String not validating properly.

[ScotchandTweed]

Hi all. First time poster and beginner in programming. I'm having problems with my program seen below. It compiles and will run but I have the following 3 problems.

1. In main() in the while loop entering 'q' does not exit.

2. in the isbnValidation function. The size check for minimum length words will work, but the check for "Invalid ISBN character" doesn't.

3. Every ISBN entered returns as valid. The formula is supposed to multiply the first digit by 10, next by 9 etc (skipping all the dashes) and add up to a weighted total which if valid will divide evenly by 11.

I'm really new to this and I'm finding arrays and strings quite challenging. Any help or advise anyone is willing to give I would really appreciate.

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


#define ISBNMIN 10
#define ISBNMAX 14




void isbnValidation(char *p_isbn, char *p_orig);


int main(void)
{
    char isbn[ISBNMAX];
    char orig[ISBNMAX];
        
    printf("Enter an ISBN [q to exit]: ");    
    
    while(scanf("%s", isbn) != 'q')
    {
        strcpy(orig, isbn);        
        
        isbnValidation(isbn, orig);


        printf("Enter an ISBN [q to exit]: ");
    }
    return 0;
}


    
void isbnValidation(char *p_isbn, char *p_orig)
{
    int i, digit;
    int total = 0;
    int weight = 10;


    if(strlen(p_isbn) < ISBNMIN)
    {
        printf("ISBN must be minimum 10 digits\n");
        return;
    }


    for(i = 0; i < strlen(p_isbn-1); i++)
    {
        if(p_isbn[i] < '0' || p_isbn[i] > '9' || p_isbn[i] != '-' || p_isbn[i] != '\0')
        {
            if(toupper(p_isbn[i]) != 'X')
            {            
                printf("Invalid ISBN character\n");
                return;
            }
        }
    }    


    for(i = 0; i < strlen(p_isbn-1); i++, weight--)
    {
        if(p_isbn[i] != '-')
        {        
            if(p_isbn[i] == 'x' || p_isbn[i] == 'X')
                digit = 10;
            else        
                digit = p_isbn[i];        
        
            total += digit * weight;


        }
    }


    if(total % 11 == 0)
    {
        printf("ISBN %s is valid.\n", p_orig);
    }
    else
    {
        printf("ISBN %s is not valid.\n", p_orig);
    }
}

[std10093]

About scanf. Return value: On success, the function returns the number of items of the argument list successfully filled. Ok, so should check the isbn. If you don't succeed in first try, try to use strcmp.

[Matticus]

1. Line 22:

- "scanf()" does not return what it reads - it returns the number of items successfully read.
- A string* cannot equal a single character (unless it's an empty string).
- Strings cannot be compared using ==/!=, you must use "strcmp()" for that.

*Note: A string must be terminated with a null character ('\0')

2. If you're expecting a string of 10 characters, you need to give the character array enough space for 11 characters (see note above).

Also, your logic is self-defeating.

// invalid check

if(p_isbn[i] < '0' || p_isbn[i] > '9' || p_isbn[i] != '-' || p_isbn[i] != '\0')

According to your logic, it's invalid if: It's less than '0' OR greater than '9' OR not a hyphen ... that is simply not possible. Name a character that is between '0' and '9' and is a hyphen at the same time.

Not to mention that it's still not valid if it's somehow also not equal to 'X'.

3. I didn't even get this far...

[jwroblewski44]

while(scanf("%s", isbn) != 'q')    {
        strcpy(orig, isbn);

This is dangerous. strcpy() will copy over any size string into the destination array, even if if is too large too fit. Use strncpy(). And as others have noted, scanf() returns the number of items read, the actual items read.

[ScotchandTweed]

Thank you all for the info. It's slow going but I think I'm starting to get it.