Re: Simple question

Path	csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.datemas.de!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed2a.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path	<rosuav@gmail.com>
X-Original-To	python-list@python.org
Delivered-To	python-list@mail.python.org
X-Spam-Status	OK 0.015
X-Spam-Evidence	'H': 0.97; 'S': 0.00; '16,': 0.03; 'output': 0.05; '0.1': 0.09; 'explanation': 0.09; 'subject:question': 0.10; 'cc:addr:python-list': 0.11; '0.3': 0.16; 'from:addr:rosuav': 0.16; 'from:name:chris angelico': 0.16; 'zero.': 0.16; 'wrote:': 0.18; 'wed,': 0.18; 'bit': 0.19; '>>>': 0.22; 'cc:addr:python.org': 0.22; 'cc:2**0': 0.24; 'header:In-Reply- To:1': 0.27; 'point': 0.28; 'wondering': 0.29; 'am,': 0.29; 'message-id:@mail.gmail.com': 0.30; "i'm": 0.30; 'decimal': 0.31; 'actual': 0.34; 'something': 0.35; 'but': 0.35; 'received:google.com': 0.35; 'really': 0.36; 'subject:Simple': 0.36; 'does': 0.39; 'full': 0.61; "you're": 0.61; 'first': 0.61; 'places': 0.64; 'close': 0.67; 'fact,': 0.69; 'phil': 0.84; 'to:none': 0.92; 'incredibly': 0.96
DKIM-Signature	v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:cc :content-type; bh=Ujzq7KHS2GqqE9+Z+XZ1vU/wd/JgCIjMWXW2dKKV6dM=; b=sojIXVJkhrZoVOCxsQdovURlNsjPVQAWDkdRRciI19CnY5y9U5lu5/hZReKjWDKAi2 cvG0ka/NzUmWpqtkBhO0UeZq+Q6gdioUpwsB9kls9kN32sVNie2tETtt39FIylK+QjrV /okZTZ2lQi1S89hU/NI7Z3SrLYlPeQZQF5bZjT+THVG7jJGyf8S0NS7heb9kMNPbdSYh HZj+mmwZY3EdTZ3hgyBSFVlKCjNh52xc62Lp8juOfAmVKS2oYJtVcde2LG+0PV50Lf08 texPF6r7+oijxpj2szszor6HqjQqUelHd6jOFjLMnSBdAsUxEeADtJ0KYqxd0HHkBh7R mOBg==
MIME-Version	1.0
X-Received	by 10.220.163.3 with SMTP id y3mr2441486vcx.7.1397586429847; Tue, 15 Apr 2014 11:27:09 -0700 (PDT)
In-Reply-To	<534D7807.6050408@gmail.com>
References	<534D7807.6050408@gmail.com>
Date	Wed, 16 Apr 2014 04:27:09 +1000
Subject	Re: Simple question
From	Chris Angelico <rosuav@gmail.com>
Cc	"python-list@python.org" <python-list@python.org>
Content-Type	text/plain; charset=UTF-8
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	<https://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	<https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups	comp.lang.python
Message-ID	<mailman.9285.1397586432.18130.python-list@python.org> (permalink)
Lines	27
NNTP-Posting-Host	2001:888:2000:d::a6
X-Trace	1397586432 news.xs4all.nl 2939 [2001:888:2000:d::a6]:32942
X-Complaints-To	abuse@xs4all.nl
Xref	csiph.com comp.lang.python:70279

Show key headers only | View raw

On Wed, Apr 16, 2014 at 4:18 AM, Phil Dobbin <phildobbin@gmail.com> wrote:
> '>>> 0.1 + 0.1 + 0.1 - 0.3
> 5.55111.....'
>
> What I'm wondering is why the first calculation that arrives at
> '5.55111...' is so far out?

There's something you're not seeing here. In full, the output I see is:

>>> 0.1 + 0.1 + 0.1 - 0.3
5.551115123125783e-17

See that bit at the end? "e-17" means "times ten to the -17th power" -
that is, move the decimal point 17 places to the left. So the actual
value is:

>>> "%50.50f"%(0.1 + 0.1 + 0.1 - 0.3)
'0.00000000000000005551115123125782702118158340454102'

It's not actually as far out as you think; in fact, that's incredibly
close to zero.

A full explanation of why floating point arithmetic does this can be
found in many places on the internet, but the main thing to note is
that it really isn't showing 5.5.

ChrisA

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

Thread

Re: Simple question Chris Angelico <rosuav@gmail.com> - 2014-04-16 04:27 +1000

csiph-web