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

Fundamentals of C, Data types, Talking user input in C

## 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 -

- Read radius of circle from user. Store it in a variable say radius.
- Apply the formulas to compute diameter, circumference and area - diameter = 2 * radius, circumference = 2 * 3.14 * r and area = 3.14 * r * r.
- Print all resultant values diameter, circumference and area.

Let us now 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; }

**Important note: ** The above program contains a constant value PI. It is always advisable to use a constant variable to represent such types of constants. The constant PI is defined in math.h header file as 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 PI referred as 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; }

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

- Read more if you need to evaluate exponents. C program to find power of any number x^y

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

**Note:** %.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.

Happy coding ;)

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