level.

3. Final Answer:

   Constants start with a capital letter and are defined outside methods. -> Option C

4. Quick Check:

   Constants = Capital letter + outside methods [OK]

Hint: Constants start uppercase and live outside methods [OK]

Common Mistakes:

• Thinking constants can be changed like variables
• Defining constants inside methods
• Using lowercase letters for constants

2. Which of the following is the correct way to define a constant inside a Ruby class?

easy

A. class MyClass
def PI = 3.14
end

B. class MyClass
PI = 3.14
end

C. class MyClass
pi = 3.14
end

D. class MyClass
@PI = 3.14
end

Solution

1. Step 1: Identify constant syntax

   Constants in Ruby start with a capital letter and are assigned directly inside the class but outside any method.

2. Step 2: Check each option

   class MyClass PI = 3.14 end correctly assigns PI = 3.14 inside the class. class MyClass pi = 3.14 end uses lowercase (not constant). class MyClass def PI = 3.14 end tries to define a method named PI incorrectly. class MyClass @PI = 3.14 end uses an instance variable (@PI), not a constant.

3. Final Answer:

   class MyClass PI = 3.14 end -> Option B

4. Quick Check:

   Constants = Capital letter + direct assignment [OK]

Hint: Constants = Capital letter + direct assignment inside class [OK]

Common Mistakes:

• Using lowercase names for constants
• Defining constants inside methods
• Confusing constants with instance variables

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

class Car
  WHEELS = 4
  def wheels
    WHEELS
  end
end

car = Car.new
puts car.wheels

medium

A. 4

B. WHEELS

C. Error: uninitialized constant

D. nil

Solution

1. Step 1: Understand constant access inside instance method

   The constant WHEELS is defined inside the class Car. Inside instance methods, constants can be accessed directly by their name.

2. Step 2: Trace the method call

   Calling car.wheels returns the constant WHEELS value, which is 4. Then puts prints 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Constant accessed inside method = 4 [OK]

Hint: Constants inside class accessible by name in instance methods [OK]

Common Mistakes:

• Expecting an error for constant access
• Thinking constant name prints instead of value
• Confusing constants with instance variables

4. Identify the error in this Ruby code:

class Book
  TITLE = "Ruby Guide"
  def change_title
    TITLE = "New Title"
  end
end

medium

A. Constants must be defined inside methods.

B. Missing end keyword for class.

C. TITLE should be lowercase.

D. Cannot assign to constant TITLE inside method.

Solution

1. Step 1: Check constant assignment rules

   Constants in Ruby should not be reassigned once set. Assigning a constant inside a method is not allowed and causes a warning or error.

2. Step 2: Analyze the code

   The method change_title tries to assign a new value to TITLE constant, which is invalid.

3. Final Answer:

   Cannot assign to constant TITLE inside method. -> Option D

4. Quick Check:

   Constants cannot be reassigned inside methods [OK]

Hint: Constants cannot be reassigned inside methods [OK]

Common Mistakes:

• Trying to change constants inside methods
• Confusing constants with variables
• Ignoring Ruby warnings about constant reassignment

5. Given this Ruby code, what will puts Vehicle::TYPE print?

class Vehicle
  TYPE = "Transport"
  class Car
    TYPE = "Car"
  end
end

puts Vehicle::TYPE

hard

A. "Transport"

B. "Car"

C. Error: uninitialized constant

D. nil

Solution

1. Step 1: Understand constant lookup in nested classes

   Constants are scoped to the class they are defined in. Vehicle::TYPE refers to the constant TYPE inside Vehicle, not inside Vehicle::Car.

2. Step 2: Check the code output

   Vehicle::TYPE is "Transport". The inner class Car has its own TYPE, but it is not accessed here.

3. Final Answer:

   "Transport" -> Option A

4. Quick Check:

   Outer class constant accessed by Vehicle::TYPE = "Transport" [OK]

Hint: Outer class constant accessed by ClassName::CONSTANT [OK]

Common Mistakes:

• Confusing inner class constant with outer class constant
• Expecting error for nested constants
• Using wrong scope operator