Re: Zeller's Algorithm

["Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net>]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:1028145549.13341@master.nyc.kbcfp.com...
> No. It is required in C or C++ that for non-zero b,
> a / b * b + a % b == a
>
> When division rounds towards zero, as is usually desired
> (the C++ standard claims that this is what the Fortran
> standard requires), this forces the remainder operation
> to return negative results. Eg., we want -3 / 2 == -1,
> so we must have -3 % 2 == -1 for the requirement to hold.

You're absolute right, but with the caveat that the C standard does not
require that integer division round to zero. It is acceptable for a C
implementation to round to minus infinity, in which case (-3) / 2 == -2 and
therefore (-3) % 2 == 1.

This decision on the part of the C Standards committee is most unfortunate,
since as Mr. Rosen correctly indicates, virtually all platforms round to
zero. Life could have been made simpler for C programmers if this was
required for all implementations, but apparently there was some platform,
with politically powerful allies, that did not round to zero.