Type checking with .class and .is_a? in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of .class vs .is_a? for Integer

What is the output of the following Ruby code?

Ruby

num = 42
puts num.class
puts num.is_a?(Integer)

A

Integer
true

B

Integer
false

C

Fixnum
true

D

Fixnum
false

💡 Hint

Remember Ruby 2.4+ unified Fixnum and Bignum into Integer.

The .class method returns the exact class of the object, which is Integer in modern Ruby versions.

The .is_a?(Integer) method checks if the object is an instance of Integer or its subclasses, so it returns true.

❓ Predict Output

intermediate

Checking class inheritance with .is_a?

What will this Ruby code print?

Ruby

class Animal; end
class Dog < Animal; end
pet = Dog.new
puts pet.class
puts pet.is_a?(Animal)

A

Dog
true

B

Animal
true

C

Dog
false

D

Animal
false

💡 Hint

Remember that .class returns the object's exact class, but .is_a? checks inheritance.

pet.class returns Dog because pet is an instance of Dog.

pet.is_a?(Animal) returns true because Dog inherits from Animal.

🔧 Debug

advanced

Why does this .is_a? check fail?

Consider this Ruby code snippet. Why does the .is_a? check return false?

Ruby

class Vehicle; end
class Car < Vehicle; end
my_car = Vehicle.new
puts my_car.is_a?(Car)

A Because Car is not defined properly as a subclass

B Because .is_a? only works with modules, not classes

C Because my_car is an instance of Vehicle, not Car or its subclass

D Because .is_a? checks exact class equality, not inheritance

💡 Hint

Think about the object and class relationship and what .is_a? means.

my_car is an instance of Vehicle, not Car. The .is_a?(Car) method returns true only if the object is an instance of Car or its subclasses.

📝 Syntax

advanced

Identify the syntax error in type checking

Which option contains a syntax error when checking an object's type in Ruby?

Ruby

obj = 'hello'

A obj.is_a?(String)

B obj.is_a?(String))

C obj.class == String

D obj.class.eql?(String)

💡 Hint

Check for unmatched parentheses.

Option B has an extra closing parenthesis, causing a syntax error.

Other options are syntactically correct ways to check type.

🚀 Application

expert

Determine the output of complex type checks

What is the output of this Ruby code?

Ruby

module Mixin; end
class Parent; include Mixin; end
class Child < Parent; end
obj = Child.new
puts obj.is_a?(Mixin)
puts obj.class == Parent
puts obj.is_a?(Parent)

A

false
false
true

B

true
true
true

C

true
false
false

D

true
false
true

💡 Hint

Remember that .is_a? checks inheritance and included modules, but .class checks exact class.

obj.is_a?(Mixin) is true because Parent includes Mixin and Child inherits Parent.

obj.class == Parent is false because obj's class is Child, not Parent.

obj.is_a?(Parent) is true because Child inherits Parent.

Practice

1. What does the .class method return when called on an object in Ruby?

easy

A. Whether the object is a kind of a class

B. The exact type (class) of the object

C. The parent class of the object

D. A boolean indicating if the object is a class

Solution

1. Step 1: Understand what .class does

   The .class method returns the exact class of the object it is called on.

2. Step 2: Compare with other options

   Options A, C, and D describe behaviors of other methods or incorrect meanings.

3. Final Answer:

   The exact type (class) of the object -> Option B

4. Quick Check:

   .class returns exact type [OK]

Hint: Remember: .class gives exact type, not inheritance [OK]

Common Mistakes:

• Confusing .class with .is_a?
• Thinking .class returns boolean
• Assuming .class returns parent class

2. Which of the following is the correct syntax to check if variable x is an instance of the Integer class using .is_a??

easy

A. x.is_a?(Integer)

B. x.is_a(Integer)

C. x.is_a<Integer>

D. x.is_a? Integer

Solution

1. Step 1: Recall the syntax of .is_a?

   The method .is_a? requires a class argument inside parentheses, like .is_a?(ClassName).

2. Step 2: Check each option

   x.is_a?(Integer) uses correct syntax with parentheses and question mark. Options A, B, and D have syntax errors or missing parentheses.

3. Final Answer:

   x.is_a?(Integer) -> Option A

4. Quick Check:

   Use parentheses with .is_a? [OK]

Hint: Always use parentheses with .is_a? method [OK]

Common Mistakes:

• Omitting parentheses after .is_a?
• Using angle brackets instead of parentheses
• Leaving out the question mark

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

class Animal; end
class Dog < Animal; end

dog = Dog.new
puts dog.class
puts dog.is_a?(Animal)

medium

A. Dog
true

B. Animal
true

C. Dog
false

D. Animal
false

Solution

1. Step 1: Understand dog.class

   The dog object is an instance of Dog, so dog.class returns Dog.

2. Step 2: Understand dog.is_a?(Animal)

   Since Dog inherits from Animal, dog.is_a?(Animal) returns true.

3. Final Answer:

   Dog true -> Option A

4. Quick Check:

   .class exact type, .is_a? checks inheritance [OK]

Hint: Remember: .class exact, .is_a? true for subclasses too [OK]

Common Mistakes:

• Thinking .class returns parent class
• Assuming .is_a? false for subclasses
• Confusing output order

4. Identify the error in the following Ruby code snippet:

num = 10
if num.is_a?(Integer)
  puts "Number is integer"
else
  puts "Number is not integer"
end

medium

A. Syntax error in if statement

B. Missing parentheses in is_a? call

C. No error, code runs correctly

D. Incorrect class name Integer

Solution

1. Step 1: Check is_a? usage

   The method .is_a? is correctly called with parentheses and the class Integer.

2. Step 2: Verify syntax correctness

   The if statement and puts calls are syntactically correct.

3. Final Answer:

   No error, code runs correctly -> Option C

4. Quick Check:

   Correct syntax and usage means no error [OK]

Hint: Check parentheses and class names carefully [OK]

Common Mistakes:

• Removing parentheses from is_a?
• Using wrong class name like 'int'
• Misplacing if-end keywords

5. Given the following code, which option correctly filters only strings from the mixed array using .is_a??

items = ["apple", 42, :symbol, "banana", 3.14]
strings = items.select { |item| ??? }

hard

A. item.class != String

B. item.class == String

C. item.is_a?(Integer)

D. item.is_a?(String)

Solution

1. Step 1: Understand filtering with .is_a?

   We want to select only elements that are strings, so we check if each item is a kind of String.

2. Step 2: Compare options

   item.is_a?(String) uses .is_a?(String) which returns true for strings. item.class == String uses .class == String which also works but is less flexible if subclasses exist. Options C and D filter integers or non-strings, which is incorrect.

3. Final Answer:

   item.is_a?(String) -> Option D

4. Quick Check:

   Use .is_a?(String) to filter strings [OK]

Hint: Use .is_a?(String) to include subclasses too [OK]

Common Mistakes:

• Using .class == String and missing subclasses
• Filtering wrong types by mistake
• Forgetting question mark in .is_a?