Example: Circle Area and Circumference Calculator in C

// Circle Area and Circumference Calculator
#include <stdio.h>  // Required for input/output operations (printf, scanf)
#include <math.h>   // Required for M_PI constant and mathematical functions

// On some compilers, _USE_MATH_DEFINES is needed to expose M_PI
#ifndef M_PI
    #define M_PI 3.14159265358979323846
#endif

// Function to calculate the area of a circle
// Takes the radius as a double and returns the area as a double
double calculateArea(double radius) {
    return M_PI * radius * radius;
}

// Function to calculate the circumference of a circle
// Takes the radius as a double and returns the circumference as a double
double calculateCircumference(double radius) {
    return 2 * M_PI * radius;
}

int main() {
    // Step 1: Declare a variable to store the radius
    double radius;

    // Step 2: Prompt the user to enter the radius
    printf("Enter the radius of the circle: ");

    // Step 3: Read the radius from the user
    // %lf is used for reading double values with scanf
    scanf("%lf", &radius);

    // Step 4: Call the calculateArea function and store the result
    double area = calculateArea(radius);

    // Step 5: Call the calculateCircumference function and store the result
    double circumference = calculateCircumference(radius);

    // Step 6: Print the calculated area, formatted to two decimal places
    printf("Area of the circle: %.2f square units\n", area);

    // Step 7: Print the calculated circumference, formatted to two decimal places
    printf("Circumference of the circle: %.2f units\n", circumference);

    return 0; // Indicate successful program execution
}