check digit

i'm trying to make a check digit for my program as below

The check digit sometimes must have a value of 10, and this is indicated by putting an X in the check-digit location.

I know it has something to do with the ASCII table that X has a value of 88 and I need to work with that somehow.

How do I read if a user has entered an X and then set the X to a value of 10?

OK, so where is your "program as below" ?

Sorry I meant "as below" as in what I wanted to make as the check digit.

> How do I read if a user has entered an X and then set the X to a value of 10?
Since your question is, um... unclear

The answer could be:

Code:

---

int X = 0;
if(getchar() == 'X')
    X = 10;

---

But it's probably not ;)

your question sounds sortof, homeworky.

Oh, you mean you were referring to another message on the forum, which has since changed the order since your post, and the idea of "below" doesn't mean anything anymore.

Post your own effort.

This is the brief:

The ISBN has ten digits. These are normally broken into smaller groups with hyphens or spaces to make it easier to transcribe them. The break-up varies, because the publisher definition can be done in several ways, depending on the size of the publisher.

Some examples of ISBN’s are:
0-7897-0414-5
0-262-68053-X
0 201 54322 2

Notes:
1) There is no guarantee that there will always be four numeric groups, so you should consider that the number of characters may be any value of ten or greater.
2) The check digit sometimes must have a value of 10, and this is indicated by putting an X in the check-digit location.
3) The

Your program will accept a line of input containing an ISBN with dashes or spaces, identify the digits, and perform the above calculation. Note that the digits in the input string will be characters, with their associated ASCII value. To convert a single digit character to its corresponding internal integer value for calculation purposes, simply subtract the character code for '0'.

E.g.
char c = ‘9’;
int num = c – ‘0’;
Will put the value 9 into num.

And this is what I've done:

Code:

---

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

int validate(char ISBN[]);

int main(int argc, char** argv)
{
  printf("%d\n", validate(argv[1]));
  return 0;
}

int validate(char ISBN[])
{

  int sum = 0, i = 0;
  int numElements = strlen(ISBN);
 
  if (numElements % 10)
    return 0;
  else
    return 1;
}

---

Well it definitely sounds like homework. But what "above caluclation"?

Why not run a loop over each character, check if isdigit() or it's an X and do it that way, because a space and dashes aren't digits.

eg:

Code:

---

#include <ctype.h>
/* ... */
for(i = 0; i < len; i++)
{
    /* it's a digit */
    if(isdigit(whatever[i]) != 0)
    {
        /* do whatever with the digit */
    }else if(whatever[i] == 'X') {
        /* it's an X... which means... :o! */
    }
}

---

It is homework that I am working on and that's why I've come here to ask for help with it.

So something like

Code:

---

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

int validate(char ISBN[]);

int main(int argc, char** argv)
{
  printf("&#37;d\n", validate(argv[1]));
  return 0;
}

int validate(char ISBN[])
{

  int sum = 0, i = 0;
  int numElements = strlen(ISBN);
 
  for(i=0; i < len; i++)
  {
    if(isdigit(ISBN[i]) !=0)
    {
    /*...*/
    }
  else if(ISBN[i] == 'X'
  {
    'X' = 10
  }

  if (numElements % 10)
    return 0;
  else
    return 1;
}

---

Is that right?

Sorry, but also if X can only be the last 'digit' entered is there anyway to factor that in as well?

> 'X' = 10
Isn't right, that's like saying, 88 = 10. 'X' is constant, so is 10.

Your homework doesn't say that you need to validate it, it also says that there may be 10 or more digits...
It looks like you've just copy and pasted by example without reading, or understanding it...

I am trying to understand it I just can't get my head around it sorry. I've never done any programming before this subject so it's a little tricky for me sorry.

Quote:

---

Originally Posted by Jailan

It is homework that I am working on and that's why I've come here to ask for help with it.

---

Think about this statement a few times.

You're paying money to be taught something that you have to come here and ask total strangers for help with. It's not that we don't wish to help, but if you need to get our help because the course help is inadequate, why are you taking the course?

> but also if X can only be the last 'digit' entered is there anyway to factor that in as well?
Of course there is, use && to compare the index to see if you're at the last char
if ( i == len && char == 'X' )

>if ( i == len && char == 'X' )
Would it not be, if(i == len - 1 ... ? Since the condition in the loop is i < len, ie the loop will only run if i is less than len

I've no idea, it was merely a description of the possible, to make the OP think.
It wasn't exactly a "do this and it will work" post.

@Jailan: Have you tried searching the forum?

http://cboard.cprogramming.com/search.php

Use "ISBN" as a key word. Sure several threads are about books. Several others offer previously asked version of your same question, and likely a solution lies therein as well.

Thread titles like "Processing a book ISBN" or "validate isbn" spring to mind.

Thanks for your help I managed to nut it out after a while but I need some more help;

I need to split it up into seperate functions but each time I try I get a whole bunch of errors no matter what way I try to split it.

Code:

---

#include<stdio.h>

int main()
{
  char input[20] = { '\0' };
  char flag = 0; 
  int weight = 10;
  int index = 0; 
  int sum=0; 
  int temp=0;
  int noOfChars = 0;
 

 
  while(input[0] != '\n')
  {
      fgets(input, 20, stdin);
 
      while(index < 20)
      {
        if((input[index] >= 0x30 && input[index] <= 0x39) ||
            (input[index] == 'X' || input[index] == 'x'))
        {
            if(input[index] == 'X' || input[index] == 'x')
            {
              temp = 10;
            }
            else
            {
              temp = input[index] - '0';
            }
            temp = temp * weight;
            sum += temp;
            weight--;
            noOfChars++;
        }
        else if(input[index] == '\n')
        {
            flag = 1;
            index++;

            while(index < 20)
            {
              input[index] = '\0';
              index++;
            }
            break; 
        }
        index++;
      }

      if(flag != 1 || noOfChars != 10)
      {
        if(input[0] != '\n')
        {
            printf("Invalid ISBN number\n\n");
        }
      }
      else
      {
        temp = sum % 11;
           
        if(temp == 0)
        {
            printf("Valid ISBN number\n\n");

        }
        else
        {
            printf("Invalid ISBN number\n\n");
        }
      }
      weight = 10;
      noOfChars = 0;
      index = 0;
      sum = 0;
      flag = 0;

  }

  return 0;
}

---

Code:

---

            while(index < 20)
            {
              input[index] = '\0';
              index++;
            }

---

Why? Setting input[index] = '\0' may be fine, but there's no need to fill the entire rest of the string.

Code:

---

      weight = 10;
      noOfChars = 0;
      index = 0;
      sum = 0;
      flag = 0;

---

Stick these at the start of the code.

Code:

---

        if((input[index] >= 0x30 && input[index] <= 0x39) ||
            (input[index] == 'X' || input[index] == 'x'))

---

is easier to read (and write) as:

Code:

---

        if(isdigit(input[index]) || tolower(input[index]) == 'x')

---

Similarly, use:

Code:

---

            if(tolower(input[index]) == 'x')

---

I don't really see the point of your "flag" variable. Surely if there's no newline because the input is too long to fit in 20 chars, it will fail due to "not the right number of digits".

Code:

---

temp = temp * weight;
// why not
temp *= weight

---

--
Mats

These are the errors I am getting back:
==> Your program only contains the main function
Your program must be split into appropriate functions

==> At least one function in your code must use 'call by
You do not appear to have done this in your program.

Quote:

---

Code:

---

      weight = 10;
      noOfChars = 0;
      index = 0;
      sum = 0;
      flag = 0;

---

Stick these at the start of the code.

---

I did this but I get

==> Global variables are not allowed in this subject.

Ehm, when I said start of the code, I meant the start of the for-loop. That is your current code (as posted earlier) sets these variables at the end of the loop, rather than at the start of the loop. It makes the code easier to read if you set the variables at the start of the loop.

You obviously have some requirement to use a function, and you should break out the function to check the number. I would write it something like this :

Code:

---

int main()
{
    char str[20];
 
    do {
        printf("Enter a ISBN or empty line to exit:");
        fgets(str, sizeof(str), stdin);
        if (str[0] != '\n')  checkISBN(str);
    } while (str[0] != '\n');
    return 0;
}

---

I'll leave it to you to implement the "checkISBN" code - but essentially it is most of the code you've got in main that needs to move to the checkISBN function.

--
Mats

Removed

Whilst having inputFlush as a function is certainly a good idea, and checking digit in a function isn't bad either, I think the "correct" thing to do is to split the check of the ISBN part into a separate function that takes a string as input.

I personally have nothing against having main last - but that's probably heavily influenced by my previous Pascal experience, where code is built "bottom up", meaning that the most basic functions are declared first, the the functions calling the basic function etc. But there's no strict rule that you have to do it one way or the other in C.

The cause of errors in your second piece of code, however, are caused by the fact that a function has to be declared (as a prototype at least) before it is called.

There are some style points I'd like to point out too:
Your input flush is overcomplicated: Use something like this:

Code:

---

char ch;
while((ch = getchar()) != '\n' && ch != EOF) ;

---

Don't pass a flag to inputFlush, check the flag in the code before calling inputflush.

Code:

---

      if(flag != 1 || noOfChars != 10)

---

I'm sure that flag will only be zero if noOfChars is NOT ten, so checking both doesn't really lead to anything useful. [Because the only way that the string doesn't end on a '\n' is if there's more than 19 chars in the input, which is higher than 10].


I would also add that the only position in an ISBN that is allowed to be X is the last one, so it would be possible to enter an invalid ISBN and get an "OK" answer.

You also don't bail out if there's invalid input, e.g.
A12345B 6789AX would be accepted as 123456789X, ignoring the A's.

And you still don't use "tolower" to check if the 'X' is an x.

--
Mats

Ignoring the over complicated stuff cause I just want it to work at the moment then I'll play around with it looking better... I tried declaring the two functions at the start but just got the below errors

"isb2.c", line 73: warning: identifier redeclared: inputFlush
current : function(char) returning void
previous: function() returning void : "isb2.c", line 3
"isb2.c", line 87: warning: identifier redeclared: decodeDigit
current : function(char, pointer to int) returning int
previous: function() returning int : "isb2.c", line 4
isb2.c:73: error: conflicting types for 'inputFlush'
isb2.c:73: note: an argument type that has a default promotion can't match an em
pty parameter name list declaration
isb2.c:3: error: previous declaration of 'inputFlush' was here
isb2.c:73: error: conflicting types for 'inputFlush'
isb2.c:73: note: an argument type that has a default promotion can't match an em
pty parameter name list declaration
isb2.c:3: error: previous declaration of 'inputFlush' was here
isb2.c:87: error: conflicting types for 'decodeDigit'
isb2.c:87: note: an argument type that has a default promotion can't match an em
pty parameter name list declaration
isb2.c:4: error: previous declaration of 'decodeDigit' was here
isb2.c:87: error: conflicting types for 'decodeDigit'
isb2.c:87: note: an argument type that has a default promotion can't match an em
pty parameter name list declaration
isb2.c:4: error: previous declaration of 'decodeDigit' was here

Code:

---

#include<stdio.h>

int decodeDigit(char ch, int *temp);
void inputFlush(char flag);

int main()
{ etc etc

---

Did you make any other changes?

The error text you're showing doesn't quite match the code - you didn't by any chance do some of it and then forgot to save the final changes?

--
Mats

Removed

That isn't a good function prototype. You should have the arguments as well.

Like this:

Code:

---

int decodeDigit(char ch, int *temp);
void inputFlush(char flag);

---

--
Mats

I just realized that you have removed the arguments from your function calls in the code - you shouldn'tt do that.

--
Mats

ahhhh ok cheers!

Now I'm getting

"isb2.c", line 22: prototype mismatch: 0 args passed, 2 expected
"isb2.c", line 46: prototype mismatch: 0 args passed, 1 expected

I can't win!

Quote:

---

Originally Posted by Jailan

ahhhh ok cheers!

Now I'm getting

"isb2.c", line 22: prototype mismatch: 0 args passed, 2 expected
"isb2.c", line 46: prototype mismatch: 0 args passed, 1 expected

I can't win!

---

You were writing as I was posting as well. I would have got there quicker if I hadn't had to remove your "doublespacing" - are you using WORD or something like that for your editing?

--

Oh ok I'll go put it back... Thanks.

Thanks heaps for your help mat! It is finally going ok! Sorry for all the trouble!