Instance variables (@) in Ruby - Mini Project: Build & Apply

Using Instance Variables (@) in Ruby Classes

📖 Scenario: Imagine you are creating a simple program to keep track of a book's details in a library system. Each book has a title and an author.

🎯 Goal: You will build a Ruby class called Book that uses instance variables to store the title and author of a book. Then, you will create an object of this class and display its details.

📋 What You'll Learn

Create a class named Book

Use instance variables @title and @author inside the class

Write an initialize method to set these instance variables

Create a method details that returns a string with the book's title and author

Create an object of Book with exact title 'The Ruby Way' and author 'Hal Fulton'

Print the details of the book object

💡 Why This Matters

🌍 Real World

Instance variables are used in real-world programs to store information unique to each object, like details of books, users, or products.

💼 Career

Understanding instance variables is essential for Ruby developers working on web applications, automation scripts, or any object-oriented programming tasks.

1

Create the Book class with an initialize method

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

Ruby

class Book
  def initialize(title, author)
    # Your code here
  end
end

Hint

Remember, instance variables start with @ and store data inside the object.

2

Add a method details to return book information

Inside the Book class, add a method called details that returns a string in the format: "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 details method below
  # Your code here
end

Hint

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

3

Create a Book object with given title and author

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

Ruby

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

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

# Create the my_book object below
# Your code here

Hint

Use ClassName.new(arguments) to create a new object.

4

Print the details of the my_book object

Write a puts statement to print the result of calling the details method on the my_book object.

Ruby

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

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

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

# Print the book details below
# Your code here

Hint

Use puts object.method to print the returned string.

Practice

1. What does an instance variable in Ruby always start with?

easy

A. An at symbol (@)

B. A dollar sign ($)

C. A hash symbol (#)

D. A percent sign (%)

Solution

1. Step 1: Recall Ruby instance variable syntax

   Instance variables in Ruby always begin with the at symbol (@) to distinguish them from local variables.

2. Step 2: Identify the correct symbol

   Among the options, only the at symbol (@) is used to start instance variables.

3. Final Answer:

   An at symbol (@) -> Option A

4. Quick Check:

   Instance variable starts with @ [OK]

Hint: Instance variables always start with @ in Ruby [OK]

Common Mistakes:

• Confusing instance variables with global variables ($)
• Using local variable syntax without @
• Thinking instance variables start with # or %

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

easy

A. var @name = 'Alice'

B. @name = 'Alice'

C. name = @'Alice'

D. def @name = 'Alice'

Solution

1. Step 1: Understand instance variable assignment syntax

   In Ruby, instance variables are assigned by writing @variable_name = value inside methods or constructors.

2. Step 2: Check each option

   @name = 'Alice' correctly assigns an instance variable. The other options use invalid syntax.

3. Final Answer:

   @name = 'Alice' -> Option B

4. Quick Check:

   Assign instance variable with @name = value [OK]

Hint: Assign instance variables with @name = value inside methods [OK]

Common Mistakes:

• Adding var keyword (not used in Ruby)
• Placing @ inside quotes
• Using def keyword incorrectly

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

class Person
  def initialize(name)
    @name = name
  end
  def greet
    "Hello, #{@name}!"
  end
end

p = Person.new('Bob')
puts p.greet

medium

A. Error: undefined variable

B. Hello, @name!

C. Hello, name!

D. Hello, Bob!

Solution

1. Step 1: Understand instance variable assignment in initialize

   The initialize method sets @name to 'Bob' when Person.new('Bob') is called.

2. Step 2: Evaluate greet method output

   The greet method returns a string with #{@name} replaced by the instance variable value 'Bob'.

3. Final Answer:

   Hello, Bob! -> Option D

4. Quick Check:

   Instance variable @name holds 'Bob' so output is Hello, Bob! [OK]

Hint: Instance variables keep data per object, used inside methods with @ [OK]

Common Mistakes:

• Expecting literal '@name' in output
• Confusing local and instance variables
• Forgetting to use @ in string interpolation

4. Identify the error in this Ruby class code:

class Car
  def initialize(model)
    name = model
  end
  def show
    puts @name
  end
end

car = Car.new('Tesla')
car.show

medium

A. Syntax error in initialize method

B. Method show is missing return statement

C. Instance variable @name is not assigned, so nil is printed

D. Cannot call puts inside method

Solution

1. Step 1: Check variable assignment in initialize

   The code assigns local variable name = model, not instance variable @name.

2. Step 2: Understand effect on show method

   Since @name is never set, @name is nil, so puts prints an empty line (nil).

3. Final Answer:

   Instance variable @name is not assigned, so nil is printed -> Option C

4. Quick Check:

   Local variable assigned, not instance variable @name [OK]

Hint: Use @ to assign instance variables, not plain variable names [OK]

Common Mistakes:

• Assigning local variable instead of instance variable
• Expecting puts to fail without return
• Thinking syntax error occurs here

5. You want to create a Ruby class Book that stores a title and author as instance variables. You also want a method description that returns "Title by Author". Which code correctly implements this?

hard

A. class Book
def initialize(title, author)
@title = title
@author = author
end
def description
"#{@title} by #{@author}"
end
end

B. class Book
def initialize(title, author)
title = @title
author = @author
end
def description
"#{title} by #{author}"
end
end

C. class Book
def initialize(title, author)
@title = title
@author = author
end
def description
"title by author"
end
end

D. class Book
def initialize(title, author)
@title = title
@author = author
end
def description
"#{title} by #{author}"
end
end

Solution

1. Step 1: Assign instance variables correctly in initialize

   class Book def initialize(title, author) @title = title @author = author end def description "#{@title} by #{@author}" end end assigns @title and @author from parameters correctly. class Book def initialize(title, author) title = @title author = @author end def description "#{title} by #{author}" end end reverses assignment. The remaining options misuse variables in description.

2. Step 2: Use instance variables in description method

   class Book def initialize(title, author) @title = title @author = author end def description "#{@title} by #{@author}" end end uses #{@title} and #{@author} inside string interpolation, correctly accessing instance variables.

3. Final Answer:

   class Book def initialize(title, author) @title = title @author = author end def description "#{@title} by #{@author}" end end -> Option A

4. Quick Check:

   Assign and use @variables inside methods for correct output [OK]

Hint: Assign and use instance variables with @ inside all methods [OK]

Common Mistakes:

• Assigning variables backwards (param = @var)
• Using local variables instead of instance variables in methods
• Returning literal strings without interpolation