/*------------------------------------------------------------------*/
/* circle.c                                                         */
/* Author: Bob Dondero                                              */
/* Definition of a simple main function.                            */
/*------------------------------------------------------------------*/

#include <stdio.h>
#include <stdlib.h>

int main(void)

/* Read a circle's radius from stdin and write its diameter and
   circumference to stdout.  Return 0 if successful. */

{
   const double PI = 3.14159;  /* or (4.0 * atan(1.0)) */
   int iRadius;
   int iDiam;
   double dCircum;
   int iScanfRet;

   printf("Enter the circle's radius:\n");
   iScanfRet = scanf("%d", &iRadius);
   if (iScanfRet != 1)
   {
      fprintf(stderr, "Error: Not a number.\n");
      exit(EXIT_FAILURE);
   }

   iDiam = 2 * iRadius;
   dCircum = PI * (double)iDiam;

   printf("A circle with radius %d has diameter %d\n",
      iRadius, iDiam);
   printf("and circumference %f.\n", dCircum);

   return 0;
}

/* Sample executions:

$ gcc -Wall -ansi -pedantic circle.c -o circle

$ circle
Enter the circle's radius:
5
A circle with radius 5 has diameter 10
and circumference 31.415900.

$ circle
Enter the circle's radius:
1
A circle with radius 1 has diameter 2
and circumference 6.283180.

$ circle
Enter the circle's radius:
abc
Error: Not a number.

*/