Path: csiph.com!newsfeed.hal-mli.net!feeder3.hal-mli.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!newsreader4.netcologne.de!news.netcologne.de!newsfeed.freenet.ag!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 0.030 X-Spam-Evidence: '*H*': 0.94; '*S*': 0.00; 'false.': 0.07; 'subject:test': 0.07; 'moreover,': 0.09; 'subtract': 0.09; 'suggest': 0.11; 'boolean': 0.16; 'false),': 0.16; 'iterating': 0.16; 'lambda': 0.16; 'subject:expression': 0.16; 'wrote:': 0.17; 'element': 0.17; 'integer': 0.17; 'trying': 0.21; 'elements': 0.23; 'header:In-Reply-To:1': 0.25; 'header:User-Agent:1': 0.26; 'correct': 0.28; 'loop,': 0.29; 'strings,': 0.29; 'maybe': 0.29; 'checks': 0.30; 'compatible': 0.30; 'sense': 0.31; 'lists': 0.31; 'to:addr:python-list': 0.33; 'list': 0.35; 'false': 0.35; 'faster': 0.35; 'expected': 0.35; 'pm,': 0.35; 'something': 0.35; 'list.': 0.35; 'but': 0.36; 'turn': 0.36; 'subject:: ': 0.38; 'some': 0.38; 'gives': 0.39; 'to:addr:python.org': 0.39; 'received:192': 0.39; 'received:192.168': 0.40; 'help': 0.40; 'your': 0.60; "you've": 0.61; 'results': 0.65; 'header:Reply- To:1': 0.68; 'received:74.208': 0.71; 'reply-to:no real name:2**0': 0.72; 'sets,': 0.84; 'why?': 0.84; 'reducing': 0.95 Date: Sat, 05 Jan 2013 13:58:10 -0500 From: Dave Angel User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121011 Thunderbird/16.0.1 MIME-Version: 1.0 To: python-list@python.org Subject: Re: reduce expression to test sublist References: <76cd3945-392e-40d4-9f87-d3956b9521d2@googlegroups.com> In-Reply-To: <76cd3945-392e-40d4-9f87-d3956b9521d2@googlegroups.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Provags-ID: V02:K0:9sb4ntFFxuBsJKSf674bDzLp1piUHaM6V4WqePyxK+M uuBB8b1xi4Xzntnt/rf+bmWFmd20i14fCkkRcukPW6H0VeFYQ9 PL2+5ZZBDzDBZHnFaDQidfeQ8pGvFSH01htuEVpbStY81MGPBV zPp/FdoSgim5xEG9zuM+3QroMNFPtu9hLLPiHrLdFBTyKn7eru UMBhkj64hRMEK9IemDr81ekgqaPio4Shm7wawO/jEStmsQg8Ny jq9s4h/fpYvFy/OuhlfwcYjp5mTes4G9SVQYAikCH/8/cGicZm CKkNzzA97Df1b2glLNCbkOBv0gmZGs6lVSDM1mZh6RLyxOPCg= = X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.15 Precedence: list Reply-To: d@davea.name 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: 39 NNTP-Posting-Host: 2001:888:2000:d::a6 X-Trace: 1357412309 news.xs4all.nl 6911 [2001:888:2000:d::a6]:33125 X-Complaints-To: abuse@xs4all.nl Xref: csiph.com comp.lang.python:36199 On 01/05/2013 01:25 PM, Asim wrote: > Hi All > > The following reduce expression checks if every element of list lst1 is present in list lst2. It works as expected for integer lists but for lists of strings, it always returns False. > > reduce( lambda x,y: (x in lst2) and (y in lst2), lst1) > > Moreover, for the lists of strings the following for-loop gives correct results when the above reduce expression doesn't. > > isSublist = True > for i in lst1: > isSublist = isSublist and (i in lst2) > if not isSublist: > isSublist = False > break > > > Can someone help me understand why? > > Asim reduce only makes sense if the value you're reducing to is of the same type as the elements of the list you're iterating over. Since your lambda expression returns a boolean (True or False), it'll seem to work on some lists of ints. That's just a coincidence since the bools are compatible with ints, and maybe you've got a 0 or 1 in the list. But if your list has only strings, then True will never be that list. If you're trying to make a faster loop, then I suggest you look into set differences. Turn both lists into sets, and subtract them. Something like (untested): result = not bool( set(lst1) - set(lst2) ) -- DaveA