Instance_variable_get and set in Ruby - Mini Project: Build & Apply

Using instance_variable_get and instance_variable_set in Ruby

📖 Scenario: Imagine you have a simple Ruby class representing a book. You want to access and change the book's title and author dynamically using special Ruby methods.

🎯 Goal: Learn how to use instance_variable_get and instance_variable_set to read and update instance variables of an object.

📋 What You'll Learn

Create a class Book with instance variables @title and @author

Create an object of class Book with specific title and author

Use instance_variable_get to read the @title variable

Use instance_variable_set to change the @author variable

Print the updated author name

💡 Why This Matters

🌍 Real World

Sometimes you need to access or change object data dynamically when you don't know the variable names in advance, like in debugging tools or meta-programming.

💼 Career

Understanding how to manipulate instance variables dynamically helps in advanced Ruby programming, frameworks, and libraries that use meta-programming.

1

Create the Book class with instance variables

Define a class called Book with an initialize method that sets instance variables @title and @author using parameters title and author. Then create an object called my_book with title "Ruby Basics" and author "Alice".

Ruby

# Define class Book with initialize method
# Create my_book with title "Ruby Basics" and author "Alice"
# Your code here

Hint

Use def initialize(title, author) and set @title and @author inside it. Then create my_book with Book.new.

2

Read the @title instance variable using instance_variable_get

Use instance_variable_get on my_book to read the value of the @title instance variable and store it in a variable called book_title.

Ruby

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

my_book = Book.new("Ruby Basics", "Alice")

# Use instance_variable_get to read @title into book_title
# Your code here

Hint

Call my_book.instance_variable_get(:@title) and assign it to book_title.

3

Change the @author instance variable using instance_variable_set

Use instance_variable_set on my_book to change the @author instance variable to "Bob".

Ruby

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

my_book = Book.new("Ruby Basics", "Alice")

book_title = my_book.instance_variable_get(:@title)

# Use instance_variable_set to change @author to "Bob"
# Your code here

Hint

Call my_book.instance_variable_set(:@author, "Bob") to update the author.

4

Print the updated author name

Use instance_variable_get on my_book to get the updated @author and print it.

Ruby

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

my_book = Book.new("Ruby Basics", "Alice")

book_title = my_book.instance_variable_get(:@title)

my_book.instance_variable_set(:@author, "Bob")

# Print the updated author
# Your code here

Hint

Use puts my_book.instance_variable_get(:@author) to print the new author.

Practice

1. What does the Ruby method instance_variable_get do?

easy

A. It reads the value of an instance variable by its name.

B. It deletes an instance variable from an object.

C. It creates a new instance variable with a given name.

D. It lists all instance variables of an object.

Solution

1. Step 1: Understand the method purpose

   instance_variable_get is used to access the value of an instance variable by passing its name as a symbol or string.

2. Step 2: Compare with other options

   It does not delete, create, or list variables; it only reads the value.

3. Final Answer:

   It reads the value of an instance variable by its name. -> Option A

4. Quick Check:

   instance_variable_get reads variable value [OK]

Hint: Remember: get = read value of variable [OK]

Common Mistakes:

• Confusing get with set (changing value)
• Thinking it deletes variables
• Assuming it lists all variables

2. Which of the following is the correct syntax to set an instance variable @name to "Alice" using instance_variable_set?

easy

A. object.instance_variable_set(:nam, "Alice")

B. object.instance_variable_set("nam", "Alice")

C. object.instance_variable_set(@name, "Alice")

D. object.instance_variable_set(:@name, "Alice")

Solution

1. Step 1: Understand the argument format

   The first argument to instance_variable_set must be the instance variable name as a symbol or string, including the '@' prefix.

2. Step 2: Check each option

   A: object.instance_variable_set(:nam, "Alice") misses '@'. B: object.instance_variable_set("nam", "Alice") misses '@'. C: passes a variable instead of symbol/string. D: uses :@name symbol correctly.

3. Final Answer:

   object.instance_variable_set(:@name, "Alice") -> Option D

4. Quick Check:

   Use symbol with '@' prefix for setting [OK]

Hint: Use symbol with '@' prefix to set variable [OK]

Common Mistakes:

• Omitting '@' in variable name
• Passing variable instead of symbol/string
• Using string without '@' prefix

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

class Person
  def initialize(name)
    @name = name
  end
end

p = Person.new("Bob")
puts p.instance_variable_get(:@name)

medium

A. Bob

B. nil

C. @name

D. Error

Solution

1. Step 1: Understand instance variable initialization

   The @name variable is set to "Bob" in the constructor.

2. Step 2: Use instance_variable_get to read value

   Calling instance_variable_get(:@name) returns the value "Bob" stored in @name.

3. Final Answer:

   Bob -> Option A

4. Quick Check:

   instance_variable_get returns stored value [OK]

Hint: instance_variable_get returns actual stored value [OK]

Common Mistakes:

• Expecting variable name instead of value
• Confusing with nil if variable not set
• Thinking it raises error if variable exists

4. Identify the issue in this Ruby code snippet:

class Car
  def initialize
    @speed = 0
  end

  def set_speed(value)
    instance_variable_set(:speed, value)
  end
end

car = Car.new
car.set_speed(50)
puts car.instance_variable_get(:@speed)

medium

A. The variable @speed is not initialized.

B. Cannot call instance_variable_set inside a method.

C. Missing '@' in instance_variable_set argument name.

D. instance_variable_get should use string, not symbol.

Solution

1. Step 1: Check instance_variable_set argument

   The argument to instance_variable_set must include '@' prefix to identify the instance variable.

2. Step 2: Identify the error

   Here, :speed is missing '@', so Ruby will not find the variable and will create a new one without '@', causing unexpected behavior.

3. Final Answer:

   Missing '@' in instance_variable_set argument name. -> Option C

4. Quick Check:

   Always include '@' in variable name for instance_variable_set [OK]

Hint: Always prefix variable name with '@' in set/get methods [OK]

Common Mistakes:

• Omitting '@' in variable name
• Thinking instance_variable_set can't be used in methods
• Confusing symbol and string usage

5. Given a class Box with private instance variable @content, how can you change its value to "Books" from outside the class using instance_variable_set?

hard

A. box.instance_variable_set(:conten, "Books")

B. box.instance_variable_set(:@content, "Books")

C. box.@content = "Books"

D. box.set_content("Books")

Solution

1. Step 1: Understand private variable access

   Private instance variables cannot be accessed or changed directly from outside the class using normal assignment.

2. Step 2: Use instance_variable_set with correct name

   Using instance_variable_set with the symbol including '@' allows changing the private variable from outside.

3. Step 3: Check options

   box.instance_variable_set(:@content, "Books") correctly uses :@content. box.instance_variable_set(:conten, "Books") misses '@', box.@content = "Books" is invalid syntax, box.set_content("Books") assumes a setter method which may not exist.

4. Final Answer:

   box.instance_variable_set(:@content, "Books") -> Option B

5. Quick Check:

   Use instance_variable_set with '@' to change private vars [OK]

Hint: Use '@' prefix symbol to set private variables externally [OK]

Common Mistakes:

• Omitting '@' in variable name
• Trying direct assignment to private variable
• Assuming setter method exists