validate isbn

hey ,guys,i'm stuck trying to validate a ISBN number(w/o hyphens). I want it to check the last digit for a X and update the sum. But i keep getting the wrong sum. What's wrong in my code?

[code]
#include<stdio.h>
#include<ctype.h>

Code:

---

int testISBN(char*);

int main (void)
{
        int divisible;
        char choice, trash;
        char code[15];

        do
        {
                printf("Enter an ISBN (book) number:\n");
                fgets(code,15,stdin);

                printf("The ISBN entered is:%s\n",code);
                divisible = testISBN(code);
                if (divisible == 1)
                                        printf("\n\n\tValid ISBN Number\n");
                else
                                        printf("\n\n\tInvalid ISBN Number\n");
                       
                printf("\nDo you wish to continue? (Y/N)\n");
                choice = getchar();
                trash = getchar();
        }while(toupper(choice) != 'N');
       
        return 0;
}

int testISBN(char*code)
{
        int i;
        int sum = 0;
        int value;

        for(i=9;i>=0; i--)
        {
                if((i==9) && (toupper(*(code + i)) == 'x'))
                        {
                                value = 10;
                        }
                else
                        {
                                value = (int)(*(code + i)-48);
                        }
                sum += value * (i+1);
        }
        printf("sum is %d",sum);
       
        if ((sum%11) == 0)
                return 1;
        return 0;
}

---

Code:

---

    if((i==9) && (toupper(*(code + i)) == 'x'))

---

Perhaps you want:

Code:

---

    if((i==9) && (toupper(*(code + i)) == 'X'))

---

With an uppercase x.

i try replacing with it with an uppeercase X(no good), what i would like to do is how this website explains corrrect ISBN by using mod 11

http://www.ee.unb.ca/tervo/ee4253/isbn.html

if iI input there number 0131391399, my sum does not equal to 143

I'm at work and only had about "half a cup of coffee's" time to look at your problem.

I think the problem is when you are traversing the ISBN number.

You're starting at the far end, and checking for 'X', and you traverse the number towards the beginning.

The article you cite says that you multiply the last digit times 1, and the first digit times 10. You're actually doing the opposite.

You're multipying the last digit times 10, working down to multipying the first digit times 1.

I think that is your problem but again, I only had a couple of minutes to look at it.

fgw_three is right.

Code:

---

char code[15];
fgets(code,15,stdin);

---

When you store like this.The most significant digit is stored at code[0].

Code:

---

for(i=9;i>=0; i--)
        {
                if((i==9) && (toupper(*(code + i)) == 'x'))
                        {
                                value = 10;
                        }
                else
                        {
        value = (int)(*(code + i)-48); \\\By this Yor are moving from least signicant to most significant.
                        }
                sum += value * (i+1);\\here you are multiplying with 10 with least significant,9 with.... so on.Your are doin opposite.
        }

---

I've played with it and still not getting the right sum. Ok, how do i know when it's multiply by the numbers 1-10 in string array, then i'm confused after mulitpy out, then sum them. I know it's simply, I'm not thinking the right way to do this. Please help!!

Code:

---

#include<stdio.h>
#include<ctype.h>

int testISBN(char*);

int main (void)
{
        int divisible;
        char choice, trash;
        char code[15];

        do
        {
                printf("Enter an ISBN (book) number:\n");
                fgets(code,15,stdin);

                printf("The ISBN entered is:%s\n",code);
                divisible = testISBN(code);
                if (divisible == 1)
                                        printf("\n\n\tValid ISBN Number\n");
                else
                                        printf("\n\n\tInvalid ISBN Number\n");
                       
                printf("\nDo you wish to continue? (Y/N)\n");
                choice = getchar();
                trash = getchar();
        }while(toupper(choice) != 'N');
       
        return 0;
}

int testISBN(char*code)
{
        int i;
        int sum = 0;
       

        for(i=0;i<9; i++)  /*loop thru string;checking for char
        {
                if((i==9) && (toupper(*(code + i)) == 'X'))
                        {
                                how do i assign character X to 10
                                (i.e) *(code+9) =10;(maybe)
                                then update sum
                        }
                else
                        {
                                sum = (int)(*(code + i)-48)*(i+1)/*if not char then mutilipy string with num(1-10) ,then sum*/
                        }
               
        }
        printf("sum is %d",sum);/*making sure sum is correct before
                                        returning to main*/
       
        if ((sum%11) == 0)
                return 1;
        return 0;
}

---

Why don't you use more variables to make it clearer, as well as using array notation rather than pointer notation.

Code:

---

int testISBN(char*code)
{
  int i;
  int sum = 0;

  for(i=0;i<9; i++)
  {
    int chVal;
    if( (i==9) && (toupper(code[i]) == 'X') )
    {
      chVal = 10;
    }
    else
    {
      chVal = code[i] - '0';
    }
    sum += chVal;
  }
  printf("sum is %d",sum);

  return ( sum % 11 ) == 0;
}

---

>>how do i assign character X to 10 (i.e) *(code+9) =10;(maybe) then update sum
well,i dont know what exactly you mean by this.but if you wanna assign X(ascii) to the least significant digit.
*(code+9)='X'

Code:

---

sum = (int)(*(code + i)-48)*(i+1)/*if not char then mutilipy string with num(1-10) ,then sum*/

---

You want to multiply most significant with 10 n so on?
so,it should be

Code:

---

sum = (int)(*(code + i)-48)*(10-i)

---

//plus previous sum.

Code:

---

int testISBN(char*code)
{
#define TEN 10
    int i;
    int sum = 0;

    for(i=0;i<=9; i++)  /*loop thru string;checking for char */
    {
        if (isdigit(*(code + i)))
                  sum  +=  ((*(code + i) -= '0') * (i+1));
              else
            if((i==9) && (toupper(*(code + i)) == 'X'))
            {
                //  how do i assign character X to 10
                //  (i.e) *(code+9) =10;(maybe)
                //  then update sum
                sum += TEN * i;
            }
    }
    printf("sum is %d",sum);/*making sure sum is correct before returning to main*/

    if ((sum%11) == 0)
        return 1;
    return 0;
}

---

i guess i'm not tell the truth for code, this is what i like to do
take an isbn number with last digit is an X

0 1 3 2 8 8 3 6 6 X,

if i do have a char X ,then X =10

then do this mulitiply each digit which I believe is a counting for loop

0 1 3 2 8 8 3 6 6 10 char X replace with 10

* 10 9 8 7 6 5 4 3 2 1
_____________________________
then sum
0 +9 +24+14+48+40+12+18+12+10 = 187

Any ideas?

Well you need to implement the right algorithm - it's not just a simple sum, nor everything multiplied by 10.

Code:

---

#include<stdio.h>
#include<ctype.h>

int testISBN(char*);

int main (void)
{
  int isValid;
  char code[15];
  char temp[100];

  do
  {
    printf("Enter an ISBN (book) number:\n");
    fgets(code,sizeof code,stdin);

    printf("The ISBN entered is:%s\n",code);
    isValid = testISBN(code);
    if (isValid) {
      printf("\n\n\tValid ISBN Number\n");
    } else {
      printf("\n\n\tInvalid ISBN Number\n");
    }
    printf("\nDo you wish to continue? (Y/N)\n");
    fgets( temp, sizeof temp, stdin );
  } while(toupper(temp[0]) != 'N');

  return 0;
}

/*
 * http://www.morovia.com/supportforum/forum_posts_TID_55_PN_1.html
 */
int testISBN(char*code)
{
  int i;
  int sum = 0;
  int check;

  /* sum first 9 digits, as per algorithm */
  for ( i = 0 ; i < 9 ; i++)
  {
    int chVal = code[i] - '0';  /* the numeric value of the char */
    int pos = i + 1;            /* the numeric value of its position */
    sum += chVal * pos;
  }

  /* 10th digit is the checksum */
  if ( toupper(code[9]) == 'X' ) {
    check = 10;
  } else {
    check = code[9] - '0';
  }

  printf( "Sum=%d, Sum%%11=%d, check=%d\n", sum, sum%11, check);
  /* sum % 11 should be the check */
  return ( sum % 11 ) == check;
}


// results
$ ./a.out
Enter an ISBN (book) number:
0131103628
The ISBN entered is:0131103628

Sum=107, Sum%11=8, check=8


        Valid ISBN Number

Do you wish to continue? (Y/N)
n

---

hey, thanks to all for your help, I'm almost home to finish it. I had to change some of it to understand it myself. I'm still having probably changing the character X to the number 10. Code sums to the right value when the X in not entered. I can see that when character X is entered a 40 appears(everytime). I need a 10 there, to multiply thru times number 1.

Code:

---

#include<stdio.h>
#include<ctype.h>

int testISBN(char*);

int main (void)
{
    int choice,trash;
    int isValid;
    char code[15];
 

  do
  {
    printf("Enter an ISBN (book) number:\n");
    fgets(code,15,stdin);

    printf("The ISBN entered is:%s\n",code);
    isValid = testISBN(code);
    if (isValid) {
      printf("\n\n\tValid ISBN Number\n");
    } else {
      printf("\n\n\tInvalid ISBN Number\n");
    }
    printf("\nDo you wish to continue? (Y/N)\n");
    choice=getchar();
        trash=getchar();
  } while(toupper(choice) != 'N');

  return 0;
}


int testISBN(char*code)
{
  int i;
  int sum = 0;


  /* sum first 9 digits */
  for ( i = 9 ; i>=0 ; i--)
  {
    int chVal = (*(code+i)) - 48; /* the numeric value of the char */
    int pos = 10-i;              /* the numeric value of its position */
    printf("chval is %d\n,pos is +%d\n",chVal,pos);
                                  /* i put this here to check values of 
                                        strings and position number*/

  /* 10th digit is the checksum */
  if (toupper(code[9]) == 'X' )
  {
        *(code+9)=10;
        sum+=chVal*pos;
  }
  else
  {
    sum+=chVal*pos;
  }
  }
  printf("Sum=%d", sum);
  /* sum / 11 is check */
  return ( sum /11 );

}

---

It's remakable how you can take something which works and trash it with dumb stuff like
> return ( sum /11 );

Where in the hell did you get division from when the algorithm clearly states modulus.

> /* 10th digit is the checksum */
This code was OUTSIDE the loop for a reason - why did you corrupt it and put it inside the loop?

Ghod :rolleyes:

Salem, didn't mean any harm,i'm just playing around with it, so that i may understand it. here is your code you post. For example the last digit, if a char X was entered, then it mulitplies it with a 40. I would like it to be a 10. It does check for a toupper 'X', or replaces it with a check =10. Try this number:013288366X,
my sum should be 187, 187%11 =0, then valid isbn.

Code:

---

#include<stdio.h>
#include<ctype.h>

int testISBN(char*);

int main (void)
{
  int isValid;
  char code[15];
  char temp[100];

  do
  {
    printf("Enter an ISBN (book) number:\n");
    fgets(code,sizeof code,stdin);

    printf("The ISBN entered is:%s\n",code);
    isValid = testISBN(code);
    if (isValid==0) {
      printf("\n\n\tValid ISBN Number\n");
    } else {
      printf("\n\n\tInvalid ISBN Number\n");
    }
    printf("\nDo you wish to continue? (Y/N)\n");
    fgets( temp, sizeof temp, stdin );
  } while(toupper(temp[0]) != 'N');

  return 0;
}

/*
 * http://www.morovia.com/supportforum/forum_posts_TID_55_PN_1.html
 */
int testISBN(char*code)
{
  int i;
  int sum = 0;
  int check;

  /* sum first 9 digits, as per algorithm */
  for ( i = 0 ; i<=9 ; i++)
  {
    int chVal = code[i] - '0';  /* the numeric value of the char */
    int pos = 10-i;            /* the numeric value of its position */
    sum += chVal * pos;
printf("chval is %d\n,pos is +%d\n",chVal,pos);
  }

  /* 10th digit is the checksum */
  if ( toupper(code[9]) == 'X' ) {
    check = 10;
       
  } else {
    check = code[9] - '0';
  }

  printf( "Sum=%d, Sum%%11=%d, check=%d\n", sum, sum%11, check);
  /* sum % 11 should be the check */
  return ( sum % 11 );
}

---

First, that's not Salem's code, some of your changes are still there.

The page you linked to has two different algorithms. I believe you are trying to implement the second one.

I don't understand why you find it more intuitive to count backwards with your for loop.

All you have to do is loop through the array.
Convert the element to an int.
Determine the multiplier (10 minus current position).
Keep a sum of digit * multiplier.
The only trick is: X means 10 (apparently this should only occur in the last position)
return sum % 11

Code:

---

int testISBN(char*code)
{
  int i;
  int chVal,multiplier;
  int sum = 0;

  for ( i = 0 ; i <= 9 ; i++)
  {
    if ( toupper(code[i]) == 'X' ) {
        chVal = 10;
    } else {
        chVal = code[i] - '0';
    }

    multiplier = 10-i;
    sum += chVal * multiplier;
  }

  return ( sum % 11 );
}

---

You can add validation that each elements is a digit, and the last element is a digit or 'X'

Hmm, just looked... this is actually very close to your original attempt, except for the couting backwards part.