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

Newbie here again ...


I get the "Global symbol "$counter" requires explicit package name" error with this script, and no idea why! What concept have I missed? Thanks, Jim

#!/usr/bin/perl

use strict;
use warnings;

$counter = 0;

use vars qw($counter);

open (INPUT, "< data")
     or die "Couldn't open data file for reading: $!\n";

while(<INPUT>) {
  my @a = split;               # split record into fields
  $counter++;                  #increment counter
  print "Value($counter) = $_\n" for @a; # print out all fields
  print "\n";                  # blank line between "records"
}

close (INPUT);

[download]
jamaas btinternet com

janitored by ybiC: Closed unbalanced <code> tag and removed <br> tags from codeblock

Replies are listed 'Best First'.
Re: perl doesn't like variable
by pg (Canon) on Oct 03, 2004 at 15:17 UTC

    By the way, there was no need to use $counter at all. Perl has a variable $. which tracks the counter of the last accessed file handler. Have more than one file open? okay, then use FOO->input_line_number(). Play with the following code (save this piece of code as test.pl):

    use IO::Handle;
open(MYSELF1, "<", "test.pl");
open(MYSELF2, "<", "test.pl");
while (<MYSELF1>) {
    print "after reading from myself1: $.\n";
    if ($. % 2) {
        <MYSELF2>;
        print "after reading from myself2: $.\n";
    }
    print "returned values from input_line_number: (";
    print MYSELF1->input_line_number(), ", ";
    print MYSELF2->input_line_number(), ")\n";
}
close(MYSELF1);
close(MYSELF2);

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Re: perl doesn't like variable
by Eimi Metamorphoumai (Deacon) on Oct 03, 2004 at 14:11 UTC
    I think the problem is that you're using $counter before the use vars qw($counter) line. That line basically says "Henceforth, $counter refers to the global variable $MAIN::counter". If you refer to it before that, you get the same error as if you didn't have the use vars at all.
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Re: perl doesn't like variable
by Dietz (Curate) on Oct 03, 2004 at 14:12 UTC
    Under use strict, your $counter is initialized before it is declared:
    $counter = 0;
use vars qw($counter);

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    So you have to declare it (outside of while(<INPUT>)):
    my $counter = 0;

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    Note, that you don't need use vars qw($counter); afterwards.
    Otherwise you would declare just another variable called $counter living in the symbol table:
    my $counter = 0; # declare and initialize lexical variable
use vars qw($counter); # declare global variable living in symbol tabl
+e
                                                                      
+                                                                     
+   
$counter = 0; # initialize again, just to show that $counter in 'use v
+ars' is not affected

print "\$main::counter: $main::counter\n"; # $counter living in symbol
+ table: not initialized
$main::counter = 1; # initialize $counter in symbol table
print "\$main::counter: $main::counter\n"; # prints 1
print "\$counter: $counter\n"; # prints 0

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Re: perl doesn't like variable
by jbware (Chaplain) on Oct 03, 2004 at 13:47 UTC
    You need to declare the variable before usage. Try: 
    my $counter = 0;
# (comment-out added): use vars qw($counter);

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    or (From davido) 
    use vars qw($counter);
$counter = 0;

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    -jbWare

    Update: Per davido's comment, I've modified my answer.
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      You need to declare the variable. Try:

      my $counter = 0;

      Why? He is properly declaring his variable to be a package global, and that is perfectly legit under strictures. It would be a mistake to try to also declare it to be a lexical variable, as that would create two separate variables.

      His problem is that he initialized $counter before declaring it. Here is his code: 

      $counter = 0;
use vars qw($counter);

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      The proper sequence of events is to declare the variable, and then start using it (or initialize it), like this: 

      use vars qw($counter);
$counter = 0;

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      HTH.


      Dave

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        Good catch. Chalk that up to tunnel vision on my part. I saw the usage w/o declaration and didn't even look any further.

        -jbWare
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      Thanks!


      I did that and now it does not increment within the loop.
      Do I need to declare it somewhere within the block?
      Thanks
      Jim

      jamaas btinternet com
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Re: perl doesn't like variable
by johnnywang (Priest) on Oct 03, 2004 at 18:50 UTC
    You can also use "our": 
    our $counter = 0;

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