#include <stdio.h>
#include <conio.h>
main ()
{
char isbn [11] ;
char CheckDigit (char [11]) ;
int choice ;
char dummy ;

do
   {
   clrscr () ;
   printf ("\n\n\n\n\n\n") ;
   printf ("           ISBN Checker\n") ;
   printf ("           ------------\n\n") ;
   printf ("           1 : Calculate a check digit\n") ;
   printf ("           2 : Validate a ten-character ISBN\n") ;
   printf ("           3 : Quit\n\n") ;
   printf ("           Select an action : ") ;
   scanf ("%d%c", &choice, &dummy) ;
   switch (choice)
      {
      case 1 :
          clrscr () ;
          printf ("\n\n\n\n\n\n") ;
          printf ("           Calculate a check digit\n") ;
          printf ("           -----------------------\n\n") ;
          printf ("           Enter a nine-character ISBN: ") ;
          scanf ("%s", isbn) ;
          printf ("\n           The full ISBN is ") ;
          printf ("%s-", isbn) ;
          printf ("%c\n", CheckDigit (isbn))  ;
 	       printf ("\n           Hit a key to continue ") ;
	       getch ();
          break ;
      case 2 :
          clrscr () ;
          printf ("\n\n\n\n\n\n") ;
          printf ("           Validate a ten-character ISBN\n") ;
          printf ("           -----------------------------\n\n") ;
          printf ("           Enter a ten-character ISBN: ") ;
          scanf ("%s", isbn) ;
          if (CheckDigit (isbn) == isbn [9])
             printf ("\n           Valid ISBN.\n") ;
          else
             printf ("\n           Invalid ISBN\n") ;
 	       printf ("\n           Hit a key to continue ") ;
	       getch ();
          break ;
      }
   }
while (choice < 3) ;
}
char CheckDigit (char isbn [11])
   {

   int count,
       number,
       check_digit,
       sum = 0 ;
   for (count = 0; count <9; count ++)
      {
      number = isbn [count] - '0' ;
      sum = sum + number * (10 - count) ;
      }
   check_digit = (11 - (sum % 11)) % 11 ;
   if (check_digit == 10)
      return 'X' ;
   else
      return check_digit + '0';
   }