Constants in classes in Ruby - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constant in a Ruby class?

A constant in a Ruby class is a value that is meant to stay the same throughout the program. It is written with a name starting with a capital letter and usually holds data that should not change.

beginner

How do you define a constant inside a Ruby class?

You define a constant inside a Ruby class by writing the constant name starting with a capital letter, followed by an equal sign and the value. For example: <pre>class MyClass
  MY_CONSTANT = 10
end</pre>

intermediate

Can you change the value of a constant in Ruby after it is set?

Technically, you can change a constant's value in Ruby, but it will give a warning because constants are meant to stay the same. Changing them is not recommended.

beginner

How do you access a constant defined inside a Ruby class from outside the class?

You access a constant from outside the class by using the scope resolution operator :: with the class name. For example: <pre>MyClass::MY_CONSTANT</pre>

intermediate

What happens if you define a constant with the same name in a subclass in Ruby?

If a subclass defines a constant with the same name as in its parent class, the subclass's constant will override the parent's constant when accessed from the subclass.

How do you write a constant name in Ruby?

A Starts with a lowercase letter

B Starts with a number

C Starts with an underscore

D Starts with a capital letter

In Ruby, constants must start with a capital letter.

How do you access a constant MY_CONST inside class MyClass from outside?

A MyClass::MY_CONST

B MyClass.MY_CONST

C MyClass->MY_CONST

D MY_CONST.MyClass

Use the scope resolution operator :: to access constants inside classes.

What happens if you change a constant's value in Ruby?

A Ruby deletes the constant

B Ruby throws an error and stops

C Ruby allows it but shows a warning

D Ruby ignores the change silently

Ruby allows changing constants but warns you because it's not recommended.

Where should constants be defined in Ruby?

A Only inside methods

B Inside classes or modules

C Only in global scope

D Inside local variables

Constants are usually defined inside classes or modules to organize them.

If a subclass defines a constant with the same name as its parent, which one is used inside the subclass?

A The subclass's constant

B The parent's constant

C Both constants are merged

D An error occurs

The subclass's constant overrides the parent's constant when accessed from the subclass.

Explain how to define and access constants inside Ruby classes.

Think about how you write constants and how you get their values from outside.

Describe what happens when you try to change a constant's value in Ruby and why it matters.

Consider Ruby's behavior and why constants exist.

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