zeltus has asked for the wisdom of the Perl Monks concerning the following question:

I really ought to be able to answer this for myself but it's turning out to be a long week and what's left of my brain is half-fried already.

Anyway, all I want to do is declare a global variable in the main part of my programand then manipulate it within subroutines.

This fragment shows what I am trying to do. But it fails when I try to increment the value withing the sub "modification of a read-only value..."

#/usr/bin/perl
$tt = 1;
mt($tt);

sub mt {

  my $t = shift;
  print " sub: 1 t = ${$t}\n";
  ${$timer}++;
  print " sub: 2 t = ${$t}\n";

}

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Once I have understood (with someone's help, hopefully) what I need to do to make this work, I'd like to ensure it works with "use warnings, strict" and with sub prototyping... and anything else I can think of the keep it under some sort of control.

As I said, I should be able to do this by now. I have worked on PHP code for too long I guess!

Thanks for any help

Bill

Update

Whoopsy! Yes, I did mean $t throughout. This is just the simplest code I could write off the fly to try and demonstrate what I am trying to do. I always use strict and warnings in my "proper" code - this is just some QnD to demo my problemette.

The reason I can't use it as a "real" global and deal with it directly in the subroutine is that I have x number or variables to maintain with values, not just the one in this example. And I do like to use subroutine prototyping

But thanks for the replies... I can now see where I was screwing up and I am happier now I understnad why I was screwing up. Still not entirely sure this is The Right Way To Program tho'! :-)

Bill

Replies are listed 'Best First'.
Re: global, write-able vars
by BrowserUk (Patriarch) on Oct 05, 2011 at 13:36 UTC

    Quite how you think that  ${$timer}++; is going to work when the local var is called $t and the global var $tt?

    The first thing you need to do is get (back?) into the habit of enabling strict & warnings.

    The (one) way to achieve what you want is to pass a reference to the variable:

    #/usr/bin/perl
$tt = 1;
mt( \$tt );

sub mt {

  my $t = shift;
  print " sub: 1 t = ${$t}\n";
  ${ $t }++;
  print " sub: 2 t = ${$t}\n";

}

__END__
C:\test>junk55
 sub: 1 t = 1
 sub: 2 t = 2

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    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
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Re: global, write-able vars
by keszler (Priest) on Oct 05, 2011 at 13:58 UTC

    I don't recommend this, but if you purposely use a global variable why bother passing it to a subroutine? Global is global.

    use strict;
use warnings;
my $global = 1;
mt();
print "C: global still = $global\n";

sub mt {
  print "A: global=$global\n";
  $global++;
  print "B: global=$global\n";
}
__END__

A: global=1
B: global=2
C: global still = 2

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Re: global, write-able vars
by bart (Canon) on Oct 05, 2011 at 15:26 UTC
    @_ contaisn aliases to the parameters you pass, so if you work directly on $_[0], yo ucan change its value. It's the 
    my $t = shift;

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    that makes a copy.

    There are several ways to make a variable alias to an existing value, but, shame on Perl, none are quite what I'd call "easy". For example: 

        *t = \($_[0]);

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    which uses a global variable $t. With local you can restrict the damage, but it'll still not pass strict — you still need to declare $t – but not using "my"!

    another way is using for in a non-obious way (for can make a loop variable that is an alias for each item it iterates over, and with a scalar it will loop exactly one): 

    for my $t ($_[0]) {
    # ...  the rest of the sub body
}

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    There is even a few Alias modules on CPAN, the most interesting one always looked to me to be Data::Alias but I must confess I've never used it. (I tried to, long time ago, but it wouldn't compile back then.) Actually, looking at it now, I think Lexical::Alias might be more interesting — if it works.

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