[New-bugs-announce] [issue36493] Add math.midpoint(a,b) function
Stefan Behnel
report at bugs.python.org
Sun Mar 31 09:10:06 EDT 2019
New submission from Stefan Behnel <stefan_ml at behnel.de>:
I recently read a paper¹ about the difficulty of calculating the most exact midpoint between two floating point values, facing overflow, underflow and rounding errors. The intention is to assure that the midpoint(a,b) is
- actually within the interval [a,b]
- the floating point number in that interval that is closest to the real midpoint (i.e. (a+b)/2).
It feels like a function that should be part of Python's math module.
¹ https://hal.archives-ouvertes.fr/file/index/docid/576641/filename/computing-midpoint.pdf
The author proposes the following implementation (pages 20/21):
midpoint(a,b) =
a if a == b else # covers midpoint(inf, inf)
0 if a == -b else # covers midpoint(-inf, +inf)
-realmax if a == -inf else # min. double value
+realmax if b == +inf else # max. double value
round_to_nearest_even((a - a/2) + b/2)
I guess nans should simply pass through as in other functions, i.e. midpoint(a,nan) == midpoint(nan,b) == nan.
The behaviour for [a,inf] is decidedly up for discussion. There are certainly cases where midpoint(a,+inf) would best return +inf, but +realmax as an actual finite value also seems reasonable. OTOH, it's easy for users to promote inf->realmax or realmax->inf or even inf->a themselves, just like it's easy to check for +/-inf before calling the function. It just takes a bit longer to do these checks on user side. There could also be a "mode" argument that makes it return one of: a or b (i.e. the finite bound), +/-realmax or +/-inf in the two half-infinity cases.
What do you think about this addition?
----------
components: Library (Lib)
messages: 339257
nosy: mark.dickinson, rhettinger, scoder, stutzbach
priority: normal
severity: normal
status: open
title: Add math.midpoint(a,b) function
type: enhancement
versions: Python 3.8
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<https://bugs.python.org/issue36493>
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