Everything is an object mental model in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code?

Consider this Ruby code snippet. What will it print when run?

Ruby

puts 42.class
puts "hello".class
puts :symbol.class

A

Integer
String
Symbol

B

Fixnum
String
String

C

Integer
String
String

D

Fixnum
String
Symbol

💡 Hint

Remember in modern Ruby versions, Fixnum and Bignum are unified into Integer.

In Ruby 2.4 and later, Fixnum and Bignum are unified into Integer. So 42.class returns Integer. Strings and symbols have their own classes: String and Symbol.

🧠 Conceptual

intermediate

Which statement best describes Ruby's object model?

Choose the statement that correctly explains Ruby's 'everything is an object' model.

A All values, including numbers, strings, and even classes themselves, are objects.

B Only user-defined data types are objects; primitives like numbers are not.

C Only strings and arrays are objects; numbers are treated as primitive values.

D Objects exist only when explicitly created with 'new'; literals are not objects.

💡 Hint

Think about how you can call methods on numbers and strings in Ruby.

In Ruby, everything is an object. This means numbers, strings, classes, and even nil are objects. You can call methods on all of them.

🔧 Debug

advanced

Why does this Ruby code raise an error?

Look at this Ruby code. Why does it raise a NoMethodError?

Ruby

5.upcase

A Because 'upcase' requires an argument which is missing here.

B Because 'upcase' is a private method and cannot be called directly.

C Because numbers cannot be objects in Ruby.

D Because 5 is an Integer and Integer does not have an 'upcase' method.

💡 Hint

Check which methods are available for Integer objects.

The method 'upcase' is defined for String objects, not for Integer objects. Since 5 is an Integer, calling 5.upcase raises NoMethodError.

❓ Predict Output

advanced

What is the output of this Ruby code involving classes as objects?

What will this Ruby code print?

Ruby

puts Class.class
puts Integer.class
puts 10.class.class

A

Class
Object
Class

B

Class
Integer
Integer

C

Class
Class
Class

D

Class
Class
Integer

💡 Hint

Remember that classes themselves are instances of Class.

In Ruby, Class is an object of class Class. Integer is a class, so its class is Class. The class of 10 is Integer, and Integer's class is Class.

❓ Predict Output

expert

What is the output of this Ruby code using singleton methods?

Examine this Ruby code and determine what it prints.

Ruby

obj = "42"
 def obj.description
   "The number is #{self}"
 end
puts obj.description
puts 42.respond_to?(:description)

A

NoMethodError
false

B

The number is 42
false

C

The number is 42
true

D

NoMethodError
true

💡 Hint

Think about whether the singleton method is defined on the string "42" or on the variable obj.

The singleton method 'description' is defined on the object referenced by 'obj', which is the string "42". The method is defined on the specific object 'obj' references, but 42 literals elsewhere do not have it. So 'obj.description' works, but '42.respond_to?(:description)' returns false because the literal 42 is a different object.