C++ program to calculate diameter, 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 while the diameter would be 2 * r. 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 and diamerter would be (2 * pi * r) = 31.42 cm and (2 * r) = 10 cm respectively.

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

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

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

2. Compute d = 2 * r

3. Compute a = pi * r^2

4. Compute p = 2 * pi * r

5. Declare d as the diameter, a as the area and p as the perimeter of that circle.

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

Input : Radius r of a circle

Output : Diameter D, Area A, Perimeter P of the circle

1. Procedure diameterAreaPerimeter(r):

2. D := 2 * r

3. A := pi * r^2

4. P := 2 * pi * r

5. Return D, A, P

6. End Procedure

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

Time Complexity: O(1)

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

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

#include <iostream>
#define pi 3.141592

using namespace std;

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

// take input
cout << "Enter the radius of a circle: ";
cin >> r;

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

// display result
cout << "Diameter: " << d << endl;
cout << "Area: " << a << endl;
cout << "Perimeter: " << p << endl;

return 0;
}


Output

Enter the radius of a circle: 6

Diameter: 12

Area: 113.097

Perimeter: 37.6991