http://www.perlmonks.org?node_id=114561

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

My program looks like this.

....
$var1 ...
$var2...
$var3 ...

sub get_vars { 
     local (*variables) = *main::;
    
     #-- Some checking on certain variables
}

[download]

When I call get_vars before defining $var1, $var2, $var3, then %variables does not contain them.
When I call get_vars after defining $var1, $var2, $var3 then %variables has them. Isn't *variables reading *main:: at compile time. What am I missing?

Thanks

Replies are listed 'Best First'.
Re: using symbol table lookup
by dragonchild (Archbishop) on Sep 25, 2001 at 20:04 UTC
    What on earth do you want to fiddle with the symbol table for?? It's dangerous, messy, and there's better ways to do it.

    As for your questions, *variables is bound to *main:: when you bind it. Hence, if the variable hasn't been created yet, it's not in *main::. Compilation doesn't populate anything. All it does is convert the script to opcodes. When those opcodes execute, that's when the SV's are created, and a reference entered into *main::.

    ------
    We are the carpenters and bricklayers of the Information Age.

    Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

[reply]
      My purpose is that my perl scripts read many text files
      So rather than checking each one in each script, I thought
      I would always make my filename variables always as
      "something_file". And have a standard subroutine that checks
      all %main:: scalar variables  =~ /\_file/ and do the checks
      on each entry (Like file exists, file is readable etc.)
      .
[reply]
[d/l]
        Use Exporter, or write a subroutine in each program that returns the filename, or a hash reference of values.

        Leave symbol-table mucking about for when there's really no other way to do that (basically run-time code generation).

[reply]

        Rather odd way to do it I would have thought. Is there something inherently wrong with this approach: 

        my $dir = 'c:/';
opendir DIR, $dir or die "Oops cna't open $dir $!\n";
my @files = grep { /\.pl$/ } readdir DIR;
close DIR;
$_ = $dir.$_ for @files; # add full path

print "Here are all the files:\n";
print "$_\n" for @files;

        [download]

        This finds all the files in my root dir which end in .pl but I could use the regex in the grep to match anything. I don't see any reason to mess with the symbol table. Just wrap this sort of code in a sub that you can call from wherever....

        cheers

        tachyon

        s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

[reply]
[d/l]

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