Is_a? and kind_of? for type checking in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the variable obj is a String using is_a?.

Ruby

obj = "hello"
puts obj.[1](String)

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

Ais_a?

Bkind_of?

Cinstance_of?

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'instance_of?' which only checks exact class, not subclasses.

Using 'class' which returns the class object, not a boolean.

The is_a? method checks if an object is an instance of a class or its subclass. Here, obj.is_a?(String) returns true because obj is a String.

2fill in blank

medium

Complete the code to check if num is a Numeric type using kind_of?.

Ruby

num = 42
if num.[1](Numeric)
  puts "It's a number!"
end

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

Ais_a?

Binstance_of?

Ckind_of?

Drespond_to?

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'instance_of?' which does not consider subclasses.

Using 'respond_to?' which checks for methods, not class type.

The kind_of? method checks if an object is an instance of a class or its subclass. Here, num.kind_of?(Numeric) returns true because 42 is a Numeric.

3fill in blank

hard

Fix the error in the code to correctly check if arr is an Array using is_a?.

Ruby

arr = [1, 2, 3]
if arr.[1](Array)
  puts "It's an array!"
end

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

Ais_a

Bis_a?

Ckind_of?

Dkind_of

Attempts:

3 left

💡 Hint

Common Mistakes

Omitting the question mark in the method name.

Using 'kind_of' without question mark.

The method is_a? must include the question mark. Without it, Ruby raises a NoMethodError.

4fill in blank

hard

Fill both blanks to create a method that returns true if obj is a String or a subclass of String.

Ruby

def string_check(obj)
  obj.[1](String) || obj.[2](String)
end

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

Ais_a?

Binstance_of?

Ckind_of?

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'instance_of?' which only checks exact class.

Using 'class' which returns the class object, not a boolean.

Both is_a? and kind_of? check if an object is an instance of a class or its subclass. Using either returns true for String or its subclasses.

5fill in blank

hard

Fill all three blanks to create a hash that maps objects to their type check results using is_a? and kind_of?.

Ruby

obj1 = [1, 2, 3]
obj2 = {a: 1}
obj3 = "test"
results = {
  obj1: obj1.[1],
  obj2: obj2.[2](Hash),
  obj3: obj3.[3](String)
}

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

Ais_a?

Bkind_of?

Cinstance_of?

Dclass

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'instance_of?' which does not consider subclasses.

Using 'class' expecting a boolean result.

Here, obj1.class returns the class of obj1, which is truthy but not a boolean. obj2.kind_of?(Hash) and obj3.is_a?(String) correctly check type including subclasses.

Practice

1. What does the Ruby method is_a? check when called on an object?

easy

A. If the object is equal to a given class

B. If the object responds to a given method

C. If the object is an instance of a given class or its subclass

D. If the object is a string

Solution

1. Step 1: Understand the purpose of is_a?

   The method is_a? checks if an object is an instance of a specific class or any of its subclasses.

2. Step 2: Compare with other options

   Options A, C, and D describe different checks unrelated to is_a?.

3. Final Answer:

   If the object is an instance of a given class or its subclass -> Option C

4. Quick Check:

   is_a? checks instance or subclass [OK]

Hint: Remember: is_a? checks class or subclass membership [OK]

Common Mistakes:

• Confusing is_a? with equality check
• Thinking is_a? checks method presence
• Assuming is_a? only checks exact class

2. Which of the following is the correct syntax to check if variable obj is a kind of Array in Ruby?

easy

A. obj.is_a(Array)

B. obj.kind_of?(Array)

C. obj.kind_of(Array)

D. obj.is_a?[Array]

Solution

1. Step 1: Recall method syntax

   Both is_a? and kind_of? require a question mark and parentheses around the class name.

2. Step 2: Check each option

   obj.kind_of?(Array) uses correct method name and syntax. Options B and C miss the question mark, and D has incorrect bracket usage.

3. Final Answer:

   obj.kind_of?(Array) -> Option B

4. Quick Check:

   Correct method syntax includes ? and parentheses [OK]

Hint: Use method? with parentheses: obj.kind_of?(Class) [OK]

Common Mistakes:

• Omitting the question mark in method name
• Using square brackets instead of parentheses
• Confusing is_a and kind_of syntax

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

class Animal; end
class Dog < Animal; end

dog = Dog.new
puts dog.is_a?(Animal)
puts dog.kind_of?(Dog)
puts dog.is_a?(String)

medium

A. true
true
false

B. false
true
false

C. true
false
true

D. false
false
true

Solution

1. Step 1: Understand class inheritance

   Dog inherits from Animal, so a Dog object is both a Dog and an Animal instance.

2. Step 2: Evaluate each method call

   dog.is_a?(Animal) returns true because Dog is subclass of Animal.
   dog.kind_of?(Dog) returns true because dog is instance of Dog.
   dog.is_a?(String) returns false because dog is not a String.

3. Final Answer:

   true true false -> Option A

4. Quick Check:

   Inheritance means is_a? true for parent classes [OK]

Hint: Remember inheritance: subclass instances are also parent class instances [OK]

Common Mistakes:

• Assuming is_a? false for parent classes
• Mixing up kind_of? and is_a? results
• Thinking dog is a String

4. Identify the error in this Ruby code snippet:

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

medium

A. Logic error: should use kind_of? instead of is_a?

B. Syntax error: missing parentheses in is_a

C. Runtime error: is_a? method undefined for Integer

D. No error; code runs and prints 'Number is integer'

Solution

1. Step 1: Check method usage

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

2. Step 2: Verify code logic and syntax

   The if condition is valid and will print "Number is integer" because 10 is an Integer.

3. Final Answer:

   No error; code runs and prints 'Number is integer' -> Option D

4. Quick Check:

   Correct syntax and logic means no error [OK]

Hint: Check method spelling and parentheses carefully [OK]

Common Mistakes:

• Omitting parentheses in method call
• Confusing is_a? with kind_of? as error
• Assuming is_a? is undefined for numbers

5. You have a method that accepts any object and should behave differently if the object is a String or a Numeric (including Integer and Float). Which Ruby code correctly uses is_a? or kind_of? to check this?

hard

A. if obj.is_a?(String)
puts "It's a string"
elsif obj.is_a?(Numeric)
puts "It's a number"
else
puts "Unknown type"
end

B. if obj.kind_of?(String)
puts "It's a string"
elsif obj.kind_of?(Integer)
puts "It's a number"
else
puts "Unknown type"
end

C. if obj.is_a?(String)
puts "It's a string"
elsif obj.kind_of?(Float)
puts "It's a number"
else
puts "Unknown type"
end

D. if obj.is_a?(Object)
puts "It's a string"
elsif obj.is_a?(Numeric)
puts "It's a number"
else
puts "Unknown type"
end

Solution

1. Step 1: Understand class hierarchy

   Numeric is a superclass of Integer and Float, so checking is_a?(Numeric) covers all number types.

2. Step 2: Analyze each option

   if obj.is_a?(String) puts "It's a string" elsif obj.is_a?(Numeric) puts "It's a number" else puts "Unknown type" end correctly checks for String and then Numeric.
   if obj.kind_of?(String) puts "It's a string" elsif obj.kind_of?(Integer) puts "It's a number" else puts "Unknown type" end checks Integer only, missing Float.
   if obj.is_a?(String) puts "It's a string" elsif obj.kind_of?(Float) puts "It's a number" else puts "Unknown type" end checks only Float, missing Integer.
   if obj.is_a?(Object) puts "It's a string" elsif obj.is_a?(Numeric) puts "It's a number" else puts "Unknown type" end checks Object first, which is always true, so logic fails.

3. Final Answer:

   Option A code correctly distinguishes String and all Numeric types -> Option A

4. Quick Check:

   Use Numeric superclass to cover all numbers [OK]

Hint: Check against Numeric to cover all number types [OK]

Common Mistakes:

• Checking only Integer or Float separately
• Using Object which is always true
• Mixing is_a? and kind_of? incorrectly