2.2 Variables and Constants

Definition: A variable is a named storage location used to hold a value that can change.

Purpose: To store and manipulate data during program execution.

When used: When values need to change or be reused.

Python example:

score = 0
score = score + 10

print(score)
        

Definition: A constant is a named value that should not change during program execution.

Purpose: To protect fixed values and improve readability.

When used: For values such as limits, configuration values, or mathematical constants.

Python note: Python does not enforce constants, but uppercase names are used by convention.

Python example:

MAX_USERS = 100
PI = 3.14159

print(MAX_USERS)
print(PI)
        

Definition: Data type conversion changes a value from one data type to another.

Purpose: To allow compatible operations and handle user input correctly.

Why used: Input is often read as text and must be converted.

Python example:

age_text = "16"
age = int(age_text)

price = float("9.99")

print(age)
print(price)
        

Definition: Scope determines where a variable can be accessed in a program.

Purpose: To manage memory and avoid naming conflicts.

When used: When organising code into functions or modules.

Global variables: Accessible throughout the program.
Local variables: Only accessible within the function where they are defined.

Python example:

total = 0  # Global variable

def add_number():
    number = 5  # Local variable
    return total + number

print(add_number())
        

Variables and constants are declared using standard data types and used throughout programs to store, calculate, and control logic.

Python example:

TAX_RATE = 0.2
price = 50

tax = price * TAX_RATE
total_price = price + tax

print(total_price)