Constants in classes in Ruby - Step-by-Step Execution

Concept Flow - Constants in classes

Define Class

↓

Set Constant inside Class

↓

Access Constant via Class

↓

Use Constant Value

↓

End

This flow shows how a constant is set inside a class and then accessed using the class name.

Execution Sample

Ruby

class MyClass
  MY_CONST = 10
end

puts MyClass::MY_CONST

Defines a class with a constant and prints the constant value using class scope resolution.

Execution Table

Step	Action	Code Line	State Change	Output
1	Define class MyClass	class MyClass	Class MyClass created
2	Set constant MY_CONST to 10	MY_CONST = 10	MY_CONST set to 10 inside MyClass
3	End class definition	end	Class MyClass finalized
4	Access constant MY_CONST via MyClass	puts MyClass::MY_CONST	Retrieve MY_CONST value 10	10

💡 Program ends after printing the constant value 10.

Variable Tracker

Variable	Start	After Step 2	Final
MyClass::MY_CONST	undefined	10	10

Key Moments - 2 Insights

Why do we use :: to access the constant?

The :: operator accesses constants or methods inside a class or module. Here, it tells Ruby to look inside MyClass for MY_CONST, as shown in execution_table step 4.

Can we change the value of MY_CONST after setting it?

Constants in Ruby can technically be changed but Ruby warns you. In this example, MY_CONST is set once and used as 10, as shown in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of MY_CONST after step 2?

A nil

B 10

C undefined

D error

💡 Hint

Check the 'State Change' column in row for step 2 in execution_table.

At which step is the constant MY_CONST accessed to print its value?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look for the 'Action' column mentioning 'Access constant' in execution_table.

If we tried to access MY_CONST without MyClass:: prefix, what would happen?

A It would cause an error

B It would print 10

C It would print nil

D It would print 0

💡 Hint

Constants inside classes need the class name prefix to be accessed, as shown in execution_table step 4.

Concept Snapshot

class MyClass
  MY_CONST = 10  # Define constant inside class
end

Access with MyClass::MY_CONST
Constants are fixed values inside classes
Use :: to access constants inside class scope

Full Transcript

This example shows how to define a constant inside a Ruby class and access it. First, the class MyClass is created. Then, inside it, the constant MY_CONST is set to 10. After the class ends, we access the constant using MyClass::MY_CONST and print its value. Constants inside classes are accessed with the :: operator to specify the class scope. Changing constants is possible but not recommended. The program prints 10 as the output.

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