Xah Lee <xah...@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.
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