Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!feeder.erje.net!eu.feeder.erje.net!news2.arglkargh.de!news.mixmin.net!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed1.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path: <python-python-list@m.gmane.org>
X-Original-To: python-list@python.org
Delivered-To: python-list@mail.python.org
X-Spam-Status: OK 0.002
X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'mrab': 0.05; 'float': 0.07; 'constructor': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'def': 0.12; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'true:': 0.16; 'all,': 0.19; '>>>': 0.22; 'header:User-Agent:1': 0.23; 'fraction': 0.24; 'header:X-Complaints-To:1': 0.27; 'header:In-Reply-To:1': 0.27; "i'm": 0.30; 'guess': 0.33; '8bit%:86': 0.38; 'to:addr:python- list': 0.38; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'break': 0.61; 'more': 0.64; 'subject:results': 0.91
X-Injected-Via-Gmane: http://gmane.org/
To: python-list@python.org
From: Serhiy Storchaka <storchaka@gmail.com>
Subject: Re: Unexpected results comparing float to Fraction
Date: Mon, 29 Jul 2013 22:34:04 +0300
References: <51f68d9c$0$30000$c3e8da3$5496439d@news.astraweb.com> <51F693B4.9000201@mrabarnett.plus.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
X-Gmane-NNTP-Posting-Host: 37.19.137.81
User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130623 Thunderbird/17.0.7
In-Reply-To: <51F693B4.9000201@mrabarnett.plus.com>
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.5254.1375126459.3114.python-list@python.org>
Lines: 29
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1375126459 news.xs4all.nl 15931 [2001:888:2000:d::a6]:45877
X-Complaints-To: abuse@xs4all.nl
Xref: csiph.com comp.lang.python:51478

29.07.13 19:09, MRAB написав(ла):
> I'm surprised that Fraction(1/3) != Fraction(1, 3); after all, floats
> are approximate anyway, and the float value 1/3 is more likely to be
> Fraction(1, 3) than Fraction(6004799503160661, 18014398509481984).

 >>> def approximate_fraction(f):
     prev_numer, numer = 0, 1
     prev_denom, denom = 1, 0
     r = f
     while True:
         i = math.floor(r)
         prev_numer, numer = numer, i * numer + prev_numer
         prev_denom, denom = denom, i * denom + prev_denom
         if i == r or numer / denom == f:
             break
         r = 1 / (r - i)

     return Fraction(numer, denom)

 >>> approximate_fraction(1/3)
Fraction(1, 3)
 >>> approximate_fraction(1e-17)
Fraction(1, 100000000000000000)
 >>> approximate_fraction(math.pi)
Fraction(245850922, 78256779)

I guess the Fraction constructor is more faster than this function.