Step by Step solution about How to resolve the algorithm ISBN13 check digit step by step in the C programming language

The provided C code implements a function, check_isbn13, that checks the validity of a 13-digit ISBN (International Standard Book Number) and a main function, main, that tests the function with several ISBNs.

• Header Files:

  • The code includes the necessary standard header file, <stdio.h>, for input and output operations.

• Function check_isbn13:

  • This function takes a constant character array isbn as input and returns an integer indicating whether the ISBN is valid (1) or not (0) based on the following criteria:
    • ISBN must contain exactly 13 digits (excluding hyphens or spaces).
    • ISBN must consist of digits only (0-9).
    • ISBN must follow the specific weighted sum algorithm, where each digit (except the checksum digit) is assigned a weight based on its position. The weight alternates between 1 and 3, starting with 1 for the first digit and 3 for the second digit, and so on. The sum of the weighted digits should be divisible by 10.

• Implementation of check_isbn13:

  • It initializes the current character ch to the first character of the isbn, count to 0, and sum to 0.
  • It iterates through the characters of the isbn until the end is reached (ch != 0).
  • Within the loop:
    • It skips hyphens or spaces (ch == ' ' || ch == '-') in the ISBN.
    • It checks if the current character is a digit; if not, it returns 0, indicating an invalid ISBN.
    • It calculates the weighted sum based on the position of the digit (odd positions have a weight of 3, even positions have a weight of 1).
  • After processing all characters, it checks if the count is equal to 13, which is the expected number of digits in an ISBN.
  • Finally, it checks if the sum is divisible by 10 (i.e., sum % 10 == 0). If it is, the ISBN is considered valid, and the function returns 1; otherwise, it returns 0.

• Function main:

  • This is the entry point of the program.
  • It defines an array of four constant character arrays, isbns, each representing a different ISBN.
  • It iterates through the ISBNs, passes each one to the check_isbn13 function, and prints whether the ISBN is valid or not.

• Usage:

  • The code demonstrates how to check the validity of ISBNs and displays the results for the provided ISBNs in isbns.

#include <stdio.h>

int check_isbn13(const char *isbn) {
    int ch = *isbn, count = 0, sum = 0;
    /* check isbn contains 13 digits and calculate weighted sum */
    for ( ; ch != 0; ch = *++isbn, ++count) {
        /* skip hyphens or spaces */
        if (ch == ' ' || ch == '-') {
            --count;
            continue;
        }
        if (ch < '0' || ch > '9') {
            return 0;
        }
        if (count & 1) {
            sum += 3 * (ch - '0');
        } else {
            sum += ch - '0';
        }
    }
    if (count != 13) return 0;
    return !(sum%10);
}

int main() {
    int i;
    const char* isbns[] = {"978-1734314502", "978-1734314509", "978-1788399081", "978-1788399083"};
    for (i = 0; i < 4; ++i) {
        printf("%s: %s\n", isbns[i], check_isbn13(isbns[i]) ? "good" : "bad");
    }
    return 0;
}