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

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

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

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

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

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

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.

Practice

1. What is a key characteristic of Ruby class variables (@@)?

easy

A. They are unique to each instance of a class.

B. They are shared among a class and all its subclasses.

C. They are only accessible inside instance methods.

D. They behave like local variables inside methods.

Solution

1. Step 1: Understand class variable scope

   Class variables (@@) are shared across a class and all its subclasses, meaning they hold the same value for all related classes.

2. Step 2: Compare with other variable types

   Instance variables are unique per object, local variables are method-specific, so they don't share data like class variables do.

3. Final Answer:

   They are shared among a class and all its subclasses. -> Option B

4. Quick Check:

   Class variables share data across class hierarchy [OK]

Hint: Remember @@ means shared across class and subclasses [OK]

Common Mistakes:

• Confusing class variables with instance variables
• Thinking @@ variables are local to instances
• Assuming @@ variables are private to one class

2. Which of the following is the correct syntax to define and use a class variable in Ruby?

easy

A. class MyClass
@@count = 0
end

B. class MyClass
@count = 0
end

C. class MyClass
$count = 0
end

D. class MyClass
count = 0
end

Solution

1. Step 1: Identify class variable syntax

   Class variables start with @@, so @@count = 0 is correct for defining a class variable.

2. Step 2: Differentiate other variable types

   @count is an instance variable, $count is a global variable, and count alone is a local variable.

3. Final Answer:

   class MyClass @@count = 0 end -> Option A

4. Quick Check:

   Class variable syntax uses @@ [OK]

Hint: Class variables always start with @@ [OK]

Common Mistakes:

• Using @ instead of @@ for class variables
• Confusing global variables ($) with class variables
• Defining variables without any prefix

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

class Parent
  @@value = 1
  def self.value
    @@value
  end
  def self.value=(v)
    @@value = v
  end
end

class Child < Parent
end

Parent.value = 5
puts Child.value

medium

A. 5

B. Error

C. nil

D. 1

Solution

1. Step 1: Understand class variable sharing

   The class variable @@value is shared between Parent and Child classes.

2. Step 2: Trace the assignment and output

   Setting Parent.value = 5 changes @@value to 5 for both classes. So, Child.value returns 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Class variables shared, so Child.value = 5 [OK]

Hint: Class variables share values across subclasses [OK]

Common Mistakes:

• Assuming Child has its own separate @@value
• Expecting default value 1 after change
• Confusing class variables with instance variables

4. Identify the problem in this Ruby code using class variables:

class A
  @@count = 0
  def initialize
    @@count += 1
  end
  def self.count
    @@count
  end
end

class B < A
  def initialize
    @@count += 10
  end
end

A.new
B.new
puts A.count

medium

A. The code has a syntax error in class B definition.

B. The class variable @@count is not initialized properly.

C. The class variable @@count is shared and modified differently causing unexpected count.

D. The method self.count is missing a return statement.

Solution

1. Step 1: Analyze class variable usage in inheritance

   Both classes A and B share the same @@count. A#initialize adds 1, B#initialize adds 10.

2. Step 2: Understand the effect on count

   Creating A.new adds 1, then B.new adds 10, so total @@count becomes 11, which may be unexpected if counting instances separately.

3. Final Answer:

   The class variable @@count is shared and modified differently causing unexpected count. -> Option C

4. Quick Check:

   Shared @@count causes tricky bugs [OK]

Hint: Shared @@ variables cause unexpected cross-class changes [OK]

Common Mistakes:

• Thinking @@count is separate per class
• Ignoring inheritance effect on @@ variables
• Assuming initialize methods don't affect shared vars

5. You want to track the number of instances created separately for two subclasses without interference. Which approach avoids the dangers of class variables (@@)?

hard

A. Use a local variable inside the initialize method to count instances.

B. Use a global variable ($count) shared by all classes.

C. Use class variables (@@count) but reset them in each subclass initialize.

D. Use class instance variables (@count) in each subclass instead of @@count.

Solution

1. Step 1: Understand the problem with class variables

   Class variables (@@count) are shared across subclasses, causing counts to mix and produce wrong totals.

2. Step 2: Use class instance variables for separate counts

   Class instance variables (@count) belong to each class separately, so each subclass can track its own count safely.

3. Final Answer:

   Use class instance variables (@count) in each subclass instead of @@count. -> Option D

4. Quick Check:

   Class instance vars avoid shared @@ dangers [OK]

Hint: Use @count per class to avoid shared @@count bugs [OK]

Common Mistakes:

• Using global variables which mix counts globally
• Trying to reset @@count which is error-prone
• Using local variables which don't persist counts