Problems with references using GD::Graph::Data

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

I am doing something improperly with references, but I can't see it! This is the error I get.

Can't call method "gif" on an undefined value at test2.pl line 80.
That line is 3rd from the bottom on the snippet below.
this is the piece of code that generates the above error:

   my $data = GD::Graph::Data->new();
   my $graph = GD::Graph::bars->new();
   my $format = $graph->export_format;
   if ( $dbh = DBI->connect("DBI:ODBC:$INI{DSN}",$INI{DBuser},$INI{DBp
+ass}) ) { 
      my $gotdata = 0;
      my $stmt = 'SELECT SysID FROM MeterEvent WHERE Sysid >= 2 and Sy
+sid < = 10';
      if ( my $sth = $dbh->prepare($stmt) ) { 
         if ( my $rows = $sth->execute ) { 
            while (my @row = $sth->fetchrow_array) {
               $data->add_point(@row);
               $gotdata = 1;
            }
         } 
      } 
      $dbh->disconnect(); 
   } else {
    print "Database Error$br\n";
   }
   my $gd = $graph->plot($data);
   open(IMG, '>file.gif') or die $!;
   binmode IMG;
   print IMG $gd->gif;  # this is the line # 80
   if ( defined $gd ) { print " 4.gd defined\n"; } else {print " 4.gd 
+NOT defined\n"; }
   close IMG;
[download]

The GD::Graph::Data manual states the following in the examples section

  use GD::Graph::Data; 
  use GD::Graph::bars; 
  my $data = GD::Graph::Data->new(); 
  $data->read(file => '/data/sales.dat', delimiter => ','); 
  $data = $data->copy(wanted => [2, 4, 5]); 
  # Add the newer figures from the database 
  use DBI; 
  # do DBI things, like connecting to the database, statement 
  # preparation and execution 
  while (@row = $sth->fetchrow_array) 
  { 
          $data->add_point(@row); 
  } 
  my $chart = GD::Graph::bars->new(); 
  my $gd = $chart->plot($data);
[download]

Thanks for your help!

Comment on Problems with references using GD::Graph::Data Select or Download Code

Replies are listed 'Best First'.
Re: Problems with references using GD::Graph::Data by buckaduck (Chaplain) on Apr 01, 2002 at 18:56 UTC
Presumably, `$gd` is undefined. I'm guessing that the GD::Graph::bars module is unhappy with the fact that you're trying to create a graph out of a one-dimensional set of data. At least your data looks one-dimensional to me, based on the SQL query that you're using. You're adding a series of X values with no associated Y values. What sort of graph are you hoping to produce from this? Now if you included some Y values, maybe things would be different... buckaduck	[reply] [d/l]
Re: Re: Problems with references using GD::Graph::Data by gary kuipers (Beadle) on Apr 01, 2002 at 22:09 UTC
You are right! When I use X and Y values ( by saying SELECT SysID, SysID FROM ..." the graph prints without problems. I made a bad assumption ... that the module would provide a sequential X value. Not so! Anyway, thanks!	[reply]
Re: Problems with references using GD::Graph::Data by metadoktor (Hermit) on Apr 01, 2002 at 18:49 UTC
I assume that you're using warnings and use strict. Are you using? `use diagnostics` [download] Also perhaps you might want to rearrange some of your code so that: `print IMG $gd->gif; if (defined $gd)` [download] becomes `if (defined $gd) { print IMG $gd->gif; } else { ... }` [download] conditional. metadoktor "The doktor is in."	[reply] [d/l] [select]
Re: Problems with references using GD::Graph::Data by mla (Beadle) on Apr 01, 2002 at 20:37 UTC
You need to add more error checking. Often the man pages are very terse and skip it, but it becomes really valuable once you begin writing code of any significant length. In this case, a quick look at the GD::Graph::Data POD suggests that calling the error() method might tell you what went wrong. Lots of additional error info is discussed in the GD::Graph::Error POD, including the $GD::Graph::Error::Debug variable which might also give you a better hint. And use RaiseError on your DBI handle! See the DBI docs if you aren't familiar with it. It makes your error handling both simpler and more robust since you don't have to add error checks, and you don't have to remember to add error checks.	[reply]