C Program to Find Perimeter of Circle

To find the perimeter of a circle in C, use the formula perimeter = 2 * 3.14159 * radius and write a program that takes radius as input and prints the perimeter.

Examples

Inputradius = 1

OutputPerimeter of circle = 6.28318

Inputradius = 5

OutputPerimeter of circle = 31.41590

Inputradius = 0

OutputPerimeter of circle = 0.00000

How to Think About It

To find the perimeter of a circle, you need the radius. The perimeter is the distance around the circle, calculated by multiplying 2 by pi (approximately 3.14159) and then by the radius. So, you get the radius from the user, multiply it by 2 and pi, and then show the result.

Algorithm

1

Get the radius value from the user

2

Calculate perimeter using formula perimeter = 2 * pi * radius

3

Print the calculated perimeter

Code

c

#include <stdio.h>

int main() {
    float radius, perimeter;
    const float pi = 3.14159f;

    printf("Enter radius of circle: ");
    scanf("%f", &radius);

    perimeter = 2 * pi * radius;

    printf("Perimeter of circle = %.5f\n", perimeter);
    return 0;
}

Output

Enter radius of circle: 5
Perimeter of circle = 31.41590

Dry Run

Let's trace the example where radius = 5 through the code

1

Input radius

User enters 5, so radius = 5

2

Calculate perimeter

perimeter = 2 * 3.14159 * 5 = 31.4159

3

Print result

Output: Perimeter of circle = 31.41590

radius	perimeter
5	31.4159

Why This Works

Step 1: Getting radius input

The program asks the user to enter the radius, which is stored in a variable for calculation.

Step 2: Calculating perimeter

It uses the formula 2 * pi * radius to find the perimeter, where pi is approximated as 3.14159.

Step 3: Displaying the result

The program prints the perimeter with 5 decimal places to show a clear and precise output.

Alternative Approaches

Using math.h constant M_PI

c

#include <stdio.h>
#include <math.h>

int main() {
    double radius, perimeter;
    printf("Enter radius of circle: ");
    scanf("%lf", &radius);
    perimeter = 2 * M_PI * radius;
    printf("Perimeter of circle = %.5lf\n", perimeter);
    return 0;
}

This uses the constant M_PI from math.h for better precision but requires linking with math library.

Using double type for more precision

c

#include <stdio.h>

int main() {
    double radius, perimeter;
    const double pi = 3.141592653589793;
    printf("Enter radius of circle: ");
    scanf("%lf", &radius);
    perimeter = 2 * pi * radius;
    printf("Perimeter of circle = %.10lf\n", perimeter);
    return 0;
}

Using double type and more precise pi value gives more accurate results for large radius values.

Complexity: O(1) time, O(1) space

Time Complexity

The program performs a fixed number of operations regardless of input size, so it runs in constant time O(1).

Space Complexity

Only a few variables are used to store input and output, so space complexity is constant O(1).

Which Approach is Fastest?

All approaches run in constant time; using math.h's M_PI may add slight overhead but improves precision.

Approach	Time	Space	Best For
Basic float with manual pi	O(1)	O(1)	Simple programs and beginners
Using math.h M_PI constant	O(1)	O(1)	Precision and standard constants
Double type with precise pi	O(1)	O(1)	High precision calculations