Hi Lee,
On Fri, 28 Jun 2013 15:15:27 +0200
lee <l...@yun.yagibdah.de> wrote:
> timothy adigun <2teezp...@gmail.com> writes:
>
> >>>> use strict;
> >>>> use warnings;
> >>>>
> >>>>
> >>>> sub test {
> >>>> print $counter . "\n";
> >>>> }
> >>>>
> >>>>
> >>>> my $counter = 0;
> >>>> while($counter < 5) {
> >>>> test();
> >>>> $counter++;
> >>>> }
> >>>>
> >>>>
> >>>> It says "Global symbol "$counter" requires explicit package name ...".
> >>>> When I put the subroutine after the 'while' loop, it works just fine, so
> >>>> what's the problem?
> >>>
> >>> The subroutine does not see the lexical $counter variable because it was
> >>> declared after its scope.
> >>
> >> The subroutine is never called before $counter is declared, so it is
> >> always available to the subroutine. There should be an error only for
> >> instances when the subroutine is called before $counter is declared.
> >>
> >>> For more information, see:
> >>>
> >>> * http://www.plover.com/FAQs/Namespaces.html
> >>
> >> Ok, so perl has a totally broken design with variables :( What's the
> > I don't think so. I think you are not really getting perl and Perl.
> > How would you this in another language say C or C++?
>
> I would either declare global variables or, much better, make a data
> structure a pointer to which can be handed over to functions easily and
> efficiently.
>
> Declaring the variables on top didn't seem a good idea in perl, but
> re-reading the documentation it seems that they might never be
> accessible from outside the program when declared with 'my'. If that is
> so, declaring them at the top wouldn't be different from declaring
> global variables in C.
>
> >> solution to this problem?
> > Below is one way of solving this problem:
> > <CODE>
> > use strict;
> > use warnings;
> >
> > sub test {
> > print shift(@_) . "\n";
> > }
> >
> >
> > my $counter = 0;
> > while($counter < 5) {
> > test($counter); # pass the variable
> > $counter++;
> > }
> > </CODE>
> >
> > You can write your test subroutine like this:
> > <CODE>
> > sub test{
> > my $count = shift(@_);
> > print $count,$/;
> > }
> > </CODE>
> > I would of course not call the subroutine test, using a more
> > descriptive name would be better.
> > Hope this helps.
>
> Passing a number of variables around to every function that might need
> them is what I'm trying to avoid. When you later modify one of the
> functions and it needs more variables, you have to go through all
> functions that call the modified function, modify the all the calls of
> the modified function and add these variables to the calling functions
> as well just so they can pass them over.
>
> When you pass a pointer to a data structure around, you have only one
> place to modify (besides modifying the function), which is the
> declaration of the data structure. (And you can get away without global
> variables.)
>
> What's the equivalent to passing around a pointer in perl? You could
> pass a hash. Doing that would create a copy of the hash every time it's
> handed over, wouldn't it? Passing just a pointer is far more beautiful
> and doesn't involve the overhead of creating copies.
You can either pass a reference to a hash (which will cause the hash not to be
copied) or better yet - look into object-oriented programming in Perl:
http://perl-begin.org/topics/object-oriented/
(= a link to my site.)
Passing an object allows you to pass a single scalar which holds several
fields/slots/attributes/member variables/etc.
Regards,
Shlomi Fish
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