Re: math.frexp

From	Random832 <random832@fastmail.com>
Newsgroups	comp.lang.python
Subject	Re: math.frexp
Date	2016-07-15 14:28 -0400
Message-ID	<mailman.24.1468607289.2307.python-list@python.org> (permalink)
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On Fri, Jul 15, 2016, at 12:32, Steven D'Aprano wrote:
> I can take the geometric mean of: 
> 
> [2.0**900, 2.0**920, 2.0**960, 2.0**980, 2.0**990]
> 
> and get to within a relative error of 1e-14 of the correct answer,
> 2**950:
> 
> py> geometric_mean([2.0**900, 2.0**920, 2.0**960, 2.0**980, 2.0**990])
> 9.516908214257811e+285
> py> 2.0**950
> 9.516908214257812e+285
>
> Here's one that's exact:
> 
> py> geometric_mean([2e300, 4e300, 8e300, 16e300, 32e300])
> 8e+300

My own attempt gave an exact result for both test cases.

from math import frexp as _frexp, ldexp

def frexp(x):
    if type(x) is tuple:
        m, e1 = x
        m, e2 = _frexp(m)
        return m, e1 + e2
    else:
        return _frexp(x)

def _mul(a, b):
    ma, ea = frexp(a)
    mb, eb = frexp(b)
    return frexp((ma * mb, ea + eb))

def _product(values):
    return reduce(_mul, values)

def geometric_mean(values):
    n = len(values)
    pm, pe = _product(values)
    me, re = divmod(pe, n)
    pm = ldexp(pm, re)
    return ldexp(pm**(1./n), me)

Thread

math.frexp Steven D'Aprano <steve@pearwood.info> - 2016-07-15 21:39 +1000
  Re: math.frexp Chris Angelico <rosuav@gmail.com> - 2016-07-15 21:48 +1000
    Re: math.frexp Steven D'Aprano <steve@pearwood.info> - 2016-07-16 02:32 +1000
      Re: math.frexp Chris Angelico <rosuav@gmail.com> - 2016-07-16 02:44 +1000
      Re: math.frexp Random832 <random832@fastmail.com> - 2016-07-15 14:28 -0400
      Re: math.frexp Paul Rubin <no.email@nospam.invalid> - 2016-07-15 13:24 -0700
        Re: math.frexp Steven D'Aprano <steve@pearwood.info> - 2016-07-16 20:21 +1000
          Re: math.frexp Paul Rubin <no.email@nospam.invalid> - 2016-07-16 12:49 -0700
  Re: math.frexp Nobody <nobody@nowhere.invalid> - 2016-07-15 17:39 +0100
  Re: math.frexp Vlastimil Brom <vlastimil.brom@gmail.com> - 2016-07-17 01:18 +0200

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