Hi all, Visual C++ 2015 supports this one:
https://msdn.microsoft.com/en-us/library/h0dff77w.aspx In any case, this is easy to implement an efficient fallback in C, unlike the fma() function we discussed some time ago. To put this in a bit wider perspective: would it be useful to investigate how much of the C99 math library could be supported in Python in general? Stephan 2017-02-04 12:52 GMT+01:00 <tritium-l...@sdamon.com>: > The presence of the function in C99’s math.h isn’t strictly useful unless > it is also in the MSVC math.h. MSVC only supports a subset of C99 > > > > *From:* Python-ideas [mailto:python-ideas-bounces+tritium-list=sdamon.com@ > python.org] *On Behalf Of *Juraj Sukop > *Sent:* Saturday, February 4, 2017 6:31 AM > *To:* python-ideas@python.org > *Subject:* [Python-ideas] math.nextafter > > > > Hello! > > > > Function `nextafter(x, y)` returns the next representable value of `x` in > the direction of `y`, and if `x` equals to `y`, `y` is returned. [1] > > > > It is useful for incrementing/decrementing floating-point number by the > smallest amount possible or for testing if two numbers are closest to each > other without being the same. > > > > The following snippet written by Mark Dickinson emulates the functionality > in pure Python [2]: > > > > import math > > import struct > > > > def next_up(x): > > # NaNs and positive infinity map to themselves. > > if math.isnan(x) or (math.isinf(x) and x > 0): > > return x > > > > # 0.0 and -0.0 both map to the smallest +ve float. > > if x == 0.0: > > x = 0.0 > > > > n = struct.unpack('<q', struct.pack('<d', x))[0] > > if n >= 0: > > n += 1 > > else: > > n -= 1 > > return struct.unpack('<d', struct.pack('<q', n))[0] > > > > def next_down(x): > > return -next_up(-x) > > > > def next_after(x, y): > > # If either argument is a NaN, return that argument. > > # This matches the implementation in decimal.Decimal > > if math.isnan(x): > > return x > > if math.isnan(y): > > return y > > > > if y == x: > > return y > > elif y > x: > > return next_up(x) > > else: > > return next_down(x) > > > > Other implementations can be found at [3], [4] or [5], for example. > > > > It would be useful to have `math.nextafter` function available in standard > library, it also is provided by C99 <math.h>, and is similar in spirit to > `math.copysign` or `math.isclose`. > > > > As to why to include it by default when the above snippet works just fine, > a C implementation is likely to be much faster than using `struct.pack` and > `struct.unpack`. > > > > Thank you for considering this proposal and any feedback is greatly > welcomed! > > > > Juraj Sukop > > > > [1] http://en.cppreference.com/w/c/numeric/math/nextafter > > [2] http://stackoverflow.com/a/10426033 > > [3] http://git.musl-libc.org/cgit/musl/tree/src/math/nextafter.c > > [4] https://github.com/android/platform_bionic/blob/master/ > libm/upstream-freebsd/lib/msun/src/s_nextafter.c > > [5] https://github.com/golang/go/blob/master/src/math/nextafter.go > > > > _______________________________________________ > Python-ideas mailing list > Python-ideas@python.org > https://mail.python.org/mailman/listinfo/python-ideas > Code of Conduct: http://python.org/psf/codeofconduct/ >
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