Area and Circumference of a Circle in C

Algorithm

1. Read the radius as a decimal number.
2. Area = π × r × r.
3. Circumference = 2 × π × r.
4. Print both values with two decimal places.

C program

#include <stdio.h>

#define PI 3.14159

int main(void) {
    double radius, area, circumference;

    printf("Enter radius:\n");
    if (scanf("%lf", &radius) != 1 || radius < 0) {
        printf("Error: radius must be a non-negative number\n");
        return 1;
    }

    area = PI * radius * radius;
    circumference = 2 * PI * radius;

    printf("Area = %.2f\n", area);
    printf("Circumference = %.2f\n", circumference);
    return 0;
}

Sample input and output

7

Enter radius:
Area = 153.94
Circumference = 43.98

0

Enter radius:
Area = 0.00
Circumference = 0.00

How the program works

The area uses the square of the radius and the circumference uses the radius once, so the two formulas differ in how the radius is multiplied. PI is a macro so the constant is written in one place.

A negative radius is rejected because it has no geometric meaning, even though the formulas would still produce a number.

Viva questions

Common mistakes

• Using int for the radius, which makes 2.5 read as 2.
• Writing PI * radius ^ 2 in C, where ^ is bitwise XOR, not a power operator.