Constants in classes in Ruby - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a constant named GREETING inside the class.

Ruby

class Welcome
  [1] = "Hello!"
end

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

AGreeting

BgREETING

Cgreeting

DGREETING

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase letters for constant names.

Using variable names instead of constants.

The constant name must start with a capital letter. Here, GREETING is the correct constant name.

2fill in blank

medium

Complete the code to access the constant PI inside the class MathConstants.

Ruby

class MathConstants
  PI = 3.14

  def self.get_pi
    [1]
  end
end

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

API

BMathConstants.pi

Cself.PI

Dpi

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase letters to access constants.

Trying to call constants like methods.

Constants are accessed by their exact name. Inside the class method, PI refers to the constant.

3fill in blank

hard

Fix the error in accessing the constant MAX_SPEED from outside the class Vehicle.

Ruby

class Vehicle
  MAX_SPEED = 120
end

speed = [1]

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

AVehicle->MAX_SPEED

BVehicle.MAX_SPEED

CVehicle::MAX_SPEED

DVehicle.MAXspeed

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation instead of double colon.

Using incorrect operator like '->'.

In Ruby, constants from outside a class are accessed using the double colon :: operator.

4fill in blank

hard

Fill both blanks to define and access a constant SPEED_LIMIT inside the class Car.

Ruby

class Car
  [1] = 80

  def speed_limit
    [2]
  end
end

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

ASPEED_LIMIT

Bspeed_limit

Dself.SPEED_LIMIT

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for constants.

Trying to access constants with dot notation inside instance methods.

The constant SPEED_LIMIT is defined and accessed by its name inside the class.

5fill in blank

hard

Fill all three blanks to create a constant NAME, access it inside a method, and access it from outside the class Person.

Ruby

class Person
  [1] = "Alice"

  def name
    [2]
  end
end

puts [3]

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

ANAME

CPerson::NAME

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase names for constants.

Trying to access constants like instance variables.

Using dot notation outside the class.

The constant NAME is defined and accessed inside the class by its name, and from outside using Person::NAME.

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