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Re: sum accuracy

From Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups comp.lang.python
Subject Re: sum accuracy
Date 2016-04-15 11:10 +0100
Organization A noiseless patient Spider
Message-ID <87d1prw5b7.fsf@bsb.me.uk> (permalink)
References <570E78F9.8050409@chamonix.reportlab.co.uk> <1460567110.3933817.577783969.165F6097@webmail.messagingengine.com> <5710B365.5040903@chamonix.reportlab.co.uk> <CAHVvXxSub2Z2j6eedm3zQr7v2aYWMhmGYzXTtx+SPrcAFgXFtw@mail.gmail.com> <mailman.11.1460712984.6324.python-list@python.org>

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Oscar Benjamin <oscar.j.benjamin@gmail.com> writes:

> On 15 April 2016 at 10:24, Robin Becker <robin@reportlab.com> wrote:
<snip>
>> yes indeed summation is hard :(
>
> Not with Fraction it isn't:
>
> 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.

-- 
Ben.

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Thread

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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