4.6. Private Variables in SubroutinesBut if Perl can give us a new @_ for every invocation, can't it give us variables for our own use as well? Of course it can. By default, all variables in Perl are global variables; that is, they are accessable from every part of the program. But you can create private variables called lexical variables at any time with the my operator: sub max { my($a, $b); # new, private variables for this block ($a, $b) = @_; # give names to the parameters if ($a > $b) { $a } else { $b } } These variables are private (or scoped) to the enclosing block; any other $a or $b is totally unaffected by these two. And that goes the other way, too -- no other code can access or modify these private variables, by accident or design.[108] So, we could drop this subroutine into any Perl program in the world and know that we wouldn't mess up that program's $a and $b (if any).[109]
It's also worth pointing out that, inside the if's blocks, there's no semicolon needed after the return value expression. Although Perl allows for the last semicolon in a block to be omitted, in practice that's omitted only when the code is so simple that the block is written in a single line, like the previous ones. The subroutine in the previous example could be made even simpler. Did you notice that the list ($a, $b) was written twice? That my operator can also be applied to a list of variables enclosed in parentheses, so it's more customary to combine those first two statements in the subroutine: my($a, $b) = @_; # Name the subroutine parameters That one statement creates the private variables and sets their values, so the first parameter now has the easier-to-use name $a and the second has $b. Nearly every subroutine will start with a line much like that one, naming its parameters. When you see that line, you'll know that the subroutine expects two scalar parameters, which we'll call $a and $b inside the subroutine. Copyright © 2002 O'Reilly & Associates. All rights reserved. |
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