Type checking with .class and .is_a? in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the .class method do in Ruby?

It returns the exact class of an object. For example, <code>5.class</code> returns <code>Integer</code>.

intermediate

What is the difference between .class and .is_a? in Ruby?

<code>.class</code> checks the exact class of an object, while <code>.is_a?</code> checks if the object is an instance of a class or any of its parent classes (inheritance).

intermediate

How does .is_a? help with inheritance?

It returns <code>true</code> if the object is an instance of the given class or any class it inherits from, making it useful to check for types in a family of classes.

beginner

Example: What is the output of "hello".class and "hello".is_a?(String)?

The output of "hello".class is String. The output of "hello".is_a?(String) is true.

intermediate

Can .is_a? return true for a superclass?

Yes, <code>.is_a?</code> returns <code>true</code> if the object is an instance of the class or any of its superclasses.

What does obj.class return in Ruby?

A Whether obj is a string

B True if obj is a class

C The superclass of obj

D The exact class of obj

.class returns the exact class of the object.

Which method checks if an object is an instance of a class or its parent classes?

A .is_a?

B .class

C .to_s

D .superclass

.is_a? checks for the class or any superclass.

What will 5.is_a?(Numeric) return?

A false

B nil

C true

D error

5 is an Integer, which inherits from Numeric, so .is_a?(Numeric) returns true.

If obj.class == String is true, what does obj.is_a?(Object) return?

A false

B true

C nil

D error

All classes in Ruby inherit from Object, so .is_a?(Object) returns true.

Which method would you use to check if an object is exactly a certain class, not a subclass?

A .instance_of?

B .kind_of?

C .is_a?

D .class

.instance_of? checks if the object is exactly that class, not subclasses.

Explain the difference between .class and .is_a? in Ruby with examples.

Think about checking exact type vs family of types.

How can you use .is_a? to safely check an object's type in a program?

Consider when you want to accept objects of related types.

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?