Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!feeder.news-service.com!newsfeed.xs4all.nl!newsfeed5.news.xs4all.nl!xs4all!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 0.002 X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'skip': 0.04; 'received:verizon.net': 0.07; 'terry': 0.07; 'filter,': 0.09; 'func': 0.09; 'generators': 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; 'def': 0.15; 'argument': 0.15; 'considerably': 0.16; 'hammer': 0.16; 'reedy': 0.16; 'what?': 0.16; 'wrote:': 0.16; 'jan': 0.19; '(which': 0.19; 'seems': 0.20; 'header:In-Reply-To:1': 0.22; 'pm,': 0.24; 'string': 0.26; 'function': 0.27; 'problem': 0.28; 'yield': 0.29; '---': 0.29; 'example': 0.30; 'not.': 0.30; 'list).': 0.30; 'values,': 0.30; 'subject:?': 0.31; 'seem': 0.31; 'michael': 0.31; 'values': 0.32; 'list': 0.32; 'usually': 0.32; 'actually': 0.33; 'to:addr:python- list': 0.33; 'header:User-Agent:1': 0.34; 'header:X-Complaints- To:1': 0.35; 'list.': 0.35; 'problem.': 0.36; 'sequence': 0.37; 'passed': 0.37; 'using': 0.37; 'list,': 0.37; 'but': 0.37; 'not,': 0.38; 'think': 0.38; 'received:org': 0.38; 'should': 0.38; 'subject:: ': 0.39; 'header:Mime-Version:1': 0.39; 'to:addr:python.org': 0.39; 'where': 0.40; 'more': 0.60; 'your': 0.61; 'hope': 0.61; 'grab': 0.66; 'making': 0.67; '"for': 0.67; 'applicable': 0.68; 'saw': 0.69; 'pre-existing': 0.84; 'value):': 0.84 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Terry Reedy Subject: Re: Fast recursive generators? Date: Fri, 28 Oct 2011 22:27:40 -0400 References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: pool-74-109-121-73.phlapa.fios.verizon.net User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:6.0) Gecko/20110812 Thunderbird/6.0 In-Reply-To: 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: 1319855279 news.xs4all.nl 6888 [2001:888:2000:d::a6]:40179 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:15127 On 10/28/2011 8:49 PM, Michael McGlothlin wrote: >> Better to think of a sequence of values, whether materialized as a 'list' or >> not. > > The final value will actually be a string but it seems it is usually > faster to join a list of strings than to concat them one by one. .join() takes an iterable of strings as its argument >> Comprehensions combine map and filter, both of which conceptually work on >> each item of a pre-existing list independently. (I am aware that the >> function passed can stash away values to create dependence. > > The problem is I don't really have a pre-existing list. So, as I said, map, filter, and comprehensions are not applicable to your problem. >> def do(func, N, value): >> yield value >> for i in range(1,N): >> value = func(value) >> yield value >> >> For more generality, make func a function of both value and i. >> If you need a list, "l = list(do(f,N,x))", but if you do not, you can do >> "for item in do(f,N,x):" and skip making the list. > > Generators seem considerably slower than using comprehension or > map/filter So what? A saw may cut faster than a hammer builds, but I hope you don't grab a saw when you need a hammer. > /reduce. Do you actually have an example where ''.join(reduce(update, N, start)) is faster than ''.join(update_gen(N, start))? Resuming a generator n times should be *faster* than n calls to the update function of reduce (which will actually have to build a list). --- Terry Jan Reedy