C++ Program to Find Area of Circle with Example

To find the area of a circle in C++, use the formula area = 3.14159 * radius * radius and print the result. For example: double area = 3.14159 * radius * radius;

Examples

Inputradius = 0

OutputArea of circle: 0

Inputradius = 5

OutputArea of circle: 78.53975

Inputradius = 10

OutputArea of circle: 314.159

How to Think About It

To find the area of a circle, first get the radius from the user. Then multiply the radius by itself and by the constant pi (about 3.14159). This gives the area. Finally, show the result.

Algorithm

1

Get the radius value from the user

2

Calculate area using formula: area = pi * radius * radius

3

Display the calculated area

Code

cpp

#include <iostream>
using namespace std;

int main() {
    double radius, area;
    const double pi = 3.14159;
    cout << "Enter radius: ";
    cin >> radius;
    area = pi * radius * radius;
    cout << "Area of circle: " << area << endl;
    return 0;
}

Output

Enter radius: 5
Area of circle: 78.53975

Dry Run

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

1

Input radius

User enters radius = 5

2

Calculate area

area = 3.14159 * 5 * 5 = 78.53975

3

Print area

Output: Area of circle: 78.53975

Variable	Value
radius	5
area	78.53975

Why This Works

Step 1: Get radius input

We ask the user to enter the radius, which is the distance from the center to the edge of the circle.

Step 2: Calculate area

We use the formula area = pi * radius * radius because the area of a circle depends on the square of its radius.

Step 3: Display result

We print the calculated area so the user can see the size of the circle's surface.

Alternative Approaches

Use M_PI constant from <cmath>

cpp

#include <iostream>
#include <cmath>
using namespace std;

int main() {
    double radius, area;
    cout << "Enter radius: ";
    cin >> radius;
    area = M_PI * radius * radius;
    cout << "Area of circle: " << area << endl;
    return 0;
}

Using <cmath> and M_PI gives a more precise value of pi but requires including the cmath header.

Use float instead of double

cpp

#include <iostream>
using namespace std;

int main() {
    float radius, area;
    const float pi = 3.14159f;
    cout << "Enter radius: ";
    cin >> radius;
    area = pi * radius * radius;
    cout << "Area of circle: " << area << endl;
    return 0;
}

Using float uses less memory but is less precise than double.

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 radius and area, so space complexity is O(1).

Which Approach is Fastest?

All approaches run in constant time; using M_PI is slightly more precise but does not affect speed significantly.

Approach	Time	Space	Best For
Using constant pi	O(1)	O(1)	Simple and clear code
Using M_PI from	O(1)	O(1)	More precision with standard constant
Using float type	O(1)	O(1)	Less memory use, less precision