Do you want to learn **how to write a C program that can calculate the perimeter and area of a circle with a given radius. **In this program, I will show you how to do that in a few simple steps. You will also learn some basic concepts of C programming, such as variables, constants, input/output, and math functions.

#### What is a Circle?

A circle is a geometric shape that consists of all the points that are equidistant from a fixed point called the center. The distance from the center to any point on the circle is called the radius. The perimeter of a circle is the length of the boundary of the circle, and it is also known as the circumference. The area of a circle is the amount of space enclosed by the circle.

#### How to Calculate the Perimeter and Area of a Circle?

To calculate the perimeter and area of a circle, we need to use two mathematical formulas:

- Perimeter = 2 * PI * radius
- Area = PI * radius * radius

PI is a constant that represents the ratio of the circumference of a circle to its diameter. It is approximately equal to **3.14159.**

#### How to Write a C Program to Calculate the Perimeter and Area of a Circle?

To write a C program that can calculate the perimeter and area of a circle with a given radius, we need to follow these steps:

- Include the header files stdio.h and math.h. These files contain the functions and constants that we need for input/output and math operations.
- Define a macro named PI with the value
**3.14159**. This will allow us to use PI as a constant in our program. - Declare three variables of type double: radius, perimeter, and area. These variables will store the input and output values.
- Prompt the user to enter the radius of the circle using the printf function.
- Read the input value from the user using the scanf function and store it in the radius variable.
- Calculate the perimeter of the circle using the formula:
**perimeter = 2 * PI * radius.** - Calculate the area of the circle using the formula:
**area = PI * pow(radius, 2)**. The pow function is defined in math.h and it returns the value of**x**raised to the power of**y**. - Display the output values using the printf function with two decimal places.
- Return 0 from the main function to indicate successful execution.

#### C Code

**Here is how our C program looks like:**

#include <stdio.h>

#include <math.h>

#define PI 3.14159

int main()

{

double radius, perimeter, area;

printf("Enter the radius of the circle: ");

scanf("%lf", &radius);

perimeter = 2 * PI * radius;

area = PI * pow(radius, 2);

printf("The perimeter of the circle is %.2f\n", perimeter);

printf("The area of the circle is %.2f\n", area);

return 0;

}

#### Output

We will see something like this on our screen:

Enter the radius of the circle:5The perimeter of the circle is 31.42

The area of the circle is 78.54

We can change the input value and see how our program calculates different output values.

#### Conclusion

In this program, you learned **how to write a C program that can calculate the perimeter and area of a circle with a given radius.** You also learned some basic concepts of C programming, such as variables, constants, input/output, and math functions.

I hope you found this tutorial helpful and informative. If you want to learn more about C programming, you can check out some of my other posts on this topic. Thank you for reading and happy coding!

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