luc070.c

Metadata

• Author — Amit Dutta
• Last updated — 08 Feb 2026
• License — MIT License (See the LICENSE file for details)

Problem Statement

Problem Statement

Write a program that receives a 10-digit ISBN number, computes the checksum (d1 + 2d2 + 3d3 + ... + 10d10), and reports whether the ISBN number is correct (sum divisible by 11).

Source Code

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

int main()
{
    char isbn[15];
    int i, sum = 0, digit;

    printf("Enter 10-digit ISBN number: ");
    scanf("%s", isbn);

    /* The formula given is: d1 + 2d2 + 3d3 + ... + 10d10
       where di is the ith digit from the RIGHT.
       
       If input is "007462542X" (Length 10):
       isbn[0] is d10 (Weight 10)
       isbn[1] is d9  (Weight 9)
       ...
       isbn[9] is d1  (Weight 1)
    */

    for (i = 0; i < 10; i++)
    {
        // Handle 'X' which represents 10 in ISBN
        if (isbn[i] == 'X' || isbn[i] == 'x')
            digit = 10;
        else
            digit = isbn[i] - '0';

        // Weight is (10 - i)
        sum += digit * (10 - i);
    }

    printf("Calculated Checksum: %d\n", sum);

    if (sum % 11 == 0)
        printf("The ISBN number is Correct.\n");
    else
        printf("The ISBN number is Incorrect.\n");

    return 0;
}