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.

Enter radius: 10

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 –

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()
{
    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.

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 😉