Path: csiph.com!usenet.pasdenom.info!weretis.net!feeder1.news.weretis.net!feeder.erje.net!eu.feeder.erje.net!xlned.com!feeder1.xlned.com!news2.euro.net!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail Return-Path: X-Original-To: python-list@python.org Delivered-To: python-list@mail.python.org X-Spam-Status: OK 0.000 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'python.': 0.02; 'operator': 0.03; 'correct.': 0.07; 'overflow': 0.07; 'see.': 0.07; 'integers': 0.09; 'modules.': 0.09; 'received:mail- vc0-f174.google.com': 0.09; 'underlying': 0.09; 'cc:addr:python- list': 0.10; '2.7.3': 0.16; 'cc:name:python list': 0.16; 'double)': 0.16; 'integers,': 0.16; 'non-integers': 0.16; 'precision,': 0.16; 'shorthand': 0.16; 'storing': 0.16; 'url:decimal': 0.16; 'wrote:': 0.17; 'basically': 0.17; 'integer': 0.17; 'library,': 0.17; 'examples': 0.18; '>>>': 0.18; 'memory': 0.18; '(not': 0.20; 'suggested': 0.20; 'import': 0.21; 'operations.': 0.22; 'subject:problem': 0.22; 'runs': 0.22; 'cc:2**0': 0.23; 'work.': 0.23; "python's": 0.23; 'seems': 0.23; 'machine': 0.24; 'cc:addr:python.org': 0.25; 'header:In-Reply- To:1': 0.25; 'wrote': 0.26; 'library.': 0.27; 'message- id:@mail.gmail.com': 0.27; '(unless': 0.29; 'arithmetic': 0.29; 'cases.': 0.29; 'decimal': 0.29; 'received:209.85.220.174': 0.29; 'strings,': 0.29; 'class': 0.29; "i'm": 0.29; 'normally': 0.30; 'performing': 0.30; 'function': 0.30; 'url:python': 0.32; 'int': 0.33; 'operations': 0.33; 'received:google.com': 0.34; 'third': 0.34; 'built-in': 0.35; 'pm,': 0.35; 'received:209.85.220': 0.35; "won't": 0.35; 'received:209.85': 0.35; 'there': 0.35; 'but': 0.36; 'url:org': 0.36; 'url:library': 0.36; "didn't": 0.36; 'uses': 0.37; 'received:209': 0.37; 'subject:: ': 0.38; 'some': 0.38; 'url:docs': 0.38; 'url:en': 0.38; 'instead': 0.39; 'think': 0.40; 'your': 0.60; 'range': 0.60; 'high': 0.61; 'url:index': 0.61; 'more': 0.63; 'results': 0.65; 'friend': 0.81; '*really*': 0.84; '2013': 0.84; 'oscar': 0.84; 'hand,': 0.97 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:mime-version:in-reply-to:references:from:date:message-id :subject:to:cc:content-type; bh=eQ67IXKixZXh/lmtMbcqjt3pzqWJR9D4G1xeNc0ur4o=; b=x5HnW7HpaqoejnRUepXOW7GqtC1Ew1PMDtHwO0mL2pfJx+OuHUQ2derjXwccsPxZEm Zr9ut6K7Pmq6agJStM0ZKIfaVHgaDQwFUKdesLUO+vcCo4hTp0+XLbpro62TXwR1XYcH tpKHBYbr8zepZWvA5bf4qn2Md46N3/iagby3iJ9jGPu41pBUg5eeM02CRD8JjhzJI4Tb XSK7mlJTwSWlBaqUWm84QZxjNNSv7K0z4Xrdfqr29WfOm45yP2l/BkQJWo5PFmlcLUnB iAJ9XW1sNTrHIUbdmRbvs4hIO5l7QaKMWgJuQ0/WnKkFTN6MoUntMuA0OGUMR8/x1jMp oqRA== X-Received: by 10.58.171.38 with SMTP id ar6mr34737020vec.23.1361487256120; Thu, 21 Feb 2013 14:54:16 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: From: Oscar Benjamin Date: Thu, 21 Feb 2013 22:53:55 +0000 Subject: Re: Confusing math problem To: Schizoid Man Content-Type: text/plain; charset=ISO-8859-1 Cc: Python List X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 51 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1361487258 news.xs4all.nl 6882 [2001:888:2000:d::a6]:47828 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:39481 On 21 February 2013 22:39, Schizoid Man wrote: > "Dave Angel" wrote in message >> >> On 02/21/2013 02:33 PM, Schizoid Man wrote: >> However, there is an important inaccuracy in math.pow, because it uses >> floats to do the work. If you have very large integers, that means some of >> them won't be correct. The following are some examples for 2.7.3 on Linux: >> >> a b math.pow(a,b) a**b >> 3 34 1.66771816997e+16 16677181699666569 >> 3 35 5.0031545099e+16 50031545098999707 >> ... >> 5 23 1.19209289551e+16 11920928955078125 >> >> The built-in pow, on the other hand, seems to get identical answers for >> all these cases. So use pow() instead of math.pow() > > I see. I thought using the ** was shorthand for math.pow() and didn't think > that one would be integer operations and the other floats. Then you want operator.pow: >>> import operator >>> operator.pow(3, 2) 9 math.pow is basically the (double)pow(double, double) function from the underlying C library. operator.pow(a, b) is precisely the same as a**b. > I'm performing > some large integer arithmetic operations. I would normally do this my > writing my own multiplication class and storing results as strings, but a > friend suggested that I look at Python. There's no need to use strings if you're working with integers in Python. The results with int (not float) will be exact and will not overflow since Python's ints have unlimited range (unless your machine runs out of memory but that only happens with *really* big integers). If you want to do computations with non-integers and high precision, take a look at the decimal and fractions modules. http://docs.python.org/2/library/decimal.html http://docs.python.org/2/library/fractions.html There is also a good third party library, sympy, for more complicated exact algebra: http://sympy.org/en/index.html Oscar