2. Function Declaration (Optional):

• You can declare functions that your program will use. The main function is the entry point of a C program and is mandatory. For example:

int add(int a, int b); // Function declaration

3. Main Function:

• The main function is where the program execution begins. It should return an integer value to indicate the program’s status (usually 0 for success). For example:

int main() {
    // Your code here
    return 0;
}

4. Statements and Expressions:

• Within the main function, you write statements and expressions to perform tasks. For example:

int result = add(5, 3); // Function call
printf("The result is %d\n", result); // Output

5. Comments:

• Comments are used to add explanatory notes to your code. They are preceded by // for single-line comments or enclosed between /* and */ for multi-line comments. For example:

// This is a single-line comment
/*
This is a multi-line comment.
It can span multiple lines.
*/

Real-World Example: Calculating the Area of a Circle

Let’s create a simple C program to calculate the area of a circle using the formula Area = π * r^2. We’ll include preprocessor directives to use the standard I/O library for input and output.

#include <stdio.h>
#define PI 3.14159265359
int main() {
    double radius, area;
    printf("Enter the radius of the circle: ");
    scanf("%lf", &radius);
    area = PI * radius * radius;
    printf("The area of the circle with radius %.2lf is %.2lf\n", radius, area);
    return 0;
}

In this example, we use #include to include the <stdio.h> library for input and output functions. We define the constant PI, declare variables, take user input for the radius, and calculate and display the area of the circle.