Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.python > #44194

Re: Nested iteration?

Path csiph.com!usenet.pasdenom.info!dedibox.gegeweb.org!gegeweb.eu!nntpfeed.proxad.net!proxad.net!feeder1-1.proxad.net!ecngs!feeder2.ecngs.de!novso.com!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Return-Path <ian.g.kelly@gmail.com>
X-Original-To python-list@python.org
Delivered-To python-list@mail.python.org
X-Spam-Status OK 0.026
X-Spam-Evidence '*H*': 0.95; '*S*': 0.00; 'explicitly': 0.05; 'nested': 0.07; 'latter': 0.09; 'list).': 0.09; 'worked.': 0.09; '(x,': 0.16; '23,': 0.16; 'iterable': 0.16; 'iteration': 0.16; 'iterator': 0.16; 'iterator,': 0.16; 'looping': 0.16; 'object)': 0.16; 'roy': 0.16; 'wrote:': 0.18; '(not': 0.18; 'mention': 0.26; 'header:In-Reply-To:1': 0.27; 'am,': 0.29; '(like': 0.30; 'message-id:@mail.gmail.com': 0.30; 'code': 0.31; 'getting': 0.31; 'object.': 0.31; 'file': 0.32; 'received:209.85': 0.35; 'case,': 0.35; 'received:google.com': 0.35; "didn't": 0.36; 'subject:?': 0.36; 'two': 0.37; 'received:209': 0.37; 'somebody': 0.38; 'to:addr:python-list': 0.38; 'itself': 0.39; 'to:addr:python.org': 0.39; 'break': 0.61; 'today,': 0.61; 'simply': 0.61; 'different': 0.65; 'smith': 0.68; 'results': 0.69; 'legal': 0.71; 'guaranteed': 0.75; "else's": 0.84; 'replicate': 0.84; '2013': 0.98
DKIM-Signature v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=x-received:mime-version:in-reply-to:references:from:date:message-id :subject:to:content-type; bh=sQKkPJ0SOoS32PRvCSwxsNmdo7iSmPFurRWsUvdfWGM=; b=c3kToLGKZTW68bZ8byJ8nWycSdBOhY0QnbFISZ42gZGH1zxgRKr4R/IKJoX3PFZnJp WM9+0vjzGgaAdzuZTPsuB4w8/anRGZH8O7gNYeHH2WVw7F2bNJmNieMYZk+v2NAatYWb 25e7d6u+3lFTDq9iTfzRb9QoF/2Sk4kVFwO3Je/41lCGSj/WQWLr79tAZxBuLK8YBRKn pSUQVhHAY8lx4oSFsqJNU2e+M/fjB5pJvQofaJERoDi9V7AxCZxPkttJTFlNE8R8MgIm HbV1iFDaiyYACHtyQO5AIzTmqSqxmOMl0h2hA93uCkpYmc0HGSlsWOd0LdxFFyA4vJjj w2mA==
X-Received by 10.68.176.197 with SMTP id ck5mr41331658pbc.165.1366733166425; Tue, 23 Apr 2013 09:06:06 -0700 (PDT)
MIME-Version 1.0
In-Reply-To <kl6a1f$k2l$1@panix2.panix.com>
References <kl6a1f$k2l$1@panix2.panix.com>
From Ian Kelly <ian.g.kelly@gmail.com>
Date Tue, 23 Apr 2013 10:05:26 -0600
Subject Re: Nested iteration?
To Python <python-list@python.org>
Content-Type text/plain; charset=ISO-8859-1
X-BeenThere python-list@python.org
X-Mailman-Version 2.1.15
Precedence list
List-Id General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe <http://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive <http://mail.python.org/pipermail/python-list/>
List-Post <mailto:python-list@python.org>
List-Help <mailto:python-list-request@python.org?subject=help>
List-Subscribe <http://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups comp.lang.python
Message-ID <mailman.981.1366733175.3114.python-list@python.org> (permalink)
Lines 32
NNTP-Posting-Host 2001:888:2000:d::a6
X-Trace 1366733175 news.xs4all.nl 2244 [2001:888:2000:d::a6]:55630
X-Complaints-To abuse@xs4all.nl
Xref csiph.com comp.lang.python:44194

Show key headers only | View raw

On Tue, Apr 23, 2013 at 9:40 AM, Roy Smith <roy@panix.com> wrote:
> In reviewing somebody else's code today, I found the following
> construct (eliding some details):
>
>     f = open(filename)
>     for line in f:
>         if re.search(pattern1, line):
>             outer_line = f.next()
>             for inner_line in f:
>                 if re.search(pattern2, inner_line):
>                     inner_line = f.next()
>
> Somewhat to my surprise, the code worked.  I didn't know it was legal
> to do nested iterations over the same iterable (not to mention mixing
> calls to next() with for-loops).  Is this guaranteed to work in all
> situations?

Yes, although the results will be different depending on whether the
iterable stores its iteration state on itself (like a file object) or
in the iterator (like a list).  In the latter case, you would simply
have two independent simultaneous iterations of the same object.  You
can replicate the same effect in the latter case though by getting an
iterator from the object and explicitly looping over the same
iterator, like so:

i = iter(range(10))
for x in i:
    if x % 4 == 1:
        for y in i:
            if y % 4 == 3:
                print("%d + %d = %d" % (x, y, x+y))
                break

Back to comp.lang.python | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Nested iteration? roy@panix.com (Roy Smith) - 2013-04-23 11:40 -0400
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 17:05 +0100
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:05 -0600
  Re: Nested iteration? Peter Otten <__peter__@web.de> - 2013-04-23 18:15 +0200
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:21 +1000
  Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:35 +0000
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:30 -0600
  Re: Nested iteration? Ian Kelly <ian.g.kelly@gmail.com> - 2013-04-23 10:39 -0600
  Re: Nested iteration? Chris Angelico <rosuav@gmail.com> - 2013-04-24 02:42 +1000
    Re: Nested iteration? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2013-04-23 16:53 +0000
  Re: Nested iteration? Terry Jan Reedy <tjreedy@udel.edu> - 2013-04-23 16:49 -0400
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:14 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 22:29 +0100
  Re: Nested iteration? Joshua Landau <joshua.landau.ws@gmail.com> - 2013-04-23 22:41 +0100
  Re: Nested iteration? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2013-04-23 23:42 +0100

csiph-web