Class instance variables as alternative in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class instance variable usage

What is the output of this Ruby code using class instance variables?

Ruby

class Counter
  @count = 0

  def self.increment
    @count += 1
  end

  def self.count
    @count
  end
end

Counter.increment
Counter.increment
puts Counter.count

A 0

B Error: undefined method `+' for nil:NilClass

C 2

D nil

💡 Hint

Class instance variables belong to the class itself, not instances.

The class instance variable @count is initialized to 0. Each call to increment adds 1. After two increments, count returns 2.

❓ Predict Output

intermediate

Difference between class variable and class instance variable

What will this Ruby code output?

Ruby

class Example
  @@var = 1
  @var = 2

  def self.class_var
    @@var
  end

  def self.class_instance_var
    @var
  end
end

puts Example.class_var
puts Example.class_instance_var

A 1\n2

B 1\nnil

C nil\n2

D 2\n1

💡 Hint

Class variables start with @@ and are shared across the class hierarchy.

The class variable @@var is 1. The class instance variable @var is 2. Both are accessed correctly by their methods.

🔧 Debug

advanced

Why does this class instance variable return nil?

This Ruby code prints nil. Why?

Ruby

class Sample
  @value = 10

  def value
    @value
  end
end

obj = Sample.new
puts obj.value

A Class instance variables are not accessible inside instance methods by default.

B Syntax error because @value is not initialized in constructor.

C The method value is missing a return statement.

D Instance method accesses instance variable of object, but @value is only set on class, not instance.

💡 Hint

Class instance variables belong to the class object, not to instances.

The @value set on the class is different from the @value on the instance. The instance's @value is never set, so it is nil.

🧠 Conceptual

advanced

Class instance variables and inheritance behavior

Given this Ruby code, what will be the output?

Ruby

class Parent
  @data = 'parent data'

  def self.data
    @data
  end
end

class Child < Parent
  @data = 'child data'
end

puts Parent.data
puts Child.data

A child data\nchild data

B parent data\nchild data

C parent data\nparent data

D nil\nnil

💡 Hint

Class instance variables are not shared between parent and child classes.

Each class has its own @data variable. Parent.data returns 'parent data', Child.data returns 'child data'.

📝 Syntax

expert

Identify the syntax error in class instance variable usage

Which option contains a syntax error when trying to define and access a class instance variable?

Ruby

class Test
  @var = 5

  def self.get_var
    @var
  end
end

A

class Test
  @var = 5
  def get_var
    self.@var
  end
end

B

class Test
  @@var = 5
  def self.get_var
    @var
  end
end

C

class Test
  @var = 5
  def self.get_var
    @var
  end
end

D

class Test
  @var = 5
  def self.get_var
    @@var
  end
end

💡 Hint

Class instance variables are accessed in class methods, not instance methods.

Option A contains a syntax error: 'self.@var' is invalid Ruby syntax. Instance variables (including class instance variables in class methods) cannot be accessed using 'self.@var'; use just '@var'.