Re: Array of variables

[Jenda]

There are two places in Perl that may confuse people to think arrays contain variables. While @array = ($one, $two, $three); does make copies of the values of those variables for my $x ($one, $two, $three) { doesn't make copies, but rather aliases $x to the variables one at a time:

my $one = 1;
my $two = 2;
my $three = 3;
my @arr = ($one, $two, $three);
$arr[0] += 10;
print "\$one = $one\n"; # still 1
for my $x ($one, $two, $three) {
  $x += 0.5;
}
print "\$one = $one\n"; # changed to 1.5
for my $x (@arr) {
  $x += 0.1;
}
print "\$one = $one\n"; # still 1.5, no link between $arr[0] and $one

So arrays contain values, but for loops through the "things" specified in the list you specify and if that "thing" is a variable, you can change it.

There is an exception though. The @_ array used for parameters to subroutine calls:

my $one = 1;
my $two = 2;
my $three = 3;
sub direct {
  $_[0] += 10;
  $_[1] += 9;
}
sub indirect {
  my ($a,$b,$c) = @_;
  $a += 99;
  $b += 99;
}
print "\$one = $one\n"; # 1
direct($one, $two, $three);
print "\$one = $one\n"; # changed to 11. $_[0] was an alias to $one.
indirect($one, $two, $three);
print "\$one = $one\n"; # still 11. The $a was assigned a copy of the 
+value. It's not an alias