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!'.