Re: Rounding with Div and Mod operators

Johan Persson <johan.persson@mbox319.swipnet.se> wrote:


>William Rayer wrote:
>> My question is: which rounding system is preferred and does it matter?
>
>My experience of using int, mod and div operators are that if they round
>towards zero they are useless if you don't know that you only are
>dealing with positive OR negative numbers. I usually write my own
>functions that round downwards and use them instead of the built-in
>operators. I have never come across a case where rounding towards zero
>is the preferred choice.


          There is a neat hack for determining digital sums that would be
cleaner if modulo rounded down to 0. (That's assuming that the
digital sum of a number is always the same as that of its abs().)


                    int ds(int n)


                          {


                          int baseds;


                          if (n==0)
                                return 0;


                          /* Discussion Point */


                          baseds=n%9;
                          if (baseds>0)
                                return baseds;
                          else
                                return 9;
                          }


          This won't work for negative numbers as you have to abs() first,
but that doesn't work either because in a two's complement system,
abs(INT_MIN) could overflow (e.g. -32768 vs. 32767). At the
discussion point, you need something like
                          if (n<-9)
                                n+=9;
                          n=abs(n);


Sincerely,


Gene Wirchenko

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