# C Program to Find the Area of a Circle

Problem Description

Write a C program to calculate the area of a circle using the given radius.

What is the area of a circle?

The area of a circle is defined as π*r*r, where π is a constant with the value of approximately 3.142 and r is the radius of the circle.

Formula to calculate the area of a circle:

`Area = pi*r*r`

Example:
If the radius of a circle is 30, then the area of circle is pi*r*r = 3.14*30*30 = 2827.80.

Problem Solution

1. Start the program.
2. Get input for the radius of the circle (a floating-point value).
3. Calculate the area of the circle using the formula pi*(r2)
4. Print the area of the circle.
5. End the Program.

For a better understanding of the area of a circle program, consider the following two examples:

Method 1: (Naive Approach)

This approach calculates the area of a circle using the given radius and prints the result as a floating-point number.

Examples:

• If the radius of a circle is 10, then its area would be 3.141592*10*10 = 314.16 approx.
• If the radius is 11.1, then the area of the circle would be 3.141592*11.1*11.1 = 387.08 approx.
Program/Source Code

Here is source code of the C program to calculate the area of a circle. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```#include <stdio.h>
//Defining the value of pi upto 6 decimal places
#define pi 3.141592
int main()
{

// Calculate the area of the circle

// Print the area of the circle, rounded to two decimal places
return 0;
}```
Program Explanation

1. Get the radius of the circle from the user.
2. Calculate the area of the circle using the formula pi*r*r
3. Print the area of the circle, rounded to the desired precision.

Time Complexity: O(1)
The above program has no iterative statements, no function calls, and only performs input and output operations. Therefore, its time complexity is O(1).

Space Complexity: O(1)
The above program does not require any auxiliary space. Therefore, its space complexity is O(1).

Runtime Test Cases

Testcase 1: In this case, input the radius as “10” to calculate the area of the circle.

```Enter Radius of Circle:
10
The area of Circle with radius 10 is: 314.16```

Testcase 2: In this case, input the radius as “11.1” to calculate the area of the circle.

```Enter Radius of Circle:
11.1
The area of Circle with radius 11.1 is: 387.08```

Method 2: (Calculate Area of Circle using Function)

In this method, we calculate the area of a circle using the pow function and then print the result as a floating-point number.

Program/Source Code

Here is source code of the C program to calculate the area of a circle using function. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

```/*
* C program to find the area of a circle, given the radius
*/

#include <stdio.h>
#include <math.h>
#define PI 3.142

void main()
{

printf("Enter the radius of a circle \n");
area = PI * pow(radius, 2);
printf("Area of a circle = %5.2f\n", area);
}```
Program Explanation

This C program calculates the area of a circle. It first asks the user for the radius of the circle and then uses the formula PI * radius * radius to calculate the area. Finally, it prints the area of the circle to the console.

Time Complexity: O(1)
The time complexity of the above program is O(1), as there are no iterative statements.

Space Complexity: O(1)
The space complexity of the program is also O(1), as no auxiliary space is required.

Runtime Test Cases

In this case, input the radius as “15” to calculate the area of the circle.

```
Enter the radius of a circle
15
Area of a circle = 706.95```

To practice programs on every topic in C, please visit “Programming Examples in C”, “Data Structures in C” and “Algorithms in C”.

If you find any mistake above, kindly email to [email protected]