Re: Confusing math problem

References	(2 earlier) <51268867.1000800@davea.name> <mailman.2199.1361481815.2939.python-list@python.org> <aonjmqFqplbU1@mid.individual.net> <CAPTjJmoexWo4LcvqFQJmMUXSn+7+edgWm1kCXiYJ_su9ynPe=A@mail.gmail.com> <5126A0D7.3080606@davea.name>
Date	2013-02-22 10:15 +1100
Subject	Re: Confusing math problem
From	Chris Angelico <rosuav@gmail.com>
Newsgroups	comp.lang.python
Message-ID	<mailman.2210.1361488528.2939.python-list@python.org> (permalink)

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On Fri, Feb 22, 2013 at 9:33 AM, Dave Angel <davea@davea.name> wrote:
> On 02/21/2013 05:11 PM, Chris Angelico wrote:
>>
>>
>>>  <snip>
>>
>>
>> Note how, in each case, calculating three powers that have the same
>> real-number result gives a one-element set. Three to the sixtieth
>> power can't be perfectly rendered with a 53-bit mantissa, but it's
>> rendered the same way whichever route is used to calculate it.
>>
>
> But you don't know how the floating point math library (note, it's the
> machine's C-library, not Python's that used) actually calculates that.
>
> For example, if they were to calculate 2**64 by squaring the number 6 times,
> that's likely to give a different answer than multiplying by 2 63 times.
> And you don't know how the library does it.  For any integer power up to
> 128, you can do a combination of square and multiply so that the total
> operations are never more than 13, more or less.  But if you then figure a =
> a*a  and b = b/2, and do the same optimization, you might not do them
> exactly in the same order, and therefore might not get exactly the same
> answer.
>
> Even if it's being done in the coprocessor inside the Pentium, we don't have
> a documented algorithm for it.  Professor Kahn helped with the 8087, but I
> know they've tweaked their algorithms over the years (as well as repairing
> bugs).  So it might not be a difference between Python versions, nor between
> OS's, but between processor chips.

I was under the impression that, on most modern FPUs, calculations
were done inside the FPU with more precision than the 53-bit that gets
stored. But in any case, I'd find it _extremely_ surprising if the
calculation actually resulted in something that wasn't one of the two
nearest possible representable values to the correct result. And I'd
call it a CPU/FPU bug.

Of course, as we know, Intel's *never* had an FPU bug before...

ChrisA

Thread

Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 19:33 +0000
  Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 15:25 -0500
    Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 22:39 +0000
      Re: Confusing math problem Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-21 22:53 +0000
        Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 23:41 +0000
          Re: Confusing math problem Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-02-22 00:04 +0000
          Re: Confusing math problem Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-21 17:19 -0700
    Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 23:39 +0000
  Re: Confusing math problem Ian Kelly <ian.g.kelly@gmail.com> - 2013-02-21 13:42 -0700
  Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 07:46 +1100
    Re: Confusing math problem "Schizoid Man" <schiz_man@21stcentury.com> - 2013-02-21 22:44 +0000
      Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 11:29 +1100
      Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 21:19 -0500
  Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 15:49 -0500
  Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 08:23 +1100
    Re: Confusing math problem Peter Pearson <ppearson@nowhere.invalid> - 2013-02-21 21:59 +0000
      Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 09:11 +1100
      Re: Confusing math problem Dave Angel <davea@davea.name> - 2013-02-21 17:33 -0500
      Re: Confusing math problem Chris Angelico <rosuav@gmail.com> - 2013-02-22 10:15 +1100
    Re: Confusing math problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 09:16 +0000
      Re: Confusing math problem Serhiy Storchaka <storchaka@gmail.com> - 2013-02-22 13:48 +0200
  Re: Confusing math problem Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-02-22 09:27 +0000

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