Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!gegeweb.org!de-l.enfer-du-nord.net!feeder1.enfer-du-nord.net!newsfeed.eweka.nl!eweka.nl!feeder3.eweka.nl!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!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 0.017 X-Spam-Evidence: '*H*': 0.97; '*S*': 0.00; '>>>>': 0.09; '[0,': 0.09; 'lawrence': 0.09; 'portions': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; '16,': 0.15; '2.7.2': 0.16; 'better?': 0.16; 'bieber': 0.16; 'front-end.': 0.16; 'pythonwin': 0.16; 'wed,': 0.17; 'wrote:': 0.18; 'trying': 0.21; 'header:In-Reply- To:1': 0.22; 'feb': 0.22; 'important,': 0.23; 'slice': 0.23; 'function': 0.27; 'import': 0.27; 'lists': 0.28; 'bit': 0.28; "i'm": 0.28; 'print': 0.29; 'definition': 0.30; 'tail': 0.30; 'header:User-Agent:1': 0.33; 'header:X-Complaints-To:1': 0.34; 'from:addr:yahoo.co.uk': 0.34; 'lee': 0.34; 'running': 0.34; '17,': 0.34; 'to:addr:python-list': 0.35; '...': 0.35; 'starting': 0.36; 'beginning': 0.36; 'received:org': 0.36; 'sequence': 0.37; 'but': 0.37; 'uses': 0.38; 'could': 0.38; 'some': 0.38; 'i.e.': 0.39; 'possible.': 0.39; 'called': 0.40; 'received:78': 0.40; 'to:addr:python.org': 0.40; 'back': 0.60; 'more': 0.61; 'copyright': 0.62; 'thousands': 0.62; 'further': 0.64; 'chain': 0.66; '11,': 0.68; '19,': 0.68; 'dennis': 0.73; 'cycling': 0.84; 'join:': 0.84; 'received:as13285.net': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Mark Lawrence Subject: Re: Cycle around a sequence Date: Wed, 08 Feb 2012 08:26:33 +0000 References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: host-78-146-7-131.as13285.net User-Agent: Mozilla/5.0 (Windows NT 6.0; rv:10.0) Gecko/20120129 Thunderbird/10.0 In-Reply-To: X-Antivirus: avast! (VPS 120207-1, 07/02/2012), Outbound message X-Antivirus-Status: Clean X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.12 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: 45 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1328689595 news.xs4all.nl 6941 [2001:888:2000:d::a6]:46207 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:20015 On 08/02/2012 01:26, Dennis Lee Bieber wrote: > On Wed, 08 Feb 2012 01:10:28 +0000, Mark Lawrence > wrote: > >> I'm looking at a way of cycling around a sequence i.e. starting at some >> given location in the middle of a sequence and running to the end before >> coming back to the beginning and running to the start place. About the >> best I could come up with is the following, any better ideas for some >> definition of better? >> >> PythonWin 2.7.2 (default, Jun 12 2011, 15:08:59) [MSC v.1500 32 bit >> (Intel)] on win32. >> Portions Copyright 1994-2008 Mark Hammond - see 'Help/About PythonWin' >> for further copyright information. >>>>> from itertools import chain >>>>> a=range(10) >>>>> g = chain((a[i] for i in xrange(4, 10, 1)), (a[i] for i in xrange(4))) >>>>> for x in g: print x, >> ... >> 4 5 6 7 8 9 0 1 2 3 >>>>> > > How large a sequence and, more important, is it fully known at the > start... > >>>> a = range(20) >>>> a > [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19] >>>> a_shift = a[5:] + a[:5] >>>> a_shift > [5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 0, 1, 2, 3, 4] >>>> > > IOWs, just slice and join: tail first, then front-end. > The sequences are small and the start is always known but the function that uses this is called thousands of times so I was trying to avoid building lists if at all possible. -- Cheers. Mark Lawrence.