C program to calculate area and perimeter of a circle has been given below. Suppose, the radius of a circle is r unit, the area and perimeter of that circle would be (pi * r^2) unit^2 and (2 * pi * r) unit, respectively. The approximate value of pi is 3.141592.

For example, if the radius of a circle is 5 cm, then area of the circle would be (pi * 5^2) = 78.54 cm^2 while perimeter would be (2 * pi * r) = 31.42 cm.

The algorithm, pseudocode, and time-complexity of the program have also been covered below.

## 1. Algorithm to calculate area and perimeter of a circle

1. Take the radius r of a circle as input.

2. Compute a = pi * r^2

3. Compute p = 2 * pi * r

4. Declare a as the area and p as the perimeter of that circle.

## 2. Pseudocode to calculate area and perimeter of a circle

Input : Radius r of a circle

Output : Area A, Perimeter P of the circle

1. Procedure areaPerimeter(r):

2. A := pi * r^2

3. P := 2 * pi * r

4. Return A, P

5. End Procedure

## 3. Time complexity to calculate area and perimeter of a circle

Time Complexity: O(1)

## 4. C Program & Output to calculate area and perimeter of a circle

Code has been copied
/*****************************
alphabetacoder.com
C program to calculate area
and perimeter of a rectangle
******************************/

#include <stdio.h>
#define pi 3.141592

int main() {
// declare variables
float r, a, p;

// take input
printf("Enter the radius of a circle: ");
scanf("%f", & r);

// calculate area
a = pi * r * r;
// calculate perimeter
p = 2 * pi * r;

// display result upto 3 decimal places
printf("Area: %0.3f\n", a);
printf("Perimeter: %0.3f\n", p);

return 0;
}


Output

Enter the radius of a circle: 6

Area: 113.097

Perimeter: 37.699