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

## Required knowledge

Arithmetic operators, Data types, Defining constants, Basic input/output

## 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 –

- Input radius of circle from user. Store it in a variable say
`radius`. - Apply the formulas to calculate diameter, circumference and area. Use
`diameter = 2 * radius`

,`circumference = 2 * 3.14 * radius`

and`area = 3.14 * radius * radius`

. - 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()
{
float radius, diameter, circumference, area;
/*
* Input radius of circle from user
*/
printf("Enter radius of circle: ");
scanf("%f", &radius);
/*
* Calculate diameter, circumference and area
*/
diameter = 2 * radius;
circumference = 2 * 3.14 * radius;
area = 3.14 * (radius * 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()
{
float radius, diameter, circumference, area;
/*
* Input radius of circle from user
*/
printf("Enter radius of circle: ");
scanf("%f", &radius);
/*
* Calculate diameter, circumference and area of circle
*/
diameter = 2 * radius;
circumference = 2 * M_PI * radius;
area = M_PI * (radius * 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 ðŸ˜‰