## 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.

#### Required knowledge

Data Types in c, Input/Output in C, C Operators, Constants in C

#### Properties of circle

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

D=2r

C=2πr

A= πr2

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. 3.Print all resultant value diameter, circumference and area.

#### 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
#include  // 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.

##### Output:
```
Diameter of circle = 20.00 units
Circumference of circle = 62.79 units
Area of circle = 314.00 sq. units
```
```
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```