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Re: a little parsing challenge ☺

References (2 earlier) <8085725c-a600-4d64-8533-fd15505f94e1@p12g2000pre.googlegroups.com> <j04f5h$b2$1@speranza.aioe.org> <71aa9b11-5f47-4c89-bdbb-0fe4cac844e1@s33g2000prg.googlegroups.com> <mailman.1272.1311102598.1164.python-list@python.org> <fe661969-c7dc-48d8-97bd-5a3b469b13fe@en1g2000vbb.googlegroups.com>
From Ian Kelly <ian.g.kelly@gmail.com>
Date 2011-07-20 01:29 -0600
Subject Re: a little parsing challenge ☺
Newsgroups comp.lang.python
Message-ID <mailman.1283.1311146981.1164.python-list@python.org> (permalink)

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On Wed, Jul 20, 2011 at 12:29 AM, jmfauth <wxjmfauth@gmail.com> wrote:
>> Then it is hard to code precisely.
>>
>
> Not really. The trick is to count the different opener/closer
> separately.
> That is what I am doing to check balanced brackets in
> chemical formulas. The rules are howerver not the same
> as in math.

I think the difficulty is not in the algorithm, but in adhering to the
desired output when it is ambiguously described.

> But, if I want to parse a string from right to left,
> what's the trick?
> The best I found so far:
>
>>>> s = 'abcd'
>>>> for i, c in enumerate(reversed(s)):
> ...     print len(s) - 1 - i, c

That violates DRY, since you have reversal logic in the iterator
algebra and then again in the loop body.  I prefer to keep all such
logic in the iterator algebra, if possible.  This is one possibility,
if you don't mind it building an intermediate list:

>>> for i, c in reversed(list(enumerate(s))):
...

Otherwise, here's another non-DRY solution:

>>> from itertools import izip
>>> for i, c in izip(reversed(xrange(len(s))), reversed(s)):
...

Unfortunately, this is one space where there just doesn't seem to be a
single obvious way to do it.

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a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-17 00:47 -0700
  Re: a little parsing challenge ☺ Raymond Hettinger <python@rcn.com> - 2011-07-17 02:48 -0700
    Re: a little parsing challenge ☺ Robert Klemme <shortcutter@googlemail.com> - 2011-07-17 15:20 +0200
      Re: a little parsing challenge ☺ mhenn <michihenn@hotmail.com> - 2011-07-17 15:55 +0200
        Re: a little parsing challenge ☺ Robert Klemme <shortcutter@googlemail.com> - 2011-07-17 18:01 +0200
          Re: a little parsing challenge ☺ Robert Klemme <shortcutter@googlemail.com> - 2011-07-17 18:54 +0200
    Re: a little parsing challenge ☺ Thomas Boell <tboell@domain.invalid> - 2011-07-17 17:49 +0200
      Re: a little parsing challenge ☺ Raymond Hettinger <python@rcn.com> - 2011-07-17 12:16 -0700
    Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-18 07:39 -0700
      Re: a little parsing challenge ☺ Robert Klemme <shortcutter@googlemail.com> - 2011-07-20 08:23 +0200
      Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-20 03:31 -0700
        Re: a little parsing challenge ☺ "Uri Guttman" <uri@StemSystems.com> - 2011-07-20 12:31 -0400
          Re: a little parsing challenge ☺ rusi <rustompmody@gmail.com> - 2011-07-20 10:30 -0700
          Re: a little parsing challenge ☺ merlyn@stonehenge.com (Randal L. Schwartz) - 2011-07-20 12:06 -0700
            Re: a little parsing challenge ☺ Jason Earl <jearl@notengoamigos.org> - 2011-07-20 14:57 -0600
    Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-19 09:54 -0700
      Re: a little parsing challenge ☺ Thomas Jollans <t@jollybox.de> - 2011-07-19 20:07 +0200
        Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-21 05:58 -0700
          Re: a little parsing challenge ☺ Ian Kelly <ian.g.kelly@gmail.com> - 2011-07-21 08:26 -0600
            Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-21 08:36 -0700
              Re: a little parsing challenge ☺ python@bdurham.com - 2011-07-21 12:43 -0400
                Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-21 11:53 -0700
                Re: a little parsing challenge ☺ Terry Reedy <tjreedy@udel.edu> - 2011-07-21 18:37 -0400
          Re: a little parsing challenge ☺ John O'Hagan <research@johnohagan.com> - 2011-07-25 15:57 +1000
      Re: a little parsing challenge ☺ Ian Kelly <ian.g.kelly@gmail.com> - 2011-07-19 12:08 -0600
  Re: a little parsing challenge ☺ Chris Angelico <rosuav@gmail.com> - 2011-07-17 21:34 +1000
    Re: a little parsing challenge ☺ rusi <rustompmody@gmail.com> - 2011-07-17 04:52 -0700
    Re: a little parsing challenge ☺ Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-07-17 16:15 +0200
      Re: a little parsing challenge ☺ Raymond Hettinger <python@rcn.com> - 2011-07-17 12:18 -0700
        Re: a little parsing challenge ☺ Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-07-17 22:16 +0200
          Re: a little parsing challenge ☺ Thomas Jollans <t@jollybox.de> - 2011-07-17 22:57 +0200
      Re: a little parsing challenge ☺ Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-07-17 23:43 +0200
      Re: a little parsing challenge ☺ Rouslan Korneychuk <rouslank@msn.com> - 2011-07-18 03:09 -0400
        Re: a little parsing challenge ☺ Stefan Behnel <stefan_ml@behnel.de> - 2011-07-18 09:24 +0200
          Re: a little parsing challenge ☺ Rouslan Korneychuk <rouslank@msn.com> - 2011-07-18 04:04 -0400
        Re: a little parsing challenge ☺ Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-07-18 18:46 +0200
          Re: a little parsing challenge ☺ Rouslan Korneychuk <rouslank@msn.com> - 2011-07-18 14:14 -0400
        Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-21 06:23 -0700
          Re: a little parsing challenge ☺ Rouslan Korneychuk <rouslank@msn.com> - 2011-07-21 17:54 -0400
  Re: a little parsing challenge ☺ gene heskett <gheskett@wdtv.com> - 2011-07-17 10:26 -0400
  Re: a little parsing challenge ☺ Thomas Jollans <t@jollybox.de> - 2011-07-17 08:31 -0700
    Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-19 10:49 -0700
      Re: a little parsing challenge ☺ Thomas Jollans <t@jollybox.de> - 2011-07-19 20:14 +0200
        Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-21 05:29 -0700
          Re: a little parsing challenge ☺ Thomas Jollans <t@jollybox.de> - 2011-07-21 15:21 +0200
      Re: a little parsing challenge ☺ Thomas Jollans <t@jollybox.de> - 2011-07-19 20:17 +0200
  Re: a little parsing challenge ☺ rantingrick <rantingrick@gmail.com> - 2011-07-17 18:52 -0700
  Re: a little parsing challenge ☺ Billy Mays <81282ed9a88799d21e77957df2d84bd6514d9af6@myhashismyemail.com> - 2011-07-18 13:12 -0400
    Re: a little parsing challenge ☺ Ian Kelly <ian.g.kelly@gmail.com> - 2011-07-18 12:10 -0600
      Re: a little parsing challenge ☺ Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-07-18 23:59 +0200
        Re: a little parsing challenge ☺ Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2011-07-19 08:09 +0200
        Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-19 10:32 -0700
    Re: a little parsing challenge ☺ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-07-19 09:56 +1000
      Re: a little parsing challenge ☺ Billy Mays <noway@nohow.com> - 2011-07-18 22:07 -0400
        Re: a little parsing challenge ☺ rusi <rustompmody@gmail.com> - 2011-07-18 19:50 -0700
          Re: a little parsing challenge ☺ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-07-19 13:11 +1000
            Re: a little parsing challenge ☺ rusi <rustompmody@gmail.com> - 2011-07-18 21:59 -0700
              Re: a little parsing challenge ☺ Chris Angelico <rosuav@gmail.com> - 2011-07-19 15:36 +1000
        Re: a little parsing challenge ☺ MRAB <python@mrabarnett.plus.com> - 2011-07-19 04:08 +0100
        Re: a little parsing challenge ☺ Benjamin Kaplan <benjamin.kaplan@case.edu> - 2011-07-18 20:54 -0700
        Re: a little parsing challenge ☺ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-07-19 14:30 +1000
        Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-19 01:58 -0700
    Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-19 10:14 -0700
      Re: a little parsing challenge ☺ Billy Mays <81282ed9a88799d21e77957df2d84bd6514d9af6@myhashismyemail.com> - 2011-07-19 13:33 -0400
        Re: a little parsing challenge ☺ Xah Lee <xahlee@gmail.com> - 2011-07-19 11:12 -0700
          Re: a little parsing challenge ☺ Terry Reedy <tjreedy@udel.edu> - 2011-07-19 15:09 -0400
            Re: a little parsing challenge ☺ jmfauth <wxjmfauth@gmail.com> - 2011-07-19 23:29 -0700
              Re: a little parsing challenge ☺ Ian Kelly <ian.g.kelly@gmail.com> - 2011-07-20 01:29 -0600
                Re: a little parsing challenge ☺ jmfauth <wxjmfauth@gmail.com> - 2011-07-20 00:54 -0700
                Re: a little parsing challenge ☺ Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2011-07-20 18:18 +1000
  Re: a little parsing challenge ? sln@netherlands.com - 2011-07-18 12:34 -0700
  Re: a little parsing challenge ☺ Mark Tarver <dr.mtarver@gmail.com> - 2011-07-19 22:43 -0700

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