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

Thu Mar 1 09:39:25 EST 2012

Xah Lee <xahlee at 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.