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Re: Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp

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Date Thu, 01 Mar 2012 00:07:21 -0600
From Evan Driscoll <driscoll@cs.wisc.edu>
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To Dan Stromberg <drsalists@gmail.com>
Subject Re: Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp
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On 2/29/2012 23:05, Dan Stromberg wrote:
> 
> On Wed, Feb 29, 2012 at 8:07 PM, Xah Lee <xahlee@gmail.com
> <mailto:xahlee@gmail.com>> wrote:
> 
>     This page tells you what's “In-place algorithm”, using {python, perl,
>     emacs lisp} code to illustrate.
> 
> Aren't in-place reversals rather non-functional?

There is one place where they're reasonably idiomatic in Lispy
languages, at least by my understanding. That occurs when you are
writing a function that returns a list and there is a natural recursive
way to build up the answer -- but backwards. The idiom then is to build
up a temporary list up backwards, then call an in-place reversal
function. (NREVERSE in Common Lisp. I thought there was a reverse! in
Scheme, but apparently not.)

This doesn't break the external view of a pure function because the list
that's being reversed is a fresh, temporary list, which is why this
idiom would even fit in pretty well in Scheme.

Evan

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Thread

lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp Xah Lee <xahlee@gmail.com> - 2012-02-29 20:07 -0800
  Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-03-01 05:01 +0000
    Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp Xah Lee <xahlee@gmail.com> - 2012-02-29 21:39 -0800
  Re: Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp Evan Driscoll <driscoll@cs.wisc.edu> - 2012-03-01 00:07 -0600
  Re: lang comparison: in-place algorithm for reversing a list in Perl,Python, Lisp "WJ" <w_a_x_man@yahoo.com> - 2012-03-01 06:37 +0000
    Re: lang comparison: in-place algorithm for reversing a list in Perl,Python, Lisp Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-01 22:14 +0000
    Re: lang comparison: in-place algorithm for reversing a list in Perl,Python, Lisp Xah Lee <xahlee@gmail.com> - 2012-03-01 14:04 -0800
      Re: lang comparison: in-place algorithm for reversing a list in Perl,Python, Lisp Chris Angelico <rosuav@gmail.com> - 2012-03-02 18:11 +1100
        Re: lang comparison: in-place algorithm for reversing a list in Perl,Python, Lisp Xah Lee <xahlee@gmail.com> - 2012-03-02 03:30 -0800
  Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp Rainer Weikusat <rweikusat@mssgmbh.com> - 2012-03-01 14:39 +0000
  Re: lang comparison: in-place algorithm for reversing a list in Perl,Python, Lisp "WJ" <w_a_x_man@yahoo.com> - 2012-03-02 07:17 +0000

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