# C program to find diameter, circumference and area of circle

Write a C program to input radius of a circle from user and find diameter, circumference and area of the circle. How to calculate diameter, circumference and area of a circle whose radius is given by user in C programming. Logic to find diameter, circumference and area of a circle in C.

Example
Input

`Enter radius: 10`

Output

```Diameter = 20 units
Circumference = 62.79 units
Area = 314 sq. units```

## Properties of circle

Diameter, circumference and area of a circle formula is given by -

Where r is radius of the circle.

## Logic to find diameter, circumference and area of circle

Below is the step by step descriptive logic to find diameter, circumference and area of a circle -

1. Input radius of circle from user. Store it in a variable say radius.
2. Apply the formulas to calculate diameter, circumference and area. Use `diameter = 2 * radius`, `circumference = 2 * 3.14 * radius` and `area = 3.14 * radius * radius`.
3. Print all resultant value diameter, circumference and area.

Let us code the solution.

## Program to find diameter, circumference and area of circle

``````/**
* C program to calculate diameter, circumference and area of circle
*/

#include <stdio.h>

int main()
{

/*
* Input radius of circle from user
*/

/*
* Calculate diameter, circumference and area
*/
circumference = 2 * 3.14 * radius;

/*
* Print all results
*/
printf("Diameter of circle = %.2f units \n", diameter);
printf("Circumference of circle = %.2f units \n", circumference);
printf("Area of circle = %.2f sq. units ", area);

return 0;
}``````

`\n` is an escape sequence character used to add new line (move to next line).

Important note: The above program contains a constant value 3.14. It is always recommended to use constant variable to represent such constants. The constant PI is already defined in `math.h` header file with name M_PI.

Let us rewrite the above program using constant value.

## Program to find diameter, circumference and area of circle using PI constant

``````/**
* C program to calculate diameter, circumference and area of circle
*/

#include <stdio.h>
#include <math.h> // Used for M_PI

int main()
{

/*
* Input radius of circle from user
*/

/*
* Calculate diameter, circumference and area of circle
*/
circumference = 2 * M_PI * radius;

/*
* Print all results
*/
printf("Diameter of circle = %.2f units \n", diameter);
printf("Circumference of circle = %.2f units \n", circumference);
printf("Area of circle = %.2f sq. units ", area);

return 0;
}``````

`%.2f` is used to print the fractional value up to two decimal places. You can also use `%f` to print up to default 6 decimal places.

Important note: C doesn't provides any exponential operator. Hence never play with operations like `radius ^ 2` to find power or to evaluate exponents.

Read more - Program to find power of any number.

Output

```Enter radius of circle: 10
Diameter of circle = 20.00 units
Circumference of circle = 62.79 units
Area of circle = 314.00 sq. units```

Happy coding 😉

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