Re: sum accuracy

From	Matt Wheeler <m@funkyhat.org>
Newsgroups	comp.lang.python
Subject	Re: sum accuracy
Date	2016-04-15 18:04 +0100
Message-ID	<mailman.30.1460740251.6324.python-list@python.org> (permalink)
References	(2 earlier) <5710B365.5040903@chamonix.reportlab.co.uk> <CAHVvXxSub2Z2j6eedm3zQr7v2aYWMhmGYzXTtx+SPrcAFgXFtw@mail.gmail.com> <mailman.11.1460712984.6324.python-list@python.org> <87d1prw5b7.fsf@bsb.me.uk> <CAG93HwGWHV9+SHmTd=jvXhhUaVfU759Rfet1U=Gqc54nGxb4dg@mail.gmail.com>

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So we could build on this

On 15 April 2016 at 11:10, Ben Bacarisse <ben.usenet@bsb.me.uk> wrote:
>> from fractions import Fraction
>>
>> def exact_sum(nums):
>>     return sum(map(Fraction, nums))
>>
>> This will give you the exact result with precisely zero rounding
>> error. You can convert it to float at the end.
>
> Just a word of warning for people new to numerical work: there's no
> rounding error, but unless you start with Fraction objects you still
> have input or conversion errors.  The uninitiated might expect
>
>   exact_sum([0.3, 0.7])
>
> to be 1.

and make

def not_exact_but_probably_the_sum_you_wanted(nums):
    return sum(map(lambda x:Fraction(x).limit_denominator(), nums))


-- 
Matt Wheeler
http://funkyh.at

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Re: sum accuracy Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-04-15 10:36 +0100
  Re: sum accuracy Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-04-15 11:10 +0100
    Re: sum accuracy Tony van der Hoff <tony@vanderhoff.org> - 2016-04-15 11:39 +0100
      Re: sum accuracy Jussi Piitulainen <jussi.piitulainen@helsinki.fi> - 2016-04-15 14:31 +0300
    Re: sum accuracy Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2016-04-15 08:11 -0400
    Re: sum accuracy Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-04-15 14:49 +0100
      Re: sum accuracy Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-04-15 17:54 +0100
    Re: sum accuracy Matt Wheeler <m@funkyhat.org> - 2016-04-15 18:04 +0100

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