Subj : Re: possible overflow?
To   : comp.programming
From : jimmaureenrogers@worldnet.att.net
Date : Sun Aug 21 2005 09:04 am


mr_semantics@hotmail.com wrote:
> I am still fiddling with bitwise operations - this time I have been
> working
> on a program that accepts input (an unsigned integral) from the user,
> and
> then after some 'conversion' outputs that number in binary format.  I
> have
> one problem that is vexing me though:
>
> #include <stdio.h>
>
> int main(void)
> {
>         unsigned int input = 8;
>         unsigned long int i;
>         unsigned long int j = 0;
>         size_t number_of_bits = sizeof(j) * 8;
>
>
>         j = (1 << (number_of_bits - 1));
>         for (i = 1; i != 0; i <<= 1) {
>                 if (input & j) {
>                        (void) putchar('1');
>                 } else {
>                        (void) putchar('0');
>                 }
>                 j >>= 1;
>         }
>         puts("");
>         return 0;
> }
>
> That is  a working program that gives an example of my problem - its
> output
> is:
>
> 0000000000000000000000000000111111111111111111111111111111111000
>
> instead of:
>
> 0000000000000000000000000000000000000000000000000000000000001000
>

It appears to me that both the answer you are getting and your
expected answer are wrong.

Each time through your loop you shift I left and J right. There is
no combination of shifting, as your program is written, that should
output a 1.

The following Ada program achieves the correct results:

-----------------------------------------------------------------------
-- Binary shifting using shifting
-----------------------------------------------------------------------
with Ada.Text_Io;
use Ada.Text_Io;
with Interfaces;
use Interfaces;

procedure Binary_Shift2 is
   I, J : Unsigned_64;
   Result : Unsigned_64;
begin
   J := Shift_Left(1, (Unsigned_64'Size - 1));
   I := 1;
   while I /= 0 loop
      Result := I and J;
      if Result /= 0 then
         Put('1');
      else
         Put('0');
      end if;
      I := Shift_Left(I, 1);
      J := Shift_Right(J, 1);
   end loop;
   New_Line;
end Binary_Shift2;

0000000000000000000000000000000000000000000000000000000000000000


Jim Rogers

.