Class variables (@@) and their dangers in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class variable @@count initialized to 0.

Ruby

class Example
  [1] = 0
end

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Acount

B@count

C$count

D@@count

Attempts:

3 left

💡 Hint

Common Mistakes

Using @count instead of @@count

Using $count which is a global variable

Not using any symbol before count

Class variables in Ruby start with @@. So @@count is the correct way to define it.

2fill in blank

medium

Complete the method to increase the class variable @@count by 1.

Ruby

class Example
  @@count = 0
  def self.increment
    [1]
  end
end

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A@@count += 1

B@count += 1

Ccount += 1

D$count += 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using @count which is an instance variable

Using $count which is a global variable

Forgetting to use += 1

To increase the class variable, use @@count += 1. Instance or global variables won't work here.

3fill in blank

hard

Fix the error in accessing the class variable inside an instance method.

Ruby

class Example
  @@count = 0
  def increment
    [1]
  end
end

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A@@count += 1

Bself.count += 1

Ccount += 1

D@count += 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using @count which is an instance variable

Trying to call self.count without a method

Using count without any prefix

Class variables must be accessed with @@ even inside instance methods.

4fill in blank

hard

Fill both blanks to show how class variables are shared across subclasses.

Ruby

class Parent
  @@count = 0
  def self.increment
    @@count [1] 1
  end
  def self.count
    @@count
  end
end

class Child < Parent
  def self.increment_child
    @@count [2] 1
  end
end

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A+=

B-=

C*=

D/=

Attempts:

3 left

💡 Hint

Common Mistakes

Using -= which subtracts instead of adds

Using *= or /= which change the value differently

Class variables are shared among a class and its subclasses, so both increments use += to add 1.

5fill in blank

hard

Fill both blanks to demonstrate the danger of class variables being shared unexpectedly.

Ruby

class Parent
  @@value = 10
  def self.value
    @@value
  end
  def self.value=(val)
    @@value = val
  end
end

class Child < Parent
  def self.change_value
    @@value [1] 20
  end
end

Parent.value = 15
Child.change_value
puts Parent.value  # Output: [2]

Drag options to blanks, or click blank then click option'

A=

B+=

C-

D20

Attempts:

3 left

💡 Hint

Common Mistakes

Using += instead of = changes value differently

Expecting Parent.value to stay 15 after Child changes it

Predicting the wrong output (e.g. 15)

Changing @@value in Child changes it for Parent too, because class variables are shared. So Parent.value becomes 20.