Hi,
Im just learning Perl and was a little confused with why I couldnt prevent my subroutines from reading variables from the main program. Here is an example:
use strict;
my $var = 1;
test();
sub test { print $var; }
I had thought that a my variable would not be able to be read by the subroutine, but it can still see it. So my question is how can I prevent subroutines from having access to variables in the main program. Thanks for any help,
Common mistake. 'my' gives a variable lexical scoping, which basically defines where the variable is visible. So in a sense it appears as a 'main' variable only because your subroutine and the my'ed variable are in the same file. If you were to move your subroutine into a library file and then call the sub it would not be visible in the sub. This is one reason why people often prefer to put all subs in a library and have only a main in the script. It is also why sometimes you will see a variable's scope specifically controlled by braces, though in this case it will not help, here is an example:
{
my $count;
foreach my $file (@files) {
# do something to file
$count++;
}
}$count is not visible outside the braces even in the same file. To create a truly 'main' variable (which you don't want to do *usually*) you would need to put that variable in the package list for the 'main' by using an 'our' in the main package. Then the variable would be accessible with $main::, but your $var is not, so it is not, strictly speaking, a 'main' or global.
Excellent reading on the scoping of variables in Perl can be found here:
http://perl.plover.com/FAQs/Namespaces.html
http://danconia.org
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