Subclass with < operator in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of subclass comparison with < operator

What is the output of this Ruby code?

Ruby

class Animal
  def <(other)
    self.class < other.class
  end
end

class Dog < Animal; end
class Cat < Animal; end

puts Dog.new < Cat.new

A false

B NoMethodError

C true

D ArgumentError

💡 Hint

Remember that Ruby's < operator can be used to compare classes by their inheritance hierarchy.

The < operator compares the classes of the instances using Ruby's built-in Class#< method, which returns true if the left class (self.class) is a subclass of the right class (other.class). Dog is not a subclass of Cat, so it returns false.

❓ Predict Output

intermediate

Subclass comparison with overridden < operator

What does this Ruby code print?

Ruby

class Vehicle
  def <(other)
    self.class < other.class
  end
end

class Car < Vehicle; end
class Truck < Vehicle; end

puts Car.new < Truck.new

A NoMethodError

B false

C TypeError

D true

💡 Hint

Check if Car is subclass of Truck or vice versa.

The < operator compares the classes. Car is not a subclass of Truck, so Car < Truck returns false.

🔧 Debug

advanced

Why does this subclass < operator code raise an error?

This Ruby code raises an error. What is the cause?

Ruby

class Fruit
  def <(other)
    self < other
  end
end

class Apple < Fruit; end
class Banana < Fruit; end

puts Apple.new < Banana.new

A NoMethodError because < is undefined

B ArgumentError because of wrong argument type

C SystemStackError due to infinite recursion

D SyntaxError due to missing end

💡 Hint

Look at the < method calling itself.

The < method calls self < other, which calls the same method again, causing infinite recursion and a SystemStackError.

❓ Predict Output

advanced

Output of subclass < operator with modules included

What is the output of this Ruby code?

Ruby

module ComparableModule
  def <(other)
    self.class < other.class
  end
end

class Parent; include ComparableModule; end
class Child < Parent; end
class Sibling < Parent; end

puts Child.new < Sibling.new

A true

B TypeError

C NoMethodError

D false

💡 Hint

Check the inheritance relationship between Child and Sibling.

Child and Sibling are siblings, neither is subclass of the other, so Child < Sibling returns false.

🧠 Conceptual

expert

Understanding < operator behavior in subclass comparison

In Ruby, when you define < operator in a superclass to compare classes of instances using self.class < other.class, what will be the result of comparing an instance of a subclass with an instance of its superclass using < ?

A true, because Class#< returns true if left is subclass of right

B false, because subclass is not less than superclass

C true, because subclass is always less than superclass

D false, because Class#< returns true only if left is superclass of right

💡 Hint

Recall how Class#< works in Ruby.

Class#< returns true if the left class is a subclass of the right class. So comparing subclass instance < superclass instance returns true.