Re: FW: add long strings as unsigned integers

At 14:27 2003-11-17 -0500, Gene_Gaunt@xxxxxxxxxxxxxxx wrote:
>
> Thanks for help with the carry byte, I'm trying this:
>
> void addstring( char result[], char increment[], int length )
> {  int x, sum = 0;
>    for(  x = length-1; x >= 0; x-- )
>    {  result[x] = sum += result[x] + increment[x];
>       sum >>= 8;
>    }
> }
>

I hesitate to write this, because 
(a) I am a relative newcomer to C programming, writing more
    from what I have read than from experience.
(b) I really should not write anything for public
    consumption this late at night.
Hmm, maybe those reasons should be listed the other way
around <grin/>.  Anyway, I think your code may have a couple
of problems.

result[x] and increment[x] are each a char, and their sum is
a char, which does not have room for the carry.  Try coding
the sum as (int) result[x] + increment[x].  Casting either
operand to (int) will make the sum an (int).

To inhibit sign extension during promotion, declare or cast
result and increment to be unsigned char[].

I do not know if the result of narrowing during assignment
to a signed type is well defined.  Anyway, declaring or
casting result to be unsigned char[] avoids this suspected
problem, too.

Your loop does not terminate, at least not gracefully, if x
is unsigned.  Possible remedies are to declare x to be
signed or to rewrite the loop control as ...

   for ( x = length; x > 0; )
    {  --x;
    ...

These are things that caught my attention, looking at the
code.  Can somebody better acquainted with the C standard
comment?

HTH.

Terry.
Available for contract programming.