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Hello Monastery Dwellers,

I am a new Perl convert, still trying to master the basics of the language. Coming from Python, the concept of object references has been giving me a bit of trouble and as such, I've come to ask for some clarification.

I've started out by reading an XML file using XML::Simple, but have been urged to move into XML::LibXML.

My goal is to iterate through the loaded hash and remove key/value pairs from each hash within the array where key names are listed in a pre-defined array.

This is a sample of the XML that I'm loading

<?xml version='1.0' encoding='UTF-8' standalone='yes' ?>
<smses count="4110">
  <sms protocol="0" address="1234567890" date="1288032888762" type="1"
+ subject="null" body="Hey, I'm out of class." toa="null" sc_toa="null
+" service_center="null" read="1" status="-1" locked="0" />
  <sms protocol="0" address="0987654321" date="1288032888762" type="1"
+ subject="null" body="What are you up to this afternoon?" toa="null" 
+sc_toa="null" service_center="null" read="1" status="-1" locked="0" /
+>
</smses>
[download]

The array (via Dumper) looks like:

#$VAR1 = {
#          'count' => '5509',
#          'sms' => [
#                   {
#                     'protocol' => '0',
#                     'locked' => '0',
#                     'status' => '0',
#                     'date' => '1288194026703',
#                     'subject' => 'null',
#                     'toa' => 'null',
#                     'sc_toa' => 'null',
#                     'body' => 'You should come to dinner with us! :)
+',
#                     'read' => '1',
#                     'address' => '(123) 456-7890',
#                     'type' => '2',
#                     'service_center' => 'null'
#                   },
#                   {
#                     'protocol' => '0',
#                     'locked' => '0',
#                     'status' => '64',
#                     'date' => '1288224316833',
#                     'subject' => 'null',
#                     'toa' => 'null',
#                     'sc_toa' => 'null',
#                     'body' => 'INVADER ZIM!!',
#                     'read' => '1',
#                     'address' => '0987654321',
#                     'type' => '2',
#                     'service_center' => 'null'
#                   },
[download]

Ignoring the actual values of the keys, the above XML loads into a hashref that looks like above Dumper'd output. What I'm trying to do is iterate through all of the Hash's in the sms array. Within each hash, all I'm trying to do is delete key pairs where the key nam does NOT reside in a pre-defined list (@keysToKeep).

#! /usr/bin/env perl

use XML::Simple;
use Data::Dumper;
use strict;
use warnings;

########
###########  Variables
########

# List of info we want for each message entry
my @keysToKeep                  =               ('date', 'body', 'addr
+ess', 'type');

# XML Parser
my $xml                         =               new XML::Simple;


########
###########  Subroutines
########

# Filepath is passed to load a hashref
# a CLEANED Hashref is returned of the xml we've loaded
sub loadFileToHashRef
{
        my $xmlRef      =       $xml->XMLin($_[0]);

        # Clean our hash
        for my $textRef ( @{$xmlRef->{sms}} )
        {   
                foreach my $key (keys %{$textRef})
                {   
                        # If your $key is *not* what we're looking for
+, remove it from the hash
                        if ( !grep ($key, @keysToKeep) )
                        {   
                                delete $textRef->{$key};
                        }   
                }   
        }   

        # Return our hash ref
        return $xmlRef;
}

my $hashRef = loadFileToHashRef("xml/sms-20110704030000.xml");
print Dumper $hashRef;
[download]

Now, this code doesn't remove elements from the hash and I know why. Using keys only returns a list of the pairs that I'm modifying. I'm not actually modifying the hashref. My question becomes this: How do I modify the hashref (%hashRef) directly?

Given that the above doesn't do what I'm trying to do, I started working with 'each'. Here's what I came up with.

#! /usr/bin/env perl

use XML::Simple;
use Data::Dumper;
use strict;
use warnings;
use 5.012;

########
###########  Variables
########
# List of info we want for each message entry
my @keysToKeep                  =               ('date', 'body', 'addr
+ess', 'type');

# XML Parser
my $xml                         =               new XML::Simple;


########
###########  Subroutines
########

# Filepath is passed to load a hashref
# a CLEANED Hashref is returned of the xml we've loaded
sub loadFileToHashRef
{
        my $xmlRef      =       $xml->XMLin($_[0]);

        while (($key, $value) = each %{$xmlRef})
        {   
                print "Key: $key, Value: $value \n"; 
        }   

        # Return our hash ref
        return $xmlRef;
}

my $hashRef = loadFileToHashRef("xml/sms-20110704030000.xml");
print Dumper $hashRef;
[download]

However, I continually get the following, and I don't understand why.

Global symbol "$key" requires explicit package name at ./parse.pl line
+ 29.
Global symbol "$value" requires explicit package name at ./parse.pl li
+ne 29.
Global symbol "$key" requires explicit package name at ./parse.pl line
+ 31.
Global symbol "$value" requires explicit package name at ./parse.pl li
+ne 31.
Execution of ./parse.pl aborted due to compilation errors.
[download]

I also now know that this won't do what I'm trying to do either as the each function doesn't allow for element removal from the hash.

I highly suspect that my PHP and Python ideologies are getting in the way of my understanding of how Perl handles Hashes and Arrays (as it pertains to references). Ideally, I'm looking for the following feedback

The "right" way to accomplish what I'm trying to do (I realize this is subjective to a degree)
How do I accomplish this in the most resource efficient way possible?
How to tell when you'll be working with a reference or a literal (eg: SimpleXML returns a reference, but I assume other Libraries/Functions wont?)

-- Libra

In reply to Parsing an Array of Hashes, Modifying Key/Value Pairs by librarat

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