Path: csiph.com!usenet.pasdenom.info!dedibox.gegeweb.org!gegeweb.eu!nntpfeed.proxad.net!proxad.net!feeder1-2.proxad.net!usenet-fr.net!nerim.net!novso.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 0.001 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; '21,': 0.07; 'python': 0.09; 'list...': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'section,': 0.09; 'subject:method': 0.09; 'terry': 0.09; 'assume': 0.11; 'slightly': 0.15; 'doc,': 0.16; 'duplicates': 0.16; 'element.': 0.16; 'index.': 0.16; 'itertools': 0.16; 'losing': 0.16; 'received:80.91.229.3': 0.16; 'received:plane.gmane.org': 0.16; 'recipe': 0.16; 'reedy': 0.16; 'set()': 0.16; 'subject:Problem': 0.16; 'task.': 0.16; 'wingide': 0.16; 'wed,': 0.16; 'wrote:': 0.17; 'duplicate': 0.17; 'element': 0.17; 'odd': 0.17; 'jan': 0.18; 'code,': 0.18; 'written': 0.20; 'all,': 0.21; 'trying': 0.21; 'fine,': 0.22; 'elements': 0.23; "i've": 0.23; 'header:In-Reply-To:1': 0.25; 'header:User-Agent:1': 0.26; 'looks': 0.26; 'am,': 0.27; 'change,': 0.27; 'set.': 0.27; 'header:X-Complaints-To:1': 0.28; 'chris': 0.28; 'points': 0.29; "i'm": 0.29; 'basic': 0.30; 'function': 0.30; 'error': 0.30; 'running': 0.32; "aren't": 0.33; 'instead,': 0.33; 'problem': 0.33; 'to:addr:python-list': 0.33; 'nov': 0.35; 'doing': 0.35; "won't": 0.35; 'something': 0.35; 'received:org': 0.36; 'really': 0.36; 'but': 0.36; 'method': 0.36; 'subject:with': 0.36; 'keeps': 0.37; 'uses': 0.37; 'maintaining': 0.37; 'rather': 0.37; 'subject:: ': 0.38; 'sure': 0.38; 'several': 0.39; 'to:addr:python.org': 0.39; 'header:Received:5': 0.40; 'help': 0.40; 'your': 0.60; 'remove': 0.61; "you've": 0.61; 'first': 0.61; "you'll": 0.62; 'relatively': 0.62; 'alvaro': 0.84; 'combo': 0.84; 'exercise.': 0.84; 'order:': 0.84; 'received:fios.verizon.net': 0.84; 'sighted': 0.84; 'approach.': 0.91; 'inefficient': 0.91; 'technique': 0.93 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Terry Reedy Subject: Re: Problem with list.remove() method Date: Tue, 20 Nov 2012 15:32:21 -0500 References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: pool-173-75-251-66.phlapa.fios.verizon.net User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:16.0) Gecko/20121026 Thunderbird/16.0.2 In-Reply-To: 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: 44 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1353443565 news.xs4all.nl 6984 [2001:888:2000:d::a6]:49042 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:33655 On 11/20/2012 9:14 AM, Chris Angelico wrote: > On Wed, Nov 21, 2012 at 12:56 AM, Alvaro Combo wrote: >> Hi All, >> >> I'm relatively new to Python... but I have found something I cannot explain... and I'm sure you can help me. >> >> I have the following function that serves for removing the duplicates from a list... It's a simple and (almost) trivial task. >> >> I'm using WingIDE as editor/debugger and have Python 2.7.3. >> >> When running this I have an error when trying to remove cpy_lst[4]... and ONLY THAT!!! Even in interactive mode!!! > > Several points here. You've written a beautiful O(N^2) duplicates > remover... Python has a really fast way of doing it, if you don't mind > losing order: > > cpy_lst = list(set(lst)) > > But let's assume you're doing this for the exercise. Your technique is > fine, if inefficient on large lists, but the remove() method looks for > the first occurrence of an element by its value - what you want is: > > del cpy_lst[i] > > which will remove one element by index. > > With that change, you'll have a slightly odd duplicate remover that > keeps the *last* of any given element. That's rather unusual. Instead, > you may want to consider maintaining a set of "items I've already > seen", and keeping all elements that aren't in that set. I won't give > you all the code, but here's the basic set operations: > > sighted = set() > sighted.add(some_element) > if some_element in sighted: # condition is True if you've already > seen this element The itertools doc, in the last section, has a recipe for this problem that uses the above approach. -- Terry Jan Reedy