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Template::Tutorial::Datafile(3)User Contributed Perl DocumentationTemplate::Tutorial::Datafile(3)

NAME
       Template::Tutorial::Datafile - Creating Data Output Files Using the Template Toolkit

DESCRIPTION
Introducing the Template Toolkit
       There are a number of Perl modules that are universally recognised as The Right Thing To
       Use for certain tasks. If you accessed a database without using DBI, pulled data from the
       WWW without using one of the LWP modules or parsed XML without using XML::Parser or one of
       its subclasses then you'd run the risk of being shunned by polite Perl society.

       I believe that the year 2000 saw the emergence of another 'must have' Perl module - the
       Template Toolkit. I don't think I'm alone in this belief as the Template Toolkit won the
       'Best New Module' award at the Perl Conference last summer. Version 2.0 of the Template
       Toolkit (known as TT2 to its friends) was recently released to the CPAN.

       TT2 was designed and written by Andy Wardley <abw@wardley.org>.	It was born out of Andy's
       previous templating module, Text::Metatext, in best Fred Brooks 'plan to throw one away'
       manner; and aims to be the most useful (or, at least, the most used) Perl templating
       system.

       TT2 provides a way to take a file of fixed boilerplate text (the template) and embed
       variable data within it. One obvious use of this is in the creation of dynamic web pages
       and this is where a lot of the attention that TT2 has received has been focussed. In this
       article, I hope to demonstrate that TT2 is just as useful in non-web applications.

Using the Template Toolkit
       Let's look at how we'd use TT2 to process a simple data file.  TT2 is an object oriented
       Perl module. Having downloaded it from CPAN and installed it in the usual manner, using it
       in your program is as easy as putting the lines

	   use Template;
	   my $tt = Template->new;

       in your code. The constructor function, "new", takes a number of optional parameters which
       are documented in the copious manual pages that come with the module, but for the purposes
       of this article we'll keep things as simple as possible.

       To process the template, you would call the "process" method like this

	   $tt->process('my_template', \%data)
	       || die $tt->error;

       We pass two parameters to "process", the first is the name of the file containing the
       template to process (in this case, my_template) and the second is a reference to a hash
       which contains the data items that you want to use in the template. If processing the
       template gives  any kind of error, the program will die with a (hopefully) useful error
       message.

       So what kinds of things can go in %data? The answer is just about anything. Here's an
       example showing data about English Premier League football teams.

	   my @teams = ({ name	 => 'Man Utd',
			  played => 16,
			  won	 => 12,
			  drawn  => 3,
			  lost	 => 1 },
			{ name	 => 'Bradford',
			  played => 16,
			  won	 => 2,
			  drawn  => 5,
			  lost	 => 9 });

	   my %data = ( name   => 'English Premier League',
			season => '2000/01',
			teams  => \@teams );

       This creates three data items which can be accessed within the template, called "name",
       "season" and "teams". Notice that "teams" is a complex data structure.

       Here is a template that we might use to process this data.

	   League Standings

	   League Name: [% name %]
	   Season     : [% season %]

	   Teams:
	   [% FOREACH team = teams -%]
	   [% team.name %] [% team.played -%]
	    [% team.won %] [% team.drawn %] [% team.lost %]
	   [% END %]

       Running this template with this data gives us the following output

		       League Standings

	   League Name: English Premier League
	   Season     : 2000/01

	   Teams:
	   Man Utd 16 12 3 1
	   Bradford 16 2 5 9

       Hopefully the syntax of the template is simple enough to follow. There are a few points to
       note.

       o   Template processing directives are written using a simple language which is not Perl.

       o   The keys of the %data have become the names of the data variables within the template.

       o   Template processing directives are surrounded by "[%" and "%]" sequences.

       o   If these tags are replaced with "[%-" "-%]" then the preceding or following linefeed
	   is suppressed.

       o   In the "FOREACH" loop, each element of the "teams" list was assigned, in turn, to the
	   temporary variable "team".

       o   Each item assigned to the "team" variable is a Perl hash.  Individual values within
	   the hash are accessed using a dot notation.

       It's probably the first and last of these points which are the most important. The first
       point emphasises the separation of the data acquisition logic from the presentation logic.
       The person creating the presentation template doesn't need to know Perl, they only need to
       know the data items which will be passed into the template.

       The last point demonstrates the way that TT2 protects the template designer from the
       implementation of the data structures.  The data objects passed to the template processor
       can be scalars, arrays, hashes, objects or even subroutines. The template processor will
       just interpret your data correctly and Do The Right Thing to return the correct value to
       you. In this example each team was a hash, but in a larger system each team might be an
       object, in which case "name", "played", etc. would be accessor methods to the underlying
       object attributes. No changes would be required to the template as the template processor
       would realise that it needed to call methods rather than access hash values.

   A more complex example
       Stats about the English Football League are usually presented in a slightly more complex
       format than the one we used above. A full set of stats will show the number of games that
       a team has won, lost or drawn, the number of goals scored for and against the team and the
       number of points that the team therefore has.  Teams gain three points for a win and one
       point for a draw. When teams have the same number of points they are separated by the goal
       difference, that is the number of goals the team has scored minus the number of team
       scored against them. To complicate things even further, the games won, drawn and lost and
       the goals for and against are often split between home and away games.

       Therefore if you have a data source which lists the team name togther with the games won,
       drawn and lost and the goals for and against split into home and away (a total of eleven
       data items) you can calculate all of the other items (goal difference, points awarded and
       even position in the league). Let's take such a file, but we'll only look at the top three
       teams. It will look something like this:

	   Man Utd,7,1,0,26,4,5,2,1,15,6
	   Arsenal,7,1,0,17,4,2,3,3,7,9
	   Leicester,4,3,1,10,8,4,2,2,7,4

       A simple script to read this data into an array of hashes will look something like this
       (I've simplified the names of the data columns - w, d, and l are games won, drawn and lost
       and f and a are goals scored for and against; h and a at the front of a data item name
       indicates whether it's a home or away statistic):

	   my @cols = qw(name hw hd hl hf ha aw ad al af aa);

	   my @teams;
	   while (<>) {
	       chomp;

	       my %team;

	       @team{@cols} = split /,/;

	       push @teams, \%team;
	   }

       We can then go thru the teams again and calculate all of the derived data items:

	   foreach (@teams) {
	       $_->{w} = $_->{hw} + $_->{aw};
	       $_->{d} = $_->{hd} + $_->{ad};
	       $_->{l} = $_->{hl} + $_->{al};

	       $_->{pl} = $_->{w} + $_->{d} + $_->{l};

	       $_->{f} = $_->{hf} + $_->{af};
	       $_->{a} = $_->{ha} + $_->{aa};

	       $_->{gd} = $_->{f} - $_->{a};
	       $_->{pt} = (3 * $_->{w}) + $_->{d};
	   }

       And then produce a list sorted in descending order:

	   @teams = sort {
	       $b->{pt} <=> $b->{pt} || $b->{gd} <=> $a->{gd}
	   } @teams;

       And finally add the league position data item:

	   $teams[$_]->{pos} = $_ + 1
	       foreach 0 .. $#teams;

       Having pulled all of our data into an internal data structure we can start to produce
       output using out templates. A template to create a CSV file containing the data split
       between home and away stats would look like this:

	   [% FOREACH team = teams -%]
	   [% team.pos %],[% team.name %],[% team.pl %],[% team.hw %],
	   [%- team.hd %],[% team.hl %],[% team.hf %],[% team.ha %],
	   [%- team.aw %],[% team.ad %],[% team.al %],[% team.af %],
	   [%- team.aa %],[% team.gd %],[% team.pt %]
	   [%- END %]

       And processing it like this:

	   $tt->process('split.tt', { teams => \@teams }, 'split.csv')
	     || die $tt->error;

       produces the following output:

	   1,Man Utd,16,7,1,0,26,4,5,2,1,15,6,31,39
	   2,Arsenal,16,7,1,0,17,4,2,3,3,7,9,11,31
	   3,Leicester,16,4,3,1,10,8,4,2,2,7,4,5,29

       Notice that we've introduced the third parameter to "process".  If this parameter is
       missing then the TT2 sends its output to "STDOUT". If this parameter is a scalar then it
       is taken as the name of a file to write the output to. This parameter can also be (amongst
       other things) a filehandle or a reference to an object w hich is assumed to implement a
       "print" method.

       If we weren't interested in the split between home and away games, then we could use a
       simpler template like this:

	   [% FOREACH team = teams -%]
	   [% team.pos %],[% team.name %],[% team.pl %],[% team.w %],
	   [%- team.d %],[% team.l %],[% team.f %],[% team.a %],
	   [%- team.aa %],[% team.gd %],[% team.pt %]
	   [% END -%]

       Which would produce output like this:

	   1,Man Utd,16,12,3,1,41,10,6,31,39
	   2,Arsenal,16,9,4,3,24,13,9,11,31
	   3,Leicester,16,8,5,3,17,12,4,5,29

Producing XML
       This is starting to show some of the power and flexibility of TT2, but you may be thinking
       that you could just as easily produce this output with a "foreach" loop and a couple of
       "print" statements in your code. This is, of course, true; but that's because I've chosen
       a deliberately simple example to explain the concepts. What if we wanted to produce an XML
       file containing the data? And what if (as I mentioned earlier) the league data was held in
       an object? The code would then look even easier as most of the code we've written earlier
       would be hidden away in "FootballLeague.pm".

	   use FootballLeague;
	   use Template;

	   my $league = FootballLeague->new(name => 'English Premier');

	   my $tt = Template->new;

	   $tt->process('league_xml.tt', { league => $league })
	       || die $tt->error;

       And the template in "league_xml.tt" would look something like this:

	   <?xml version="1.0"?>
	   <!DOCTYPE LEAGUE SYSTEM "league.dtd">

	   <league name="[% league.name %]" season="[% league.season %]">
	   [% FOREACH team = league.teams -%]
	     <team name="[% team.name %]"
		   pos="[% team.pos %]"
		   played="[% team.pl %]"
		   goal_diff="[% team.gd %]"
		   points="[% team.pt %]">
		<stats type="home">
		       win="[% team.hw %]"
		       draw="[%- team.hd %]"
		       lose="[% team.hl %]"
		       for="[% team.hf %]"
		       against="[% team.ha %]" />
		<stats type="away">
		       win="[% team.aw %]"
		       draw="[%- team.ad %]"
		       lose="[% team.al %]"
		       for="[% team.af %]"
		       against="[% team.aa %]" />
	     </team>
	   [% END -%]
	   &/league>

       Notice that as we've passed the whole object into "process" then we need to put an extra
       level of indirection on our template variables - everything is now a component of the
       "league" variable.  Other than that, everything in the template is very similar to what
       we've used before. Presumably now "team.name" calls an accessor function rather than
       carrying out a hash lookup, but all of this is transparent to our template designer.

Multiple Formats
       As a final example, let's suppose that we need to create output football league tables in
       a number of formats. Perhaps we are passing this data on to other people and they can't
       all use the same format. Some of our users need CSV files and others need XML. Some
       require data split between home and away matches and other just want the totals. In total,
       then, we'll need four different templates, but the good news is that they can use the same
       data object. All the script needs to do is to establish which template is required and
       process it.

	   use FootballLeague;
	   use Template;

	   my ($name, $type, $stats) = @_;

	   my $league = FootballLeague->new(name => $name);

	   my $tt = Template->new;

	   $tt->process("league_${type}_$stats.tt",
			{ league => $league }
			"league_$stats.$type")
	       || die $tt->error;

       For example, you can call this script as

	   league.pl 'English Premier' xml split

       This will process a template called "league_xml_split.tt" and put the results in a file
       called "league_split.xml".

       This starts to show the true strength of the Template Toolkit.  If we later wanted to add
       another file format - perhaps we wanted to create a league table HTML page or even a LaTeX
       document - then we would just need to create the appropriate template and name it
       according to our existing naming convention. We would need to make no changes to the code.

       I hope you can now see why the Template Toolkit is fast becoming an essential part of many
       people's Perl installation.

AUTHOR
       Dave Cross <dave@dave.org.uk>

VERSION
       Template Toolkit version 2.19, released on 27 April 2007.

COPYRIGHT
       Copyright (C) 2001 Dave Cross <dave@dave.org.uk>

       This module is free software; you can redistribute it and/or modify it under the same
       terms as Perl itself.

perl v5.16.3				    2011-12-20		  Template::Tutorial::Datafile(3)
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