Class instance variables as alternative in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class instance variable in Ruby.

Ruby

class MyClass
  [1] = 10
end

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

A@@value

B@@count

C@value

D@count

Attempts:

3 left

💡 Hint

Common Mistakes

Using '@@' instead of '@' for class instance variables.

Defining variables inside methods instead of the class body.

Class instance variables start with a single @ inside the class body but outside any method.

2fill in blank

medium

Complete the code to access a class instance variable inside a class method.

Ruby

class MyClass
  @count = 5

  def self.get_count
    [1]
  end
end

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

A@count

B@@count

Cself.@count

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using '@@count' which refers to class variables, not class instance variables.

Trying to use 'self.@count' which is invalid syntax.

Inside a class method, you can access class instance variables directly with @variable.

3fill in blank

hard

Fix the error in the code to correctly initialize and access a class instance variable.

Ruby

class MyClass
  [1] = 0

  def self.get_value
    @value
  end
end

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

A@value

B@@value

Cself.@value

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Initializing class instance variables inside instance methods.

Using '@@' for class instance variables.

To initialize a class instance variable, assign it with '@value' inside the class body or class methods, not inside instance methods like initialize.

4fill in blank

hard

Fill both blanks to create a class instance variable and a class method that returns it.

Ruby

class MyClass
  [1] = 100

  def self.get_value
    [2]
  end
end

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

A@value

B@@value

D@@count

Attempts:

3 left

💡 Hint

Common Mistakes

Using '@@' instead of '@' for class instance variables.

Trying to access class instance variables with wrong syntax.

Class instance variables use a single '@' and can be accessed inside class methods with '@variable'.

5fill in blank

hard

Fill all three blanks to define a class instance variable, a class method to set it, and a class method to get it.

Ruby

class MyClass
  [1] = nil

  def self.set_value(val)
    [2] = val
  end

  def self.get_value
    [3]
  end
end

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

A@value

D@@value

Attempts:

3 left

💡 Hint

Common Mistakes

Using '@@value' instead of '@value'.

Trying to set class instance variables inside instance methods.

Class instance variables use '@value'. You can set and get them inside class methods using '@value'.

Practice

1. What is the main purpose of using class instance variables in Ruby?

easy

A. To store data shared by the class itself, not by individual objects

B. To store data unique to each object instance

C. To define methods that all instances can use

D. To create global variables accessible everywhere

Solution

1. Step 1: Understand class instance variables

   Class instance variables belong to the class object itself, not to each instance.

2. Step 2: Compare with instance variables

   Instance variables store data unique to each object, while class instance variables store data unique to the class object, not shared with instances.

3. Final Answer:

   To store data shared by the class itself, not by individual objects -> Option A

4. Quick Check:

   Class instance variables = data for the class object [OK]

Hint: Class instance variables hold data for the class, not objects [OK]

Common Mistakes:

• Confusing class instance variables with instance variables
• Thinking class instance variables are global variables
• Assuming class instance variables are shared with subclasses automatically

2. Which of the following is the correct way to define a class instance variable in Ruby?

easy

A. def @count; @count; end

B. @count = 0

C. @@count = 0

D. self.@count = 0

Solution

1. Step 1: Identify class instance variable syntax

   Class instance variables start with a single @ and are defined inside the class but outside methods.

2. Step 2: Check each option

   @count = 0 uses @count = 0 correctly inside the class body. @@count = 0 uses class variables (@@), which are different. def @count; @count; end is invalid method syntax. self.@count = 0 is invalid syntax.

3. Final Answer:

   @count = 0 -> Option B

4. Quick Check:

   Class instance variable = single @ [OK]

Hint: Class instance variables use single @ inside class body [OK]

Common Mistakes:

• Using @@ instead of @ for class instance variables
• Trying to use self.@ which is invalid
• Defining variables inside methods without self

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

class Dog
  @count = 0

  def self.increment
    @count += 1
  end

  def self.count
    @count
  end
end

Dog.increment
Dog.increment
puts Dog.count

medium

A. Error

B. 0

C. nil

D. 2

Solution

1. Step 1: Understand class instance variable usage

   @count is a class instance variable initialized to 0. The class methods increment and count access it.

2. Step 2: Trace method calls

   Calling Dog.increment twice adds 1 each time, so @count becomes 2. Dog.count returns 2.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   Increment twice, count = 2 [OK]

Hint: Class methods access class instance variables with @ [OK]

Common Mistakes:

• Expecting instance variable @count to be nil or 0
• Confusing class instance variables with class variables @@count
• Assuming instance methods can access @count directly

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

class Cat
  @count = 0

  def increment
    @count += 1
  end

  def self.count
    @count
  end
end

cat = Cat.new
cat.increment
puts Cat.count

medium

A. Missing initialization of @count

B. Class method count cannot access @count

C. Instance method increment cannot access class instance variable @count

D. Syntax error in method definitions

Solution

1. Step 1: Check variable scopes

   @count is a class instance variable, accessible only in class methods, not instance methods.

2. Step 2: Analyze increment method

   increment is an instance method trying to update @count, but this @count is an instance variable, not the class instance variable.

3. Final Answer:

   Instance method increment cannot access class instance variable @count -> Option C

4. Quick Check:

   Class instance vars not accessible in instance methods [OK]

Hint: Instance methods can't access class instance variables directly [OK]

Common Mistakes:

• Assuming @count in instance method is same as class @count
• Trying to access class instance variables without class methods
• Confusing class variables @@count with class instance variables

5. How can you modify this Ruby class to keep track of how many objects have been created using class instance variables?

class Bird
  def initialize
    # increment count here
  end

  def self.count
    @count
  end
end

hard

A. Add @count = 0 outside methods and increment @count inside a class method called from initialize

B. Use @@count variable and increment it inside initialize

C. Define @count inside initialize and increment it

D. Add self.count += 1 inside initialize

Solution

1. Step 1: Initialize class instance variable

   Define @count = 0 inside the class but outside methods to store count.

2. Step 2: Increment count via class method

   Create a class method to increment @count and call it inside initialize to update count on each new object.

3. Final Answer:

   Add @count = 0 outside methods and increment @count inside a class method called from initialize -> Option A

4. Quick Check:

   Class instance var + class method increment = object count [OK]

Hint: Use class method to update class instance variable from initialize [OK]

Common Mistakes:

• Using class variables @@count instead of class instance variables
• Trying to increment @count directly inside initialize without class method
• Defining @count inside initialize which resets it each time