On Thursday, June 29 at 10:00 AM, quoth Ingo van Lil:
>> mpfr_set_ui(temp, 0, GMP_RNDN);
>> mpfr_div(temp, first, second, GMP_RNDN);
>> mpfr_rint(temp, temp, GMP_RNDZ);
>> mpfr_mul(output, temp, second, GMP_RNDN);
>> mpfr_sub(output, first, output, GMP_RNDN);
>
>What's the first line good for?

Hmm, not much, come to think of it.

>> Although after reading about it, I'm not sure. So, the problem is
>> how to deal with negative numbers. For example, should -340 % 60 be
>> -40 or -20?
>
>-20 is not an option. The question is: When performing the integer
>division of -340 and 60, do you round the result by discarding the
>fractional part (-5, leaving you with a remainder of -40) or by picking
>the next-lower integer number (-6, leaving you with a remainder of
>+20). It's getting even more complicated if the divisor is negative:
>340/-60 could be either -5 (remainder 40) or -6 (remainder -20).
>So, unless both the dividend and the divisor are positive we have to
>pick one of two equally valid modulo results (one positive, one
>negative). We have four possible ways to do this:
>
>1. Always use the positive one; that's what my algorithm does:
>
>-340 % 60 == 20; 340 % -60 == 40; -340 % -60 == 20
>
>2. Use the one that's got the same sign as the dividend; that's what
>your algorithm does and what the C99 standard requires:
>
>-340 % 60 == -40; 340 % -60 == 40; -340 % -60 == -40
>
>3. Use the one that's got the same sign as the divisor, always leaving
>you with a modulo result between 0 and the divisor:
>
>-340 % 60 == 20; 340 % -60 == -20; -340 % -60 == -40
>
>4. Use the sign of the quotient, i.e. give the negative result if
>either the dividend and the divisor are negative, and use the
>positive result in case both are:
>
>-340 % 60 == -40; 340 % -60 == -20; -340 % -60 == 20
>
>I like the fourth solution, but I'm not yet sure how to implement it
>most simple. If we want to be C99 conformant, we should stick with your
>solution.

Hmm, well, I tend to like not being restrictive, and I also like
maintaining the existing behavior. In Wcalc 1.x, this was implemented
as the C % operator, so I'd like to keep it that way. On the other
hand, I can see a valid need for something more interesting, like the
"rem" operator in Ada. Perhaps I should add your method as the
implementation of that operator...?

~Kyle
--
I created a cron job to remind me of the Alamo.
-- Arun Rodriguez