Tim, Does 754 say anything about complex numbers?
It strikes me that what is needed is a defined behavior for complex numbers, rather than expecting them to magically work for all the edge cases simply by applying the algebraic rules with two floats. Your example of a complex infinity that should be one thing is a good one. Maybe the same for complex zero :-) Anyway, Python is probably not the place to define a standard like this, but if there's a good one out there, then using it in Python could be nice. -CHB On Tue, Feb 22, 2022 at 6:41 PM Tim Peters <tim.pet...@gmail.com> wrote: > I have to say that signed zeroes and signed infinities don't work well > in 754 for complex numbers. I know Kahan _wanted_ signed zeroes to > help sort out branch cuts for complex functions, but he appears to be > alone in the world in finding them useful for that ;-) > > Complex multiplication for values with signed zero components isn't > even associative. At least not under any implementation I've tried. I > pointed that out on David Hough's "numeric-interest" mailing list > while 754 was still being hammered out, but it was "already too late" > to do anything about it. > > Here's a short driver: > > from itertools import product > pz = 0.0 > cs = [complex(*t) for t in product((pz, -pz), repeat=2)] > for x, y, z in product(cs, repeat=3): > t1 = (x*y)*z > t2 = x*(y*z) > if repr(t1) != repr(t2): > print(x, y, z, t1, t2) > > On my Windows 3.10.1, associativity fails in 24 (of the 64) cases: > > 0j 0j (-0-0j) -0j 0j > 0j -0j (-0+0j) (-0+0j) 0j > 0j -0j (-0-0j) -0j (-0+0j) > 0j (-0+0j) (-0+0j) -0j 0j > 0j (-0-0j) -0j -0j (-0+0j) > 0j (-0-0j) (-0-0j) (-0+0j) 0j > -0j 0j (-0+0j) (-0+0j) 0j > -0j -0j (-0-0j) (-0+0j) 0j > -0j (-0+0j) (-0+0j) (-0+0j) -0j > -0j (-0+0j) (-0-0j) -0j 0j > -0j (-0-0j) 0j (-0+0j) -0j > -0j (-0-0j) (-0+0j) -0j 0j > (-0+0j) 0j -0j 0j (-0+0j) > (-0+0j) -0j 0j 0j (-0+0j) > (-0+0j) (-0+0j) 0j 0j -0j > (-0+0j) (-0+0j) -0j -0j (-0+0j) > (-0+0j) (-0-0j) -0j 0j -0j > (-0+0j) (-0-0j) (-0-0j) -0j (-0+0j) > (-0-0j) 0j 0j 0j -0j > (-0-0j) -0j 0j (-0+0j) -0j > (-0-0j) -0j -0j 0j (-0+0j) > (-0-0j) (-0+0j) -0j 0j -0j > (-0-0j) (-0-0j) 0j 0j (-0+0j) > (-0-0j) (-0-0j) (-0+0j) (-0+0j) -0j > > That isn't just academic. My employer's FORTRAN compiler failed to > pass the US govt's validation suite at the time, because of a bizarre > test failure: the accidental signs of these zeroes can have very > visible consequences (e.g., as arguments to atan2(), which the > validation suite called on the real and imag components after raising > a complex zero to an integer power - and _which_ of the four complex > zeroes you got depended on the grouping of the multiplies). > > Don't try to sell me the idea that any of that is logical ;-) BTW, > exactly which cases in the above fail can also depend on the rounding > mode (because x + -x is +0 for any finite x, _except_ under > to-minus-infinity rounding, where the result changes to -0). > > Signed infinities are even sillier here. You want a single projective > infinity in the complex plane, not a pair of signed infinities on each > axis. And early drafts of 754 had a control bit to determine which > flavor of infinity you got. But it's one of the few bits of esoterica > that got dropped near the end. Partly because signed zeroes seemingly > demand signed infinities too, like lutefisk demands haggis ;-) > > There was much to applaud in 754, but to my eyes signed zeroes, and > NaN != NaN, caused far more trouble than they helped - and the > exception model is such a pain it's almost never been implemented as > intended (Python's `decimal` module being an exception, and Apple's > long-dead SANE environment). > > BTW, complex was added as a full-blown built-in type in 1.4b1, long > before Python had a major user base (mid-1990s). I think Guido added > it because he was trained as a mathematician, so felt obligated ;-) > Seriously, various Python versions of it (and rationals) were long in > use as test cases for hammering out rules and APIs for coercion (and > other native language features). > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/3IY3NTJRAMSQZYTCVQABTYPM2TGS3UVN/ > Code of Conduct: http://python.org/psf/codeofconduct/ >
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