note
kcott
<p>G'day [truthseeker66],</p>
<p>++[toolic]'s answers are spot on. Here's some additional information.</p>
<blockquote><em>
"When do I use 'my' before an array and when don't I?"
</em></blockquote>
<p>
While you're focussing on arrays with this specific question, the information provided applies equally to declaring other types of variables, e.g. <c>my $scalar</c>, <c>my @array</c> and <c>my %hash</c>.
</p>
<p>
So, as already stated, you declare your variable once then subsequently use it as many times as you want (without further declaration). If you declare a variable more than once you'll usually get a warning like this:
</p>
<code>
$ perl -Mstrict -Mwarnings -e '
my $x;
my $x;
'
"my" variable $x masks earlier declaration in same scope at -e line 3.
</code>
<p><em>
[Advanced usage: there are ways to declare variables with the same name more than once within the same piece of code - that's rarely, if ever, needed and best avoided as it's confusing and will often lead to unexpected errors.]
</em></p>
<p>
[http://perldoc.perl.org/functions/my.html|my] is the most frequently used way to declare variables - more in-depth information can be found in [http://perldoc.perl.org/perlsub.html|perlsub] (particularly under [http://perldoc.perl.org/perlsub.html#Private-Variables-via-my()|Private Variables via my()]).
</p>
<p>
[http://perldoc.perl.org/perlsub.html|perlsub] also has a lot of information about other ways to declare variables, including [http://perldoc.perl.org/functions/local.html|local], [http://perldoc.perl.org/functions/our.html|our], [http://perldoc.perl.org/constant.html|constant] and [http://perldoc.perl.org/functions/state.html|state].
</p>
<blockquote><em>
"Does this code '{$a <=> $b}' work with only numbers?"
</em></blockquote>
<p>
You can reverse the order of the sort by swapping <c>$a</c> and <c>$b</c>:
</p>
<code>
$ perl -Mstrict -Mwarnings -E '
my @numbers = (2, 1, 3);
my @strings = qw{B A C};
say sort { $a <=> $b } @numbers;
say sort { $b <=> $a } @numbers;
say sort { $a cmp $b } @strings;
say sort { $b cmp $a } @strings;
'
123
321
ABC
CBA
</code>
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<div class="pmsig"><div class="pmsig-861371">
<p>-- Ken</p>
</div></div>
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