[Octave-bug-tracker] [bug #31579] Floating point mod function does not match Matlab

[John W. Eaton]

Follow-up Comment #5, bug #31579 (project octave):

I checked in the following change for the copysign problem:

http://hg.savannah.gnu.org/hgweb/octave/rev/009d16b010fa

Yes, I think we should report the problem.  The current behavior of fmod does
not seem good to me, but maybe there is some reason for it?  Though if there
is, I can't really see it.

After doing some debugging, it looks like fmod is ultimately computed by the
following function in glibc, at least on my system:

http://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/ieee754/dbl-64/e_fmod.c;h=2ce613574a0d7aad233dc2ba77036309f45681fd;hb=HEAD

Yowza.  That level of bit twiddling is far beyond my ability to comprehend,
much less fix.

In any case, even if fmod is fixed, I think we still need our versions of rem
and mod so that we can be compatible with Matlab's "round if within eps"
feature.


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