perl help troubleshooting error msg using perl module

Question

I am testing perl script using this example found on the forum. I took the sample data and put it in a separate file and using an open to try it this way.

I don't understand why I would be getting undef. I am getting the msg:

Use of uninitialized value in join or string at ./h.pl line 15, line 4.

#!/usr/local/bin/perl

use warnings;
use strict;
use Data::Dumper;
use Text::CSV_XS;

    open my $fh, '<', 't.out' or die "Unable to open: $!";
    my $csv = Text::CSV_XS->new( { sep_char => "\t" } );
    my @list;
    $csv->column_names ($csv->getline ($fh));
    while ( my $hr = $csv->getline_hr($fh) ) {
        push @list, $hr->{'Ball'};
    }

    print "@list\n";
    print Dumper(\@list);

Test file (t.out)

Camera Make Camera Model    Text    Ball    Swing
a   b   c   d   e
f   g   h   i   j
k   l   m   n   o


$ od -cx t.out 
0000000   C   a   m   e   r   a       M   a   k   e  \t   C   a   m   e
        6143 656d 6172 4d20 6b61 0965 6143 656d
0000020   r   a       M   o   d   e   l  \t   T   e   x   t  \t   B   a
        6172 4d20 646f 6c65 5409 7865 0974 6142
0000040   l   l  \t   S   w   i   n   g  \n   a  \t   b  \t   c  \t   d
        6c6c 5309 6977 676e 610a 6209 6309 6409
0000060  \t   e  \n   f  \t   g  \t   h  \t   i  \t   j  \n   k  \t   l
        6509 660a 6709 6809 6909 6a09 6b0a 6c09
0000100  \t   m  \t   n  \t   o  \n  \0
        6d09 6e09 6f09 000a
0000107

Results:

$VAR1 = [
          undef,
          undef,
          undef
        ];

Question is far from clear. As you see from the comments below, the question asked isn't the one the questioner wanted an answer to. The user is really asking What I don't understand is that I was testing this same code where I got it to work. As soon as I converted it to use a file, its not working anymore. I don't understand why. I put the _DATA_ in a test file and that's when I am getting undef.. — David W.
– David W., Commented Aug 18, 2011 at 17:33

Adam Batkin · Accepted Answer · 2011-08-18 15:24:14Z

2

If $csv->getline_hr($fh) returns an undefined value, you will get the warning "Use of uninitialized value in join or string at ..." if you try to print the list @list.

You can test this explicitly with a simple program:

use strict;
use warnings;
my @list;
my $x = undef; # This is similar to what may happen in your program on the `push @list, @hr->{'Name'}` line
push @list, $x;
print "@list\n";

Also, you may want to use Data::Dumper and then print Dumper(\@list) instead if you are just doing debugging, since you won't get any warnings, and you will get a clearer picture of what is stored in your data structure.

edited Aug 18, 2011 at 15:24

answered Aug 18, 2011 at 15:16

Adam Batkin

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TLP · Accepted Answer · 2011-08-18 22:34:01Z

1

In the question you refer to, I also wrote this on the last line:

ETA: If you're going to cut & paste to try it out, make sure that the tabs are carried over in the data.

Did you make sure that your tabs survived the copy/paste into your file 't.out'?

This module is rather unforgiving about bad formats in the data file.

Update:

Be aware that a field such as 'Ball ' will be considered different than 'Ball'. I.e. if you have additional whitespace, it will screw things up.

What you may try out to correct that is adding allow_whitespace => 1, to the options of the $csv object. This will correct any subtle whitespace errors in your input file.

You can also check for format errors by printing the header keys with

print Dumper $csv->column_names();

edited Aug 18, 2011 at 22:34

answered Aug 18, 2011 at 18:38

TLP

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5 Comments

cjd143SD Over a year ago

yes, i did see that. I did make sure that output file was using tabs. will double check again. i was using your same exact test data too. thanks

cjd143SD Over a year ago

i posted my results from using that utility to check on tabs. its tabbed correctly.

TLP Over a year ago

I have updated my answer. You can also check for format errors by printing the header keys with print Dumper $csv->column_names();

TLP Over a year ago

Doesn't look like you have any bad characters in your file. And yet there must be. All I can recommend is that you debug with Data::Dumper. The script works fine on my computer, even when using a filehandle instead of *DATA.

cjd143SD Over a year ago

thanks for working with me through this. Got it working now and using another test file. The use of Dumper and printing out the contents helped.

Community · Accepted Answer · 2017-05-23 11:47:56Z

As others have mentioned, it is very possible that @list never got a value in it. If $csv->getline_hr($fh) returns an undef the first time through, you'll never push anything into @list.

There are two things you can do:

You can test if @list is uninitialized before printing it
You can loosen the use warnings pragma not to include uninitialized warnings:

Here's how you can do your testing:

if (@list) {
   print "List = " . @list . "\n";
} 
else {
   print "List has no values\n";
}

You can use scalar @list as Igor Zinov'yev shows, but that's not really necessary in this case and I think the simple if (@list) is clearer. You can also use if (defined @list) which is very clear, but has been discouraged over the last few years.

If this is a temporary thing (just seeing if your code works or not), you can loosen the use of strict:

no warnings qw(uninitialized);
print "@list\n";   #Now you won't get a warning if @list has no value
use warnings;      #Turn the warnings back on. It helps you code better

A slight Word 'o Warning: In this case, your statement won't print anything and that might confuse you even more. Always put a prompt of some sort to make sure something prints:

print "\@list = @list\n";

I prefer this as a temporary solution when I'm printing out a lot of debugging statements which I'll later remove. In fact, I'll create a special debug subroutine for these types of messages:

use strict;
use warnings;
use constant DEBUG_LEVEL => 1;  #0 = no debug messages 1 = debug messages

[...]
while (yadda, yadda, yadda) {
   push @list, $hr->{Name};
}
debug (qq(\@list = ") . join "|", @list . qq("));

[...]

sub debug {
   my $message = shift;

   return if (not DEBUG_LEVEL);
   no warnings qw(uninitialized);
   print qq(DEBUG: $message\n);
   use warnings;      #Not really needed since the code block ends
   return $message;
}

Then ask THAT question! Edit your question to make it clear what your issue really is and give us a bit more code to work with. Did you open the file? Are you sure you're reading in the file? Show us the part of the code where you're doing that.

Igor Zinov'yev · Accepted Answer · 2011-08-18 15:16:07Z

0

It seems like your @list array is undefined. Try checking it:

if (scalar @list) {
    print "@list\n";
}

If your while loop has never been executed, scalar @list will return 0, because the array is empty.

answered Aug 18, 2011 at 15:16

Igor Zinov'yev

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