Instance methods in Ruby - Mini Project: Build & Apply

Instance methods

📖 Scenario: You are creating a simple program to manage a book's information. You want to store the book's title and author, and then display this information using a method inside the book object.

🎯 Goal: Build a Ruby class called Book that stores a book's title and author. Then, create an instance method description that returns a string describing the book.

📋 What You'll Learn

Create a class named Book with an initialize method that takes title and author as parameters.

Store title and author as instance variables.

Define an instance method called description that returns a string in the format: "Title: [title], Author: [author]".

Create an instance of Book with the title "The Ruby Way" and author "Hal Fulton".

Call the description method on the instance and print the result.

💡 Why This Matters

🌍 Real World

Classes and instance methods are used to model real-world objects in programs, like books, users, or products, making code organized and reusable.

💼 Career

Understanding instance methods is essential for Ruby developers building applications, as it helps create clear and maintainable code with object-oriented design.

1

Create the Book class with initialize method

Create a class called Book with an initialize method that takes two parameters: title and author. Inside initialize, assign @title to title and @author to author.

Ruby

# Define the Book class with initialize method
# Your code here

Hint

Remember, initialize is the method that runs when you create a new object. Use @ to create instance variables.

2

Add the description instance method

Inside the Book class, add an instance method called description that returns a string formatted as: "Title: [title], Author: [author]" using the instance variables @title and @author.

Ruby

class Book
  def initialize(title, author)
    @title = title
    @author = author
  end

  # Define the description method
  # Your code here
end

Hint

Use string interpolation with #{} to include instance variables inside the string.

3

Create a Book instance

Create a variable called my_book and assign it a new instance of Book with the title "The Ruby Way" and author "Hal Fulton".

Ruby

class Book
  def initialize(title, author)
    @title = title
    @author = author
  end

  def description
    "Title: #{@title}, Author: #{@author}"
  end
end

# Create an instance of Book called my_book
# Your code here

Hint

Use Book.new with the exact title and author strings to create the instance.

4

Print the book description

Call the description method on my_book and print the result using puts.

Ruby

class Book
  def initialize(title, author)
    @title = title
    @author = author
  end

  def description
    "Title: #{@title}, Author: #{@author}"
  end
end

my_book = Book.new("The Ruby Way", "Hal Fulton")

# Print the description of my_book
# Your code here

Hint

Use puts my_book.description to print the description string.

Practice

1.

What is an instance method in Ruby?

easy

A. A method that belongs to an object created from a class

B. A method that belongs to the class itself, not objects

C. A method that runs only once when the program starts

D. A method that cannot be called on any object

Solution

1. Step 1: Understand what instance methods are

   Instance methods are defined inside a class and belong to objects created from that class.

2. Step 2: Differentiate from class methods

   Class methods belong to the class itself, not to individual objects.

3. Final Answer:

   A method that belongs to an object created from a class -> Option A

4. Quick Check:

   Instance method = belongs to object [OK]

Hint: Instance methods belong to objects, not the class itself [OK]

Common Mistakes:

• Confusing instance methods with class methods
• Thinking instance methods run only once
• Believing instance methods cannot be called on objects

2.

Which of the following is the correct way to define an instance method greet inside a Ruby class Person?

class Person
  ?
end

easy

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

B. def greet; puts 'Hello'; end

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

D. def greet() puts 'Hello' end

Solution

1. Step 1: Identify instance method syntax

   Instance methods are defined with def method_name inside the class without self..

2. Step 2: Check options

   def greet; puts 'Hello'; end uses correct syntax. def self.greet; puts 'Hello'; end and def Person.greet; puts 'Hello'; end define class methods. def greet() puts 'Hello' end misses semicolons or line breaks, causing syntax issues.

3. Final Answer:

   def greet; puts 'Hello'; end -> Option B

4. Quick Check:

   Instance method = def method_name [OK]

Hint: Instance methods use def without self. [OK]

Common Mistakes:

• Using self. which defines class methods
• Omitting semicolons or line breaks incorrectly
• Defining methods outside the class

3.

What will be the output of the following Ruby code?

class Dog
  def bark
    'Woof!'
  end
end

fido = Dog.new
puts fido.bark

medium

A. Dog

B. bark

C. Woof!

D. NoMethodError

Solution

1. Step 1: Understand the class and method

   The class Dog has an instance method bark that returns the string 'Woof!'.

2. Step 2: Trace the method call

   An object fido is created from Dog, then fido.bark calls the bark method, returning 'Woof!'.

3. Final Answer:

   Woof! -> Option C

4. Quick Check:

   Calling instance method returns 'Woof!' [OK]

Hint: Instance method returns its string when called on object [OK]

Common Mistakes:

• Expecting method name instead of return value
• Confusing class name with output
• Thinking method is undefined

4.

Find the error in this Ruby code and select the fix that will make it work:

class Cat
  def meow
    puts 'Meow!'
  end
end

kitty = Cat.new
kitty.meow()

medium

A. Remove parentheses from kitty.meow() to kitty.meow

B. Change def meow to def self.meow

C. Add return before puts in method

D. No error; code works as is

Solution

1. Step 1: Check method definition and call

   The method meow is defined correctly as an instance method and called on an object.

2. Step 2: Verify syntax and usage

   Using parentheses in method call is allowed in Ruby, so kitty.meow() is valid.

3. Final Answer:

   No error; code works as is -> Option D

4. Quick Check:

   Parentheses optional in method calls [OK]

Hint: Parentheses optional in Ruby method calls [OK]

Common Mistakes:

• Thinking parentheses cause errors
• Confusing instance and class methods
• Adding unnecessary return before puts

5.

Given the class below, what will puts user.full_name print?

class User
  def initialize(first, last)
    @first_name = first
    @last_name = last
  end

  def full_name
    "#{@first_name} #{@last_name}".upcase
  end
end

user = User.new('Jane', 'Doe')

hard

A. JANE DOE

B. Error: undefined method full_name

C. "Jane Doe"

D. Jane Doe

Solution

1. Step 1: Understand instance variables and method

   The initialize method sets @first_name and @last_name. The full_name method returns their combination in uppercase.

2. Step 2: Trace the method call

   Calling user.full_name returns "JANE DOE" because of the upcase method.

3. Final Answer:

   JANE DOE -> Option A

4. Quick Check:

   full_name returns uppercase full name [OK]

Hint: String interpolation with upcase changes output to uppercase [OK]

Common Mistakes:

• Ignoring upcase method effect
• Expecting original case output
• Confusing string quotes in output