Why Use Variables?

• To store data during program execution.
• To perform calculations and operations on data.
• To make programs flexible and easy to manage.

Rules for Naming Variables

Understanding these rules in more detail:

While declaring variables in C, we must follow these rules:

1. Variable names must begin with a letter or underscore (_)

   Valid example:

   _count = 10;

   student = 1;

   Invalid example:

   1marks = 50;   // Invalid: starts with digit
   @price = 20;   // Invalid: starts with special character

2. Variable names can contain letters, digits, and underscores

   Valid example:

   total_1 = 95;

   roll_no = 12;

   Invalid example:

   my-name = 20;   // Invalid: contains hyphen
   my marks = 90;  // Invalid: contains space

3. C is case-sensitive

   This means Total and total are different variables.

   Example:

   int Total = 50;
   int total = 100;
   
   printf("%d\n", Total);
   printf("%d", total);

   Output:

   50

   100

4. Variable names should not be C keywords

   Examples of keywords: int, while, return, for

   Example:

   int int = 10;      // Invalid
   float while = 5;  // Invalid

5. Variable names should be meaningful

   Use studentMarks instead of sm for better readability.

Example Program: Variable Usage in C

// Program to demonstrate variables in C
#include <stdio.h>

int main() {

    int salary = 15000;

    printf("Initial Salary = %d\n", salary);

    salary = 20000;

    printf("Updated Salary = %d", salary);

    return 0;
}

Real-Life Analogy:

Think of a variable as a container with a label on it. We can store a value inside the container and replace it whenever needed. Similarly, variables in C store values that can change during program execution.

Common Mistakes with Variables

Summary: