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

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From	Rainer Weikusat <rweikusat@mssgmbh.com>
Newsgroups	comp.lang.lisp, comp.emacs, comp.lang.python, comp.lang.perl.misc
Subject	Re: lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp
Date	Thu, 01 Mar 2012 14:39:25 +0000
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Cross-posted to 4 groups.

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Xah Lee <xahlee@gmail.com> writes:

[...]


> # perl
> # in-place algorithm for reversing a list.
>
> use strict;
> use Data::Dumper;
> use POSIX; # for “floor”
>
> my @listA = qw(a b c d e f g);
>
> my $listLength = scalar @listA;
>
> for ( my $i = 0; $i < floor($listLength/2); $i++ ) {
>   my $x = $listA[$i];
>   $listA[$i] = $listA[ $listLength - 1 - $i];
>   $listA[ $listLength - 1 - $i] = $x;
> }
>
> print Dumper(\@listA);

Better algorithm for that (expects an array reference as first
argument)

sub rev
{
    my $a = $_[0];
    my ($n0, $n1, $x);
    
    $n0 = 0;
    $n1 = $#$a;
    while ($n0 < $n1) {
	$x = $a->[$n0];
	$a->[$n0] = $a->[$n1];
	$a->[$n1] = $x;
	
	++$n0;
	--$n1;
    }
}

NB: The fact that a sufficiently sophisticated compiler might be able
to fix this automatically emphasizes the deficiencies of the original
attempt, it doesn't excuse them.

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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