Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #39512

Re: Confusing math problem

Path csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.freenet.ag!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path <davea@davea.name>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.008
X-Spam-Evidence '*H*': 0.98; '*S*': 0.00; 'value,': 0.03; 'intermediate': 0.05; 'converts': 0.07; 'python': 0.09; 'finite': 0.09; 'integers': 0.09; 'size,': 0.13; 'binary,': 0.16; 'carefully.': 0.16; 'decimal.': 0.16; 'determining': 0.16; 'digits)': 0.16; 'exponent': 0.16; 'integers,': 0.16; 'precision.': 0.16; 'radix': 0.16; 'symmetric': 0.16; 'wrote:': 0.17; 'differ': 0.17; 'integer': 0.17; 'discussion': 0.20; 'trying': 0.21; 'subject:problem': 0.22; 'machine': 0.24; 'least': 0.25; 'header:In-Reply-To:1': 0.25; 'header:User-Agent:1': 0.26; 'values': 0.26; 'plain': 0.27; "doesn't": 0.28; 'represent': 0.28; 'comparison': 0.29; 'decimal': 0.29; 'received:192.168.1.3': 0.29; 'sets.': 0.29; 'no,': 0.29; "i'm": 0.29; 'usually': 0.30; 'unlike': 0.30; 'suggestion': 0.32; 'point,': 0.33; 'programming,': 0.33; 'to:addr:python-list': 0.33; 'whatever': 0.35; 'compared': 0.35; 'feed': 0.35; 'doing': 0.35; 'pm,': 0.35; 'there': 0.35; 'really': 0.36; 'but': 0.36; 'compare': 0.36; "i'll": 0.36; 'two': 0.37; 'rather': 0.37; 'data': 0.37; 'subject:: ': 0.38; 'store': 0.38; 'some': 0.38; 'to:addr:python.org': 0.39; 'received:192': 0.39; 'short': 0.39; 'little': 0.39; 'where': 0.40; 'received:192.168': 0.40; 'matter': 0.61; 'mentioned': 0.63; 'different': 0.63; 'our': 0.65; 'offer': 0.65; 'frequently': 0.65; 'lose': 0.71; 'received:74.208': 0.71; 'hand': 0.82; 'algorithm,': 0.84; 'blob': 0.84; 'interesting,': 0.84; 'off,': 0.84
Date Thu, 21 Feb 2013 21:19:15 -0500
From Dave Angel <davea@davea.name>
User-Agent Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130106 Thunderbird/17.0.2
MIME-Version 1.0
To python-list@python.org
Subject Re: Confusing math problem
References <kg5sog$lfb$1@dont-email.me> <mailman.2193.1361479569.2939.python-list@python.org> <kg67ul$d4v$1@dont-email.me>
In-Reply-To <kg67ul$d4v$1@dont-email.me>
Content-Type text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding 7bit
X-Provags-ID V02:K0:WYDQTcLM9xNFzwmtylsj9bBDt5GSw0MaWdVX2Tr7MsU 2QdedG11E7d4x+Px7ciIqVSAWVBkBS+tk1T2ZP6HnJVxEWP/OO FPp8Ch6+aej06g5kAYQr68wv+Cc+imB5vo9necr9C0qCpbgnyp f2d7ZcerCkzCeLn25sNaFbcCTZ7FJvjQ/VZxixp6kH7euhwfrm l05VK9UNWMsrLEcLGNUav8bwRJshJGZ2dN4rtolAI8Gk2nchLy bXnyIznbD4zT2ZqevyQlfH3IXrthBVq0XB27MbuVSJczJ8L/mP o/wYnJ3QOQDSw53HQK1VCK0BagpZUOlcYty4C3Q2EzIY09LHHX n82MIDOqV6OYYfBja5pc=
X-BeenThere python-list@python.org
X-Mailman-Version 2.1.15
Precedence list
List-Id General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe <http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive <http://mail.python.org/pipermail/python-list/>
List-Post <mailto:python-list@python.org>
List-Help <mailto:python-list-request@python.org?subject=help>
List-Subscribe <http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups comp.lang.python
Message-ID <mailman.2233.1361499562.2939.python-list@python.org> (permalink)
Lines 41
NNTP-Posting-Host 2001:888:2000:d::a6
X-Trace 1361499562 news.xs4all.nl 6844 [2001:888:2000:d::a6]:46907
X-Complaints-To abuse@xs4all.nl
Xref csiph.com comp.lang.python:39512

Show key headers only | View raw

On 02/21/2013 05:44 PM, Schizoid Man wrote:
>   <snip>
>
>
> No, I was aware to be honest. I thought ** was just short hand for
> math.pow(). Since ** is the integer operation

It's an integer operation because you started with two ints.  Unlike 
math.pow, which converts to floats, whatever you feed it.

>
> I compared the difference and got a large blob of numbers. To make a
> proper comparison I'll need to compare the base and exponent for which
> the numbers are different rather than the numbers themselves. I'm
> following Dave's suggestion of determining the symmetric difference of
> the sets.

But once you have a few that are just plain way off, it doesn't really 
matter whether some others differ in the 17th place.  All the other 
discussion is interesting, but don't forget the main point, that trying 
to represent large integers (over 17 or so digits) as floats is going to 
lose precision.  That can happen a number of ways, and math.pow is just 
one of them.

Floats use a finite precision to store their value, and the radix is 
binary, not decimal.  So figuring where they start to lose precision is 
tricky.  If you're doing a calculation where all intermediate values are 
integers, you're usually better off sticking with int/long.

There are many other kinds of constraints that come up in programming, 
and Python usually has an answer for each.  But in a machine of finite 
size, and when we care at least a little about performance, we 
frequently have to pick our algorithm, our set of functions, and our 
data format carefully.

Someone else has mentioned the decimal package and the fractions.  Each 
of those has a lot to offer in specific situations.  But none is a panacea.


-- 
DaveA

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

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

csiph-web