sp> Someone must of needed to do this sometime:



 sp>  Anyone know how to validate the ISBN numbers from books?  I urgently

 sp> need the algorithm for validating an input field in one of my programs,

 sp> but can't find it anywhere.





Steven,



Here's some code I wrote once to do this. Note that is assumes that the
only character passed to it are the numbers, ie. you have to remove any
hyphens before calling it.





/* This code is released to the public domain */



#include 

#include 

#include 



#define TRUE    1

#define FALSE   0



char ISBN_compute(char *isbn);

int  ISBN_validate(char *isbn);

int  ISBN_sum(char *isbn, int len);



int main(void)

    {

    char isbn[10];



    strcpy(isbn, "188357725");

    printf("%s%c\n", isbn, ISBN_compute(isbn)); /* should be X */

    printf("188357725X - %d\n",
ISBN_validate("188357725X")); /* true */

    printf("1883577251 - %d\n",
ISBN_validate("1883577251")); /* false */



    strcpy(isbn, "000648019");

    printf("%s%c\n", isbn, ISBN_compute(isbn)); /* should be 5 */

    printf("0006480195 - %d\n",
ISBN_validate("0006480195")); /* true */

    printf("000648019X - %d\n",
ISBN_validate("000648019X")); /* false */



    return EXIT_SUCCESS;

    }





char ISBN_compute(char *isbn)

    {

    int     digit;

    char    rc;



    digit = 10 - ((ISBN_sum(isbn, strlen(isbn)) - 1) % 11);

    if (digit == 10)

        rc = 'X';

    else

        rc = '0' + digit;



    return rc;

    }



int ISBN_validate(char *isbn)

    {

    int     rc;

    int     sum;

    int     len;

    char    digit;



    len = strlen(isbn);



    sum = ISBN_sum(isbn, len - 1);

    digit = isbn[len - 1];

    if (digit == 'X')

        sum += 10;

    else

        sum += digit - '0';



    if (sum % 11 == 0)

        rc = TRUE;

    else

        rc = FALSE;



    return rc;

    }



int ISBN_sum(char *isbn, int len)

    {

    int digit;<

    int sum;

    int i;



    sum = 0;

    for (i = 0; i < len; i++)

        {

        digit = isbn[i] - '0';

        sum += digit * (len - i + 1);

        }

    return sum;

    }





Paul



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