Constants in classes in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing a constant inside a nested class

What is the output of this Ruby code?

Ruby

class Outer
  CONST = 10
  class Inner
    CONST = 20
    def self.show
      CONST
    end
  end
end

puts Outer::Inner.show

A nil

B 10

C 20

D NameError

💡 Hint

Remember that constants inside a nested class override outer constants with the same name.

The Inner class defines its own CONST as 20, so Outer::Inner.show returns 20.

❓ Predict Output

intermediate

Accessing a constant from an outer class

What will this Ruby code print?

Ruby

class A
  VALUE = 5
  class B
    def self.get_value
      A::VALUE
    end
  end
end

puts A::B.get_value

A 0

B nil

C NameError

D 5

💡 Hint

Use the scope resolution operator to access constants from outer classes.

A::VALUE is 5, so A::B.get_value returns 5.

❓ Predict Output

advanced

Constant lookup with inheritance and nesting

What is the output of this Ruby code?

Ruby

class Parent
  CONST = 'parent'
end

class Child < Parent
  CONST = 'child'
  class Nested
    def self.value
      CONST
    end
  end
end

puts Child::Nested.value

A NameError

B nil

C "parent"

D "child"

💡 Hint

Constants inside nested classes do not inherit from the outer class or its ancestors automatically.

Nested class does not have CONST defined and does not inherit from Child, so CONST is undefined and raises NameError.

❓ Predict Output

advanced

Constant resolution with explicit scope

What does this Ruby code print?

Ruby

module M
  CONST = 100
  class C
    CONST = 200
    def self.show
      M::CONST
    end
  end
end

puts M::C.show

A 200

B 100

C NameError

D nil

💡 Hint

Using M::CONST accesses the constant defined in the module M, not the class C.

M::CONST is 100, so M::C.show returns 100.

🧠 Conceptual

expert

Behavior of constant reassignment in Ruby classes

Consider this Ruby code snippet. What will be the output and why? class X CONST = 1 CONST = 2 def self.value CONST end end puts X.value

A Warning and 2

B 1

C 2

D Error

💡 Hint

Ruby allows constant reassignment but warns about it.

Ruby prints a warning about constant reassignment but the value is updated to 2, so output is 2 with a warning.

Practice

1. What is true about constants in Ruby classes?

easy

A. Constants start with a lowercase letter.

B. Constants can be changed anytime like variables.

C. Constants start with a capital letter and are defined outside methods.

D. Constants must be defined inside methods only.

Solution

1. Step 1: Understand constant naming rules

   In Ruby, constants always start with a capital letter and are usually defined outside methods within classes or modules.

2. Step 2: Check where constants are defined

   Constants are not meant to be changed and are not defined inside methods but at the class/module level.

3. Final Answer:

   Constants start with a capital letter and are defined outside methods. -> Option C

4. Quick Check:

   Constants = Capital letter + outside methods [OK]

Hint: Constants start uppercase and live outside methods [OK]

Common Mistakes:

• Thinking constants can be changed like variables
• Defining constants inside methods
• Using lowercase letters for constants

2. Which of the following is the correct way to define a constant inside a Ruby class?

easy

A. class MyClass
def PI = 3.14
end

B. class MyClass
PI = 3.14
end

C. class MyClass
pi = 3.14
end

D. class MyClass
@PI = 3.14
end

Solution

1. Step 1: Identify constant syntax

   Constants in Ruby start with a capital letter and are assigned directly inside the class but outside any method.

2. Step 2: Check each option

   class MyClass PI = 3.14 end correctly assigns PI = 3.14 inside the class. class MyClass pi = 3.14 end uses lowercase (not constant). class MyClass def PI = 3.14 end tries to define a method named PI incorrectly. class MyClass @PI = 3.14 end uses an instance variable (@PI), not a constant.

3. Final Answer:

   class MyClass PI = 3.14 end -> Option B

4. Quick Check:

   Constants = Capital letter + direct assignment [OK]

Hint: Constants = Capital letter + direct assignment inside class [OK]

Common Mistakes:

• Using lowercase names for constants
• Defining constants inside methods
• Confusing constants with instance variables

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

class Car
  WHEELS = 4
  def wheels
    WHEELS
  end
end

car = Car.new
puts car.wheels

medium

A. 4

B. WHEELS

C. Error: uninitialized constant

D. nil

Solution

1. Step 1: Understand constant access inside instance method

   The constant WHEELS is defined inside the class Car. Inside instance methods, constants can be accessed directly by their name.

2. Step 2: Trace the method call

   Calling car.wheels returns the constant WHEELS value, which is 4. Then puts prints 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Constant accessed inside method = 4 [OK]

Hint: Constants inside class accessible by name in instance methods [OK]

Common Mistakes:

• Expecting an error for constant access
• Thinking constant name prints instead of value
• Confusing constants with instance variables

4. Identify the error in this Ruby code:

class Book
  TITLE = "Ruby Guide"
  def change_title
    TITLE = "New Title"
  end
end

medium

A. Constants must be defined inside methods.

B. Missing end keyword for class.

C. TITLE should be lowercase.

D. Cannot assign to constant TITLE inside method.

Solution

1. Step 1: Check constant assignment rules

   Constants in Ruby should not be reassigned once set. Assigning a constant inside a method is not allowed and causes a warning or error.

2. Step 2: Analyze the code

   The method change_title tries to assign a new value to TITLE constant, which is invalid.

3. Final Answer:

   Cannot assign to constant TITLE inside method. -> Option D

4. Quick Check:

   Constants cannot be reassigned inside methods [OK]

Hint: Constants cannot be reassigned inside methods [OK]

Common Mistakes:

• Trying to change constants inside methods
• Confusing constants with variables
• Ignoring Ruby warnings about constant reassignment

5. Given this Ruby code, what will puts Vehicle::TYPE print?

class Vehicle
  TYPE = "Transport"
  class Car
    TYPE = "Car"
  end
end

puts Vehicle::TYPE

hard

A. "Transport"

B. "Car"

C. Error: uninitialized constant

D. nil

Solution

1. Step 1: Understand constant lookup in nested classes

   Constants are scoped to the class they are defined in. Vehicle::TYPE refers to the constant TYPE inside Vehicle, not inside Vehicle::Car.

2. Step 2: Check the code output

   Vehicle::TYPE is "Transport". The inner class Car has its own TYPE, but it is not accessed here.

3. Final Answer:

   "Transport" -> Option A

4. Quick Check:

   Outer class constant accessed by Vehicle::TYPE = "Transport" [OK]

Hint: Outer class constant accessed by ClassName::CONSTANT [OK]

Common Mistakes:

• Confusing inner class constant with outer class constant
• Expecting error for nested constants
• Using wrong scope operator