Variables and constants

Variables

• To create a variable, you must specify its data type and a name. This process is called declaration.

• Initialization: Assigning a value at the time of declaration.

• Naming Rules: Must start with a letter or underscore, cannot be a keyword (like int), and are case-sensitive (age and Age are different).

#include <stdio.h>

int main() {
    int score;         // Declaration
    score = 10;        // Assignment
    
    int level = 1;     // Declaration + Initialization
    
    printf("Initial Score: %d\n", score);
    
    score = 20;        // Value changed (Variable)
    printf("Updated Score: %d\n", score);

    return 0;
}

Expected Output:

Initial Score: 10
Updated Score: 20

2. Constants (const and #define)

There are two primary ways to define constants in C. Using constants makes your code "cleaner" because you avoid "magic numbers" (random numbers in your code whose purpose isn't clear).

A. The const Keyword

This creates a variable that is read-only.

B. The #define Preprocessor

This replaces every instance of a name with a value before the program even starts compiling. It does not occupy memory like a variable does.

#include <stdio.h>

// Method 1: Preprocessor constant (No semicolon at the end!)
#define PI 3.14159

int main() {
    // Method 2: The const keyword
    const int MAX_USERS = 100;

    printf("Value of PI: %.5f\n", PI);
    printf("Max Users allowed: %d\n", MAX_USERS);

    // MAX_USERS = 200; // This would cause a COMPILE ERROR
    
    return 0;
}

Expected Output:

Value of PI: 3.14159
Max Users allowed: 100