Thread: using modulus & weighting factors

What is wrong with this code:

#include <stdio.h>
#include <string.h>
#define MODULUS 11

void main()
{
char code[] = "24686", weight[] = "23456", *wgt_ptr = weight[ ( strlen( code ) - 2) ];
int loop, tot = 0;

for( loop = 0; loop < ( strlen( code ) ) - 1; loop++ )
{
tot = ( ( code[loop] - '0') * ( ( wgt_ptr-- ) - '0' ) );
}
tot = MODULUS - ( tot % MODULUS );
if( tot != code[ strlen( code ) ] )
printf("Invalid code");
}

I think the problem lies with if( tot != .....

What I want is for a function to be able to validate code[] using a modulus of 11 and a weight factor of 5432 if code[] is 5 characters long and 65432 if code[] is 6 characters long.

By the way code[] wouldn't be a fixed literal normally that was just for the sake of testing it.

T

- It has no code tags surrounding it.

- void main()
nuff said. Search the boards if you don't know why.

- *wgt_ptr = weight[(strlen(code) - 2)]
This is all a bit wrong. What are you trying to do here? A quick fix is this, but I don't know if it's doing what you want:
>*wgt_ptr = &weight[(strlen(code) - 2)];

- (wgt_ptr--) - '0')
What is this supposed to be doing? Did you mean
>(*wgt_ptr--) - '0')

There might be more...

Here's my version of the code...I'm only guessing on what you're trying to accomplish. Perhaps you should clarify:

Let's address some errors first:
1) wgt_ptr-- should be *wgt_ptr-- since you want the value of it and not the address where it's pointing to. Also when you initialized it it should be: *wgt_ptr = &weight....

2) if( tot != code[ strlen( code ) ] )
Your string has 5 characters so you will get code[5] but the 5th index contains the '\0' character. Change it to strlen(code) - 1 if you really wanted the last character in your string.

3) I don't understand your for loop. It keeps overwriting the old tot value everytime it loops. So why don't you run it once with this:
tot = (code[strlen(code)-2] - '0') * (weight[0] - '0');
It does the exact same thing as your for loop.

Code:

#include <stdio.h>
#include <string.h>
#define MODULUS 11

int main()
{
   char code[] = "24686", weight[] = "23456", *wgt_ptr = &weight[ ( strlen( code ) - 2) ];
   int loop, tot = 0;

   for( loop = 0; loop < ( strlen( code ) ) - 1; loop++ )
   {
      tot = ( ( code[loop] - '0') * ( ( *wgt_ptr-- ) - '0' ) );
   }
   tot = MODULUS - ( tot % MODULUS );
   if( tot != code[ strlen( code )-1 ] - '0')
      printf("Invalid code");
   return 0;
}

Again, I'm not sure what you're doing. Hope this gets you on the right track though.

EDIT - yeah hammer, there was more

int main()
{
char code[] = "24686", weight[] = "23456", *wgt_ptr = &weight[ ( strlen( code ) - 2) ];
int loop, tot = 0;

for( loop = 0; loop < ( strlen( code ) ) - 1; loop++ )
{
tot+= ( ( code[loop] - '0') * ( ( *wgt_ptr-- ) - '0' ) );
}
tot = MODULUS - ( tot % MODULUS );
if( tot != code[ strlen( code )-1 ] - '0')
printf("Invalid code");
return 0;
}

The purpose of the loop was to accumalate the total. It should of been tot+= not tot =
Sorry

int main()
{
char code[] = "24686", weight[] = "23456", *wgt_ptr = &weight[ ( strlen( code ) - 2) ];
int loop, tot = 0;

for( loop = 0; loop < ( strlen( code ) ) - 1; loop++ )
{
tot+= ( ( code[loop] - '0') * ( ( *wgt_ptr-- ) - '0' ) );
}
tot = MODULUS - ( tot % MODULUS );
if( tot != weight[ strlen( code )-1 ] - '0')
printf("Invalid code");
return 0;
}

/* And if( tot != code[strlen( code ) -1] - '0') should be
if(( tot != weight[ strlen( code) -1] - '0')

Sorry again.

T