Question

Write an ISBN Validator in C language

What are ISBN?

ISBN, or International Standard Book Number, is a numeric identifier for commercially printed books. Prior to 2007, they had 10 digits, but all newly issued ISBN use 13 digits. They are used to uniquely identify a book and consist of several portions. These include the registrant group, publisher, title, and a check digit. The registrant group indicates where the publisher is located. For example a registrant group of 0 or 1 indicates an English speaking area, while 960 indicates Greece, and 99936 indicates Bhutan. The publisher and title are codes issued by various country specific groups. In the US this is R.R. Bowker. Note that the length of the registrant group, title and publisher are not of a fixed width which gives flexibility to the issuers to allocate more numbers to higher demand groups. For instance there are more books published in English than in Greek, thus the 0/1 registrant group for English and 960 for Greek. In addition, the final digit of an ISBN is a check digit. This digit is calculated from the others and helps identify valid ISBN versus ones that may have a typo.

Customarily, ISBN are specified with hyphens or spaces separating the various groups, but these are aesthetic only. They do not affect the calculation of the check digit. However, to simplify reading the ISBN, you only need to handle hyphens.

ISBN-10

ISBN-10 have 9 characters specifying the groups detailed above and one 1 character for a check digit. The 9 group characters are all digits ranging from 0 through 9. The check digit is base 11, so it is represented as 0-9 with 10 represented as an X.

The check digit satisfies the following equation

10x1+9x2+8x3+7x4+6x5+5x6+4x7+3x8+2x9+x10≡0(mod11)10x1+9x2+8x3+7x4+6x5+5x6+4x7+3x8+2x9+x10≡0(mod11)

where x10x10 is the check digit. Note the relationship between the coefficient and the digit (look at their sum).

Note there can be only one X in a valid ISBN-10, in the position of the checksum digit.

ISBN-13

ISBN-13 have 13 characters. 12 of them specify the groups as detailed above with the final character a check digit. Unlike in ISBN-10, the check digit of ISBN-13 is always one of 0 through 9.

The check digit satisfies the following equation:

x1+3x2+x3+3x4+x5+3x6+x7+3x8+x9+3x10+x11+3x12+x13≡0(mod10)x1+3x2+x3+3x4+x5+3x6+x7+3x8+x9+3x10+x11+3x12+x13≡0(mod10)

where x13x13 is the check digit. Note the relationship between the coefficient and the digit read.

Note that a valid ISBN-13 does not have any X in them.

Your Program

Your program should perform the following actions:

1. Prompt the user to enter an ISBN. Your program should read this in as a C-string (NUL-terminated character array).
2. Loop through the string character by character, stopping when you see a NUL character ('\0').
   As you go, update the calculation of the ISBN-10 and ISBN13 checks.
3. At the end of the loop, if you have
   1. read the correct number of digits,
   2. and the checksum is zero,
   3. and if an X appears, it is only in the check digit of an ISBN-10
   then you have a valid ISBN.
4. Print out your result

Example Executions

User input is in bold

Please enter an ISBN: 0-486-63760-3
The input '0-486-63760-3' is:
Valid ISBN-10
Invalid ISBN-13

Please enter an ISBN: 0-521-33781-X
The input '0-521-33781-X' is:
Valid ISBN-10
Invalid ISBN-13

Please enter an ISBN: 978-3-642-43001-5
The input '978-3-642-43001-5' is:
Invalid ISBN-10
Valid ISBN-13

Please enter an ISBN: 01234567X5
X can only occur as a check digit
The input '01234567X5' is:
Invalid ISBN-10
Invalid ISBN-13

Please enter an ISBN: 0123456789
The input '0123456789' is:
Valid ISBN-10
Invalid ISBN-13

Please enter an ISBN: 012345678
The input '012345678' is:
Invalid ISBN-10
Invalid ISBN-13

Please enter an ISBN: 012345678905X
The input '012345678905X' is:
Invalid ISBN-10
Invalid ISBN-13

Please enter an ISBN: 01234569XX
Invalid number of Xs
The input '01234569XX' is:
Invalid ISBN-10
Invalid ISBN-13

Additional Tests

The following are all valid ISBN.

0-486-63760-3
981-02-4897-0
3-540-20105-X
0-521-33781-X
0-521-33781-x
978-0-486-63760-0
978-0-387-51768-1
978-3-642-43001-5
978-981-02-4897-0
0123456789
0123456789012

The following are all not valid ISBN.

012345678
01234567890
012345678901
0123456789013
01234567890X5
012345678905X
01234567X5
0123456XX7
01234569XX

You should also look around at books you have an verify the ISBN they have on the back work. However, it is possible for there to be errors, so double check they are good at another ISBN validator (Links to an external site.).

Hints

• The function isdigit() (Links to an external site.) defined in <ctype.h> is helpful when processing the characters. It is used like

  #include <ctype.h>
  // ....
  char c;
  // ... something fills in a value for c
  if (isdigit(c)) {
    printf("c is a digit\n");
  }

• When reading your string, if you see anything other than a digit, the letter X, or a hyphen, that makes the ISBN invalid.
• The X in an ISBN could be either lower case or upper case. Make sure to handle both.
• Remember to translate a character from ASCII to an integer when calculating the checksum. If the character is a digit, this can be done by

  value = c - '0';

  and if it is an X, then the value is just 10.
• As you loop through the string, it is useful to track the number of digits read. This can be used to calculate the coefficients for the checksum calculations.
• As you loop through the string, it is useful to track the number of Xs you encounter and the last digit you read. This is useful to know if you have read multiple Xs (and thus invalidate the ISBN) and to verify only the last digit read, the checksum, could be an X. Remember only ISBN-10 allow X, so any Xs invalidate ISBN-13s

Answer 1

[Note that some the data provided for testing are actually invalid ISBNs. Please do manually check before flagging the program as erroneous]

PROGRAM

//program to validate ISBN Numbers

//include header files

#include<stdio.h>

#include<ctype.h>

//main function

int main(void)

{

    //declaring variables

    int sum = 0,//hold sum of isbn numbers

        digit, //extract a digit from isbn

        digitcount, //count the digits in isbn

        i, //temporary loop variable

        xflag=0, //check the presence of character x in isbn

        xcount=0, //count occurrence of x

        invalidcharflag; //hold status of invalid character in isbn

    char isbn[20], //hold isbn

        c; //temprary variable to hold repeat choice

     //Repeat till user quits

     do{

        //read isbn

        printf("Enter an ISBN: ");

        scanf("%[^\n]",isbn);

        //set flags

        invalidcharflag=0;

        xflag=0;

        digitcount=0;

        //analyze isbn for validity of isbn10

        for (i = 0; isbn[i]!='\0'; i++)

        {

            //check for invalid characters

            if(!(isdigit(isbn[i]) || isbn[i]=='-'||toupper(isbn[i]=='X')))

            {

                invalidcharflag=1;

                break;

            }

            //check and process x

            if(toupper(isbn[i])=='X')

            {

                digit=10;

                xflag=1;

                xcount++;

            }

            else

                digit=isbn[i]-'0';

           //calculate sum

            if(digit>=0 && digit<=10)

                sum += (digit * (10 - i));

            if(isbn[i] != '-') digitcount++;

        }

        //process if isbn is a valid isbn

        printf("\nThe input %s is :",isbn);

        if(invalidcharflag==1)

            printf("\nInvalid ISBN10");

        else if((xflag ==1 && toupper(isbn[i-1])!='X') || xcount>1)

            printf("\nInvalid ISBN10");

        else if(digitcount!=10)

            printf("\nInvalid ISBN10");

        else if(sum%11==0)

            printf("\nValid ISBN10");

        //reset flags for testing for isbn13

        invalidcharflag=0;

        digitcount=0;

        sum=0;

        //processing for isbn13

        for (i = 0; isbn[i]!='\0'; i++)

        {

          //check for invalid characters

            if(!(isdigit(isbn[i]) || isbn[i]=='-'))

            {

                invalidcharflag=1;

                break;

            }

            //calculate sum

            digit=isbn[i]-'0';

            if(isbn[i]!='-')

            {

                if((i+1)%2==1)

                    sum += digit;

                else

                    sum += (digit*3);

                digitcount++;

            }

        }

        //check if the given isbn is a valid isbn13

        if(invalidcharflag==1)

            printf("\nInvalid ISBN13");

        else if(digitcount!=13)

            printf("\nInvalid ISBN13");

        else if(sum%10==0)

            printf("\nValid ISBN13");

        //printf("sum=%d",sum);

        printf("\n\nCheck another ISBN (Y/N): ");

        scanf("%*c%c%*c",&c);

    }

    while(toupper(c)=='Y');

    return 0;

}

SCREENSHOT OF PROGRAM

OUTPUT OF PROGRAM

Enter an ISBN: 0-486-63760-3

The input 0-486-63760-3 is :

Valid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 0-521-33781-X

The input 0-521-33781-X is :

Valid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 978-3-642-43001-5

The input 978-3-642-43001-5 is :

Invalid ISBN10

Valid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 01234567X5

The input 01234567X5 is :

Invalid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 0123456789

The input 0123456789 is :

Invalid ISBN10

Invalid ISBN13

Enter an ISBN: 012345678

The input 012345678 is :

Invalid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 012345678905X

The input 012345678905X is :

Invalid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 01234569XX

The input 01234569XX is :

Invalid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 0-486-63760-3

The input 0-486-63760-3 is :

Invalid ISBN10

Invalid ISBN13

Enter an ISBN: 3-540-20105-X

The input 3-540-20105-X is :

Valid ISBN10

Invalid ISBN13

Check another ISBN (Y/N): y

Enter an ISBN: 0-521-33781-X

The input 0-521-33781-X is :

Invalid ISBN10

Invalid ISBN13