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.

Example
Input

Enter radius: 10

Output

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 -
Circle formulas

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.

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

Happy coding 😉

About Pankaj

Pankaj Prakash is the founder, editor and blogger at Codeforwin. He loves to learn new techs and write programming articles especially for beginners. He works at Vasudhaika Software Sols as a Software Design Engineer and manages Codeforwin. In short Pankaj is Web developer, Blogger, Learner, Tech and Music lover.

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