On 2/23/09 Mon Feb 23, 2009 4:30 PM, "Karyn Stump" <ka...@calarts.edu>
scribbled:
> I am trying to learn subroutines. I have seen refernces to using my and
> local for passing variables so I am playing with them to try to understand
> better how this all works.
Neither 'local' nor 'my' is required for passing arguments to subroutines.
'my' is often used inside a subroutine to "localize" the scope of variables
used within the subroutine, but 'local' is rarely used for that purpose or
any other purpose.
Aliases to values (variables, constants, or expressions) in the calling
routine are passed in the built-in @_ array.
Your sample program contains two characteristics that indicate it was
written a long time ago and should be re-written: the use of 'local' to
declare local variables, and calling subroutines with an ampersand '&'. For
modern Perls, it is usually better to localize the scope of variables with
'my' and to call subroutines without using ampersands.
Check 'perldoc perlsub' for how to call subroutines and pass arguments to
them, and how the ampersand can affect the behavior of your program.
>
> I have a subroutine in the script below, printsub that errors when it is
> run with
>
> Global symbol "$name" requires explicit package name at ./user-sub.pl line
> 143.
> Global symbol "$name" requires explicit package name at ./user-sub.pl line
> 144.
> Execution of ./user-sub.pl aborted due to compilation error
>
> This is the code pulled out of easy viewing:
>
> sub printsub
> {
> local($name) = $_[0];
> print "This is the value of $name in the subroutine.\n";
> }
This syntax is very old and dates from Perl 4. The error message is because
you have the very modern 'use strict' in your script, but are using the very
old 'local'. Replace with:
my $name = $_[0];
and all will be fine. You have also found the equivalent below:
>
> The second sub in this script works using:
>
> my $name = shift(@_);
>
> another example I found online.
Either of those two forms is how subroutines arguments are passed and
accessed in subroutines. The first leaves @_ untouched, while the second
removes the first member from @_. You can also use members of @_ directly,
but you should be aware that the members are aliases, and if you change a
value of a member aliased to a variable in the calling routine, the value in
the calling routine will also be changed.
> I would appreciate some help understanding if or how local can be used in
> this context. Also if there is a better/preferred way to pass this var to
> the sub.
'local' declares a new value for a package (global) variable. When the
declaration goes out of scope, the old value is restored. 'local' is not
needed to pass arguments to subroutines, and it is usually better to limit
the scope using 'my', rather than 'local'. 'my' declares a lexically-scoped
variable. The difference is that the new value declared by 'local' will be
available to subroutines called by the local-declaring subroutine, while
variables declared with 'my' will not be accessible by other subroutines at
all.
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