**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

/*********************************** alphabetacoder.com C++ program to calculate diameter, area and perimeter of a circle ***********************************/ #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