`luc070.c`¶

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).

Metadata¶

Property	Detail
Author	Amit Dutta amitdutta4255@gmail.com
Date	08 Feb 2026
License	MIT License (See the LICENSE file for details)
Difficulty	Beginner (index: 0 / 10)

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Source Code¶

#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;
}

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    - Do NOT add extra examples.

    - Keep explanation clear and short.

    - If something is unclear, say "Not clear from code".

    - Follow the exact format below. Do NOT change headings.

    FORMAT:

    [START]

    ## What it does

    (Explain the overall purpose in 1-2 sentences)

    ## Step-by-step

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    ## Key Concepts

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    ## Notes

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    [END]

    CODE (luc070.c):

    #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;
    }

luc070.c¶