Re: Confusing math problem

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From	Peter Pearson <ppearson@nowhere.invalid>
Newsgroups	comp.lang.python
Subject	Re: Confusing math problem
Date	21 Feb 2013 21:59:54 GMT
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On Fri, 22 Feb 2013 08:23:27 +1100, Chris Angelico <rosuav@gmail.com> wrote:
> On Fri, Feb 22, 2013 at 7:49 AM, Dave Angel <davea@davea.name> wrote:
>> However, if I do:
>>
>> print 3**60, "\n", int(math.pow(3,60)), "\n", pow(3,60)
>>
>>
>> I get:
>>
>> 42391158275216203514294433201
>> 42391158275216203520420085760
>> 42391158275216203514294433201
>>
>>
>> and the middle one is the one that's wrong.
>
> In theory, a float should hold the nearest representable value to the
> exact result. Considering that only one operation is being performed,
> there should be no accumulation of error. The integer results show a
> small number (618) of collisions, eg 2**16 and 4**8; why should some
> of those NOT collide when done with floating point? My initial thought
> was "Oh, this is comparing floats for equality", but after one single
> operation, that should be not a problem.

Does this help explain it?

>>> print hex(int(math.pow(3,60))); print hex(3**60)
0x88f924eeceeda80000000000L
0x88f924eeceeda7fe92e1f5b1L


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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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