C# Program to Find Area of Circle with Output

To find the area of a circle in C#, use the formula area = Math.PI * radius * radius where radius is the circle's radius; for example, double area = Math.PI * radius * radius; calculates the area.

Examples

Inputradius = 0

OutputArea of circle: 0

Inputradius = 5

OutputArea of circle: 78.53981633974483

Inputradius = 10.5

OutputArea of circle: 346.3605900582744

How to Think About It

To find the area of a circle, first understand that the area depends on the radius squared multiplied by the constant pi. So, you take the radius, multiply it by itself, then multiply by pi. This gives the total space inside the circle.

Algorithm

1

Get the radius value from the user or input.

2

Calculate the area by multiplying pi with the square of the radius.

3

Display the calculated area.

Code

csharp

using System;

class Program {
    static void Main() {
        Console.Write("Enter radius of circle: ");
        double radius = Convert.ToDouble(Console.ReadLine());
        double area = Math.PI * radius * radius;
        Console.WriteLine("Area of circle: " + area);
    }
}

Output

Enter radius of circle: 5
Area of circle: 78.53981633974483

Dry Run

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

1

Input radius

User enters 5, so radius = 5

2

Calculate area

area = Math.PI * 5 * 5 = 3.141592653589793 * 25 = 78.53981633974483

3

Output area

Prints 'Area of circle: 78.53981633974483'

Step	Radius	Area Calculation	Area Result
1	5	N/A	N/A
2	5	3.141592653589793 * 5 * 5	78.53981633974483
3	5	N/A	78.53981633974483

Why This Works

Step 1: Use radius input

The program asks for the radius, which is the distance from the center to the edge of the circle.

Step 2: Calculate area using formula

It uses Math.PI for pi and multiplies it by the radius squared to get the area.

Step 3: Display the result

Finally, it prints the area so the user can see the space inside the circle.

Alternative Approaches

Using a method to calculate area

csharp

using System;

class Program {
    static double CalculateArea(double r) {
        return Math.PI * r * r;
    }
    static void Main() {
        Console.Write("Enter radius: ");
        double radius = Convert.ToDouble(Console.ReadLine());
        double area = CalculateArea(radius);
        Console.WriteLine("Area of circle: " + area);
    }
}

This approach separates calculation logic into a method for better code reuse and clarity.

Using inline expression with var

csharp

using System;

class Program {
    static void Main() {
        Console.Write("Radius: ");
        var r = double.Parse(Console.ReadLine());
        var area = Math.PI * r * r;
        Console.WriteLine($"Area: {area}");
    }
}

Using <code>var</code> and string interpolation makes the code shorter and easier to read.

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

Time Complexity

The calculation involves a fixed number of operations (multiplications), so it runs in constant time.

Space Complexity

Only a few variables are used, so the space needed is constant.

Which Approach is Fastest?

All approaches run in constant time and space; differences are mainly in code organization and readability.

Approach	Time	Space	Best For
Direct calculation in Main	O(1)	O(1)	Simple quick programs
Separate method for calculation	O(1)	O(1)	Reusable and clear code
Using var and interpolation	O(1)	O(1)	Concise and modern syntax