Why class-level behavior matters in Ruby - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a class method in Ruby.

Ruby

class Car
  def self.[1]
    "This is a class method"
  end
end

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

Ainfo

Bstart

Cdescription

Ddrive

Attempts:

3 left

💡 Hint

Common Mistakes

Using instance method names without self. prefix.

The method name info is used here as a class method example. It can be called on the class itself.

2fill in blank

medium

Complete the code to call the class method correctly.

Ruby

class Dog
  def self.[1]
    "Bark!"
  end
end

puts Dog.[1]

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

Asound

Bspeak

Cnoise

Dbark

Attempts:

3 left

💡 Hint

Common Mistakes

Calling class methods on instances instead of the class.

The class method bark is defined and called on the class Dog.

3fill in blank

hard

Fix the error in the code by completing the blank.

Ruby

class Cat
  def [1]
    "Meow"
  end
end

puts Cat.meow

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

Aclass.meow

Bmeow

Cself.meow

Ddef.meow

Attempts:

3 left

💡 Hint

Common Mistakes

Defining instance methods but trying to call them on the class.

To define a class method, prefix the method name with self.. Here, def self.meow defines a class method.

4fill in blank

hard

Fill in the blank to create a class method that accesses the class variable.

Ruby

class Book
  @@count = 0

  def self.[1]
    @@count
  end
end

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

Acount

Bclass_variable

Cdef

Ddef self.count

Attempts:

3 left

💡 Hint

Common Mistakes

Using instance variables instead of class variables.

Class variables start with @@. The class method count accesses and returns it.

5fill in blank

hard

Fill both blanks to define a class method that increments a class variable and returns it.

Ruby

class Counter
  @@total = 0

  def self.[1]
    @@total [2] 1
  end
end

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

Aclass_variable

Bincrement

C+=

Ddef

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to use self. for class methods.

Using =+ instead of +=.

Class variables start with @@. Define the class method increment and use += to increment and return the new value.

Practice

1. What is the main reason to use class-level behavior in Ruby classes?

easy

A. To share data and methods among all instances of the class

B. To create unique data for each object

C. To make objects run faster

D. To avoid using instance variables

Solution

1. Step 1: Understand class-level behavior

   Class-level behavior means data or methods shared by all objects of a class, not unique to each object.

2. Step 2: Identify the purpose

   Sharing data and methods avoids repeating the same information in every object and keeps code clean.

3. Final Answer:

   To share data and methods among all instances of the class -> Option A

4. Quick Check:

   Class-level behavior = shared data/methods [OK]

Hint: Class-level means shared by all objects, not individual ones [OK]

Common Mistakes:

• Confusing class-level with instance-level data
• Thinking class-level makes objects faster
• Believing class-level removes instance variables

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

easy

A. def class.method_name; end

B. def method_name; end

C. def @method_name; end

D. def self.method_name; end

Solution

1. Step 1: Recall class method syntax

   In Ruby, class methods are defined with def self.method_name inside the class.

2. Step 2: Check options

   def self.method_name; end uses def self.method_name, which is correct. def method_name; end defines an instance method. def @method_name; end uses invalid syntax. def class.method_name; end is incorrect syntax.

3. Final Answer:

   def self.method_name; end -> Option D

4. Quick Check:

   Class method syntax = def self.method_name [OK]

Hint: Class methods start with self. inside class [OK]

Common Mistakes:

• Defining class methods without self.
• Using @ for method names
• Trying def class.method_name syntax

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

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

Dog.new
Dog.new
puts Dog.count

medium

A. 2

B. 0

C. 1

D. Error

Solution

1. Step 1: Understand class variable usage

   @@count is a class variable shared by all Dog instances. It starts at 0.

2. Step 2: Count increments on each new Dog

   Each Dog.new calls initialize, which adds 1 to @@count. Two Dog.new calls increase @@count to 2.

3. Step 3: Output the class variable

   Dog.count returns @@count, which is 2, so puts prints 2.

4. Final Answer:

   2 -> Option A

5. Quick Check:

   Class variable increments twice = 2 [OK]

Hint: Class variables track shared data across all instances [OK]

Common Mistakes:

• Thinking @@count resets for each object
• Confusing class variables with instance variables
• Expecting an error from @@count usage

4. Identify the error in this Ruby class that tries to count instances:

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

Cat.new
Cat.new
puts Cat.count

medium

A. Missing def self.count method

B. Using @count instead of @@count in initialize

C. Class variable @@count not initialized

D. Syntax error in class definition

Solution

1. Step 1: Check variable usage in initialize

   The code uses @count (an instance variable) instead of @@count (class variable) to increment count.

2. Step 2: Understand effect of mistake

   Instance variable @count is nil initially, so @count += 1 causes an error or does not affect class count.

3. Step 3: Confirm other parts are correct

   Class variable @@count is initialized and count method exists, so no other errors.

4. Final Answer:

   Using @count instead of @@count in initialize -> Option B

5. Quick Check:

   Increment class variable with @@count, not @count [OK]

Hint: Class variables start with @@, instance with @ [OK]

Common Mistakes:

• Incrementing instance variable instead of class variable
• Assuming @count affects all instances
• Ignoring variable initialization errors

5. You want to track how many times a method is called across all instances of a class. Which approach correctly implements this in Ruby?

hard

A. Use a global variable $counter incremented inside the instance method

B. Use an instance variable @counter incremented inside the instance method

C. Use a class variable @@counter incremented inside the instance method

D. Use a local variable counter inside the method

Solution

1. Step 1: Understand the goal

   We want to count method calls shared by all instances, so data must be shared at class level.

2. Step 2: Evaluate options

   Class variable @@counter is shared by all instances, so incrementing it inside the instance method tracks total calls. Instance variable @counter is unique per object, local variable resets each call, global variable $counter works but is discouraged.

3. Step 3: Choose best practice

   Using @@counter is the correct and clean way to track calls across all instances.

4. Final Answer:

   Use a class variable @@counter incremented inside the instance method -> Option C

5. Quick Check:

   Class variable tracks shared count [OK]

Hint: Use @@ for shared counters, not @ or local variables [OK]

Common Mistakes:

• Using instance variables which count per object
• Using local variables which reset each call
• Using global variables which pollute global scope