Instance variables (@) in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using instance variables?

Consider the following Ruby class and code. What will be printed when this code runs?

Ruby

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

  def speak
    "Woof! My name is #{@name}."
  end
end

fido = Dog.new("Fido")
puts fido.speak

A Woof! My name is @name.

B Woof! My name is .

C Error: undefined method `speak' for nil:NilClass

D Woof! My name is Fido.

💡 Hint

Instance variables start with @ and belong to the object instance.

The instance variable @name is set in initialize and used in speak. It holds the value 'Fido', so the output includes the name.

❓ Predict Output

intermediate

What is the value of @count after running this Ruby code?

Look at this Ruby class that uses an instance variable @count. What will be the value of @count for the object after these calls?

Ruby

class Counter
  def initialize
    @count = 0
  end

  def increment
    @count += 1
  end

  def current
    @count
  end
end

c = Counter.new
c.increment
c.increment
c.current

A 2

B 1

C 0

D nil

💡 Hint

Each call to increment adds 1 to @count.

The @count starts at 0, then increment is called twice, adding 1 each time, so @count becomes 2.

🔧 Debug

advanced

Why does this Ruby code not print the name when accessing @name?

This Ruby code tries to print @name but does not print the name. Why? What is the output?

Ruby

class Person
  def initialize(name)
    name = name
  end

  def greet
    "Hello, my name is #{@name}."
  end
end

p = Person.new("Alice")
puts p.greet

A Output: Hello, my name is . (nil interpolated as empty string)

B Output: Hello, my name is Alice.

C Output: Hello, my name is @name.

D Output: Hello, my name is . (empty string)

💡 Hint

Check how the instance variable @name is assigned in initialize.

The code assigns local variable 'name' to itself, not to @name. So @name is never set and is nil. Interpolating nil in a string produces an empty string.

🧠 Conceptual

advanced

What happens to instance variables when you create multiple objects?

If you create two objects from the same Ruby class, each with different values for an instance variable @value, what will happen when you access @value on each object?

A Instance variables are global, so @value is shared across all objects and classes.

B Ruby raises an error if two objects have instance variables with the same name.

C Each object has its own separate @value, so they can hold different values independently.

D Both objects share the same @value, so changing one changes the other.

💡 Hint

Think about how instance variables belong to each object instance.

Instance variables belong to each object separately. Each object keeps its own copy of @value, so they do not affect each other.

❓ Predict Output

expert

What is the output of this Ruby code using instance variables and inheritance?

Analyze this Ruby code with inheritance and instance variables. What will be printed?

Ruby

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

  def speak
    "I am an animal named #{@name}."
  end
end

class Cat < Animal
  def speak
    super + " Meow!"
  end
end

kitty = Cat.new("Whiskers")
puts kitty.speak

A I am a cat named Whiskers. Meow!

B I am an animal named Whiskers. Meow!

C I am an animal named . Meow!

D Error: undefined method `speak' for Cat

💡 Hint

The Cat class calls super to use the Animal's speak method and adds its own text.

The Cat class inherits from Animal. The speak method calls super, which returns 'I am an animal named Whiskers.', then adds ' Meow!'. So the full output is 'I am an animal named Whiskers. Meow!'.

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