C program to find diameter, circumference and area of circle

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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 -
Circle formula

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. Read radius of circle from user. Store it in a variable say radius.
  2. Apply the formulas to compute diameter, circumference and area - diameter = 2 * radius, circumference = 2 * 3.14 * r and area = 3.14 * r * r.
  3. 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.

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

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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