In this article, we will write a C program to find diameter, circumference and area of a circle using functions.

The program takes the radius of the circle as input from the user and prints its diameter, circumference and area on the screen as an output.

**Sample Input:**

`Enter the Radius of the Circle: 3`

**Output:**

```
The diameter of the circle is: 6.000000
The circumference of the circle is: 18.840000
The Area of the Circle is: 28.260000
```

If we have a circle having a radius **r**, then its diameter, circumference, and area are given by the following formulas:

Where π is a constant having a fixed value of 22/7 or 3.14

To calculate the diameter, circumference and area of the circle, we have defined three functions `circleDiameter()`

, `circleCircumference()`

and `circleArea()`

.

Each function calculates the respective value and returns the result.

See implementation in the following C program:

// C program to calculate the diameter, // circumference and area of a circle #include <stdio.h> // User defined function to calculate circle's diameter float circleDiameter(float radius){ // Return Circle's diameter return 2 * radius; } // User defined function to calculate circle's circumference float circleCircumference(float radius){ const float PI = 3.14; // Return the circumference return 2 * PI * radius; } // User defined function to calculate circle's area float circleArea(float radius){ const float PI = 3.14; // Return the area return PI * radius * radius; } int main(){ // Declare the variables float radius, diameter, circumference, area; printf("Enter the Radius of the Circle: "); scanf("%f", &radius); // Call the functions diameter = circleDiameter(radius); circumference = circleCircumference(radius); area = circleArea(radius); // Print the result printf("The diameter of the circle is: %f", diameter); printf("\nThe circumference of the circle is: %f", circumference); printf("\nThe Area of the Circle is: %f", area); return 0; }

**Output:**

Enter the Radius of the Circle: 3 The diameter of the circle is: 6.000000 The circumference of the circle is: 18.840000 The Area of the Circle is: 28.260000

### Program Explanation:

- The program first asks the user to enter the radius of the circle which is stored in the
`radius`

variable. - To calculate the diameter, circumference and area of the circle, we have defined three functions circleDiameter(), circleCircumference(), circleArea().
- Each function takes the radius of the circle as an argument, calculates the respective value and returns the result.
- The returned values are stored in the respective variables.
- Finally, the calculated value of the diameter, circumference and area of the circle is printed on the screen using the printf() function.

Thanks for reading.