Swapping xml elements from an external file?

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

Hello Monks, This is my first foray into using xml and perl together. I found a thread about swapping xml elements, but unlike that thread, I want to swap them out from an external file. Here is what I have so far: first, an xml file with "head" elements that I want to tag up. So, I run this:

#!/usr/bin/perl -w

use XML::Twig;

my $file = $ARGV[0];
my $twig= new XML::Twig(TwigRoots => {head => 1});
$twig->parsefile($file);
$twig->print;
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which scrapes out all and only the head elements. Then I write it all to a file from the command line. That file looks something like this:

<TEI>
 <head>Note VII</head>
 <head>Title Page </head>
 <head>Copyright Page </head>
 <head>Preface </head>
 ...
</TEI>
[download]

Next, I retag the head elements in a text editor. Specifically, I find all roman numerals, and tag them with an xml element. I know I could do this in the script, but in this case, I want to do it by hand for oversight. So, I end up with a list of head elements, tagged as I want them, looking something like this:

<TEI>
 <head>Note <romanNumeral>VII</romanNumeral></head>
 <head>Title Page </head>
 <head>Copyright Page </head>
 <head>Preface </head>
 ...
</TEI>
[download]

Now, I want to go back through the file I extracted the head elements from, and replace the head tags there with the head tags I've extracted and tagged up in the external file. How would I do that? Thanks in advance for your help!

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Re: Swapping xml elements from an external file?
by Fletch (Bishop) on Oct 06, 2008 at 22:02 UTC

Kludgy, but as the problem's presented (and presuming there's a 1-to-1 mapping from original head elements to marked up head elements):

Create a hash mapping from original to extra-~~crispy~~markup head elements (presuming they're small just treat them as two text files and read them line-by-line in parallel)
Parse the original file and use XML::Twig to walk all the head elements, replacing the contents by looking up the original value in your hash and retrieving the possibly modified version
Profit!

The cake is a lie.
The cake is a lie.
The cake is a lie.

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Re: Swapping xml elements from an external file?
by Jenda (Abbot) on Oct 06, 2008 at 22:28 UTC

I'll assume the big file was not modified and it still contains all those head tags in the same order! First you'll need to load all the heads from the small file:

use strict;
use XML::Rules;

my $reader = XML::Rules->new(
    rules => {
        _default => 'raw',
        head => 'content array',
        TEI => 'pass no content',
    }
);

my $heads = $reader->parsefile( $small_file_name )->{head};
#use Data::Dumper;
#print Dumper($heads);
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my $updater = XML::Rules->new(
    style => 'filter',
    rules => {
        _default => 'raw',
        head => sub {
            my ($tag, $attr) = @_;
            $attr->{_content} = shift(@$heads);
            return $tag => $attr;
        }
    }
);

$updater->filterfile( $big_file_name, $updated_big_file_name);
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Jenda
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Defend the free world!

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Re: Swapping xml elements from an external file?
by GrandFather (Saint) on Oct 06, 2008 at 22:49 UTC

There is a foible in the following code that may be an XML::Twig bug, but aside from that the following may help:

use strict;
use warnings;
use XML::Twig;

my $org = <<XML;
<TEI>
 <head>Note VII</head>
 <head>Title Page </head>
 <head>Note VII</head>
 <head>Preface </head>
 <head>Copyright Page </head>
</TEI>
XML

my $mod = <<XML;
<TEI>
 <head>Note <romanNumeral>VII</romanNumeral></head>
 <head>Title Page </head>
 <head>Copyright Page </head>
 <head>Preface </head>
</TEI>
XML

my $twig = XML::Twig->new (twig_roots => {'head' => \&fetchNode,});
my %subs;

$twig->parse ($mod);
$twig->purge ();

$twig = XML::Twig->new (
    twig_roots               => {'head' => \&editNode,},
    twig_print_outside_roots => 1
);

$twig->parse ($org);
$twig->flush ();


sub fetchNode {
    my ($t, $elt) = @_;
    my $text = $elt->text ();

    $subs{$text} = $elt;
    $t->purge ();
}


sub editNode {
    my ($t, $elt) = @_;
    my $text = $elt->text ();

    $subs{$text}->replace ($elt);
    $t->flush      ();
}
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Prints:

<TEI>
 <head>Note <romanNumeral>VII</romanNumeral></head>
 <head>Title Page </head>
 <head>Note <romanNumeral>VII</romanNumeral></head>
 <head>Preface </head>
 <head>Copyright Page </head>
</TEI>
</TEI>
[download]

Note the ~~nested root node~~repeated close tag! This code doesn't care if the order of the nodes has changed or if nodes have been added in the main document, but that probably doesn't matter.

Update updating XML::Twig to 3.32 produces a slightly different result which can be corrected by removing the flush following $twig->parse ($org);.

Perl reduces RSI - it saves typing

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