Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!news.albasani.net!feeder.news-service.com!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 0.001 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; '>>>>': 0.09; 'none:': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:80.91.229.12': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'received:lo.gmane.org': 0.09; 'solution,': 0.09; 'def': 0.13; 'wrote:': 0.14; 'inverse': 0.16; 'received:dip.t-dialin.net': 0.16; 'received:t-dialin.net': 0.16; 'valueerror': 0.16; 'seems': 0.21; 'subject:list': 0.22; 'function': 0.27; "doesn't": 0.28; 'thanks': 0.29; 'raise': 0.29; 'problem': 0.29; 'subject:?': 0.29; 'from:addr:web.de': 0.31; 'to:addr:python-list': 0.32; "isn't": 0.34; 'header:X-Complaints- To:1': 0.34; 'describe': 0.35; 'occurs': 0.35; 'received:org': 0.38; 'to:addr:python.org': 0.39; 'header:Mime-Version:1': 0.39; 'header:Received:5': 0.40; 'twice': 0.60; 'best': 0.60; 'subject:better': 0.93 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Peter Otten <__peter__@web.de> Subject: Re: a better way to invert a list? Date: Wed, 06 Apr 2011 11:58:35 +0200 Organization: None References: <2215eefd-3677-4459-8656-aa04978f6f3f@g7g2000pro.googlegroups.com> Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit X-Gmane-NNTP-Posting-Host: p5084ba0e.dip.t-dialin.net 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: 27 NNTP-Posting-Host: 82.94.164.166 X-Trace: 1302083939 news.xs4all.nl 41102 [::ffff:82.94.164.166]:41866 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:2698 Glazner wrote: >> > def invert(p): >> > inverse = [None] * len(p) >> > for (i, j) in enumerate(p): >> > inverse[j] = i >> > return inverse >> >> Elegant. This seems like the best solution, although it isn't as much >> fun to write as a "one-liner". Thanks > > >>>> invert([1, 2, 3, 1]) > [None, 3, 1, 2] #blah 1 occurs twice in [1, 2, 3, 1] which therefore doesn't describe a permutation. In general a function has to be "bijective" to be invertable. You can catch the problem with (untested) def invert(p): inverse = [None] * len(p) for i, k in enumerate(p): if inverse[k] is not None: raise ValueError inverse[k] = i return inverse