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Tie-TwoLevelHash-1.2
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/ Tie::TwoLevelHash

NAME

Tie::TwoLevelHash - Tied interface to multi-dimensional (Two-Level) hash files

SYNOPSIS

 # Tie to Hash-o-hashes	
 use Tie::TwoLevelHash;
 tie (%hash, 'Tie::TwoLevelHash', $file, 'rw');  # Open in read/write mode

 $hash{PEOPLE} = {YOU => "me"}; # Set value YOU in hash PEOPLE withing hash %hash to "me"
	
 # Tie to hash within a Hash-o-hashes
 use Tie::TwoLevelHash:
 tie (%hash, 'Tie::TwoLevelHash', "$file, <SingHash>" 'rw');  # Open in read/write mode

 $hash{YOU} = "me"; # Set key YOU in hash <SingHash> (within HoH's) to "me"

 untie %hash;
 
 tie (%hash, 'Tie::TwoLevelHash', $file);    # Defaults to read-only mode

 ...

 untie %hash;

DESCRIPTION

This is the Tie::TwoLevelHash module. It is a TIEHASH interface which lets you tie to a text file which is a multi-dimensional (two level) hash.

To use it, tie a hash to a directory:

tie(%hash, 'Tie::TwoLevelHash', $file, 'rw');  # Open in read/write mode

If you pass 'rw' as the third parameter, you'll be in read/write mode, and any changes you make to the hash will create or modify the file. If you don't open in read/write mode you'll be in read-only mode, and any changes you make to the hash won't have any effect in the given file. It's actually useless to tie to the file in read mode and make write calls to it, or the hash you are tying to it. If you do, it may croak, depending on what you are trying. If you want to grab values and play with them, do that in your script, and get the values out of the hash name you are tying with, so you can write to a local hash, and not affect, or try to affect the hash you are tying with.

Two Level Hash Files

A two level hash file (I use a .tlh extension) is a file after the same format as the defunct(?) Windows .ini files. A simple example of a small TLH file is as follows:

    # This is a TLH file
# Comments on top of this file are allowed

COLORS
	Red: #ff0000
	Black: #000000
	White: #ffffff

PEOPLE
	Dog: Zeke
	Cat: Tigger
	PerlHacker: Randal
	Author: Kevin Meltzer

EXTRA
	Key: Val
	Test: Vest

This file is a textual representation of a two-level hash, also known as a Hash of hashes. The file itself is the main hash, and each section contains another hash. So, this file contains the hash COLORS the hash PEOPLE and the hash EXTRA. Tie::TwoLevelHash allows for you to tie to the entire hash of hashes, or directly to one of the hashes within that hash of hashes. When you make a change in your script to the tied hash, it makes that change in your file.

EXAMPLES

Tying to hash of hashes

    $file = "foo.tlh";
tie(%hash, 'Tie::TwoLevelHash', $file, 'rw');

# Set %foo to equal %hash
%foo = %hash;

# Grab value of BAR in hash FOO into $bar
$bar = $hash{FOO}->{BAR};

# Set existing value
$hash{PEOPLE} = {You => "me"};

# Set new value
$hash{COLORS} = {YELLOW => "flowery"};	

# Set new record
$hash{HATS} = {BLACK => "Cowboy", RED => "Baseball", WHITE => "Beanie"}; 

# Add new record with predefined hash
%new = (ONE => "1",
	TWO => "2",
	THREE => "3",
	FOUR => "4",
	);

$hash{NUM} = {%new}; # Works, or can use \%new instead of {%new}

# Clear, set then delete entry
$hash{PEOPLE} = {FOO => ""};
$hash{PEOPLE} = {FOO => "Bar"};
$hash{PEOPLE} = {FOO => undef};

# Added new element to existing record
$hash{PEOPLE} = {'FOO' => "FOObar"};

$hash{EXTRA} = undef;

untie %hash;


The resulting TLH file would be (assuming you began with the TLH example above):

# This is a TLH file
# Comments on top of this file are allowed

COLORS
	Black: #000000
	Red: #ff0000
	White: #ffffff
	Yellow: flowery

HATS
	BLACK: Cowboy
	RED: Baseball
	WHITE: Pope hat


NUM
	FOUR: 4
	ONE: 1
	THREE: 3
	TWO: 2

PEOPLE
	Author: Kevin Meltzer
	Cat: Tigger
	Dog: Zeke
	FOO: FOObar
	PerlHacker: Randal

Tying to a hash within a hash of hashes

    tie(%hash, 'Tie::TwoLevelHash', "$file, PEOPLE", 'rw');

# Set %foo to equal %hash
%foo = %hash;

# Grab value of FOO into $bar
$bar = $hash{FOO};

# Set existing value
$hash{Cat} = "Gizmo";

# Set new value	
$hash{Someone} = "Larry"; 

# Clear, set then delete entry
$hash{FOO} = "";
$hash{FOO} = "bar";
$hash{FOO} = undef;

untie %hash;

    The resulting TLH file would be:

    # This is a TLH file
# Comments on top of this file are allowed

COLORS
	Red: #ff0000
	Black: #000000
	White: #ffffff

PEOPLE
	Author: Kevin Meltzer
	Cat: Gizmo
	Dog: Zeke
	PerlHacker: Randal
	Someone: Larry

EXTRA
	Key: Val
	Test: Vest

Getting the value of your tied hash to your local script

When you are tied to the hash, tie doesn't actually export the values in that hash (or HoH) by default. So, Tie::TwoLevelHash exports a method that allows you to muck around with the hash's actual values within your script. This can also be useful if, for whatever reason, you don't want to write out the hash file whenever you make a change, and wish to do it at a later time, while still working with the new values.

With a hash-o-hashes

    # Tie your hash, but use a scaler to be object-like 
tie(%hash, "Tie::TwoLevelHash" , $file, 'rw');

# Make untied copy of your tied HoH
%bar = %hash;

    Now, you can generally use %bar as you would any other hash-o-hashes. Above we tied to the entire hash-o-hashes, so %bar will be filled with hash references. You could do something like the following to list all the hash names, and values:

    foreach $key (keys %bar) {
print "$key\n";
	$foo = $bar{$key};
	%foo = %$foo; # Deref the lower hash
	foreach $fookey (keys %foo) {
		print "\t$fookey\: $foo{$fookey}\n";
	}
}

    Now, say you wanted to change one of the values, but not change it in your TLH file just yet. You can do this like:

    # This line changes the value in %bar
$bar{PEOPLE}->{Comedian} = "Sienfeld";

# This line writes the new value to your tied hash (and TLH)
$hash{PEOPLE} = $bar{PEOPLE};

With a hash in a hash-o-hashes

    This way is slightly different. Right now, trying to make a copy of your hash the same way you do above, does not copy the hash correctly. So, there is a method, GetHash , which will export it correctly.

    # Tie your hash, but use a scaler to be object-like 
$foo = tie(%hash, "Tie::TwoLevelHash" , "$file, PEOPLE", 'rw');

# Now, we will call the method that exports the hash
# We will import it into the hash %bar
%bar = $foo->GetHash;

    Now, you can generally use %bar as you would any other hash. Above we tied only to one hash in the hash-o-hashes, so %bar will be one hash. You could do something like the following to list all the hash names, and values:

    foreach $key (keys %goo) {
	print "$key\: $goo{$key}\n";
}

    Now, if you wanted to change values before writting the new TLH file out via your tie:

    # This line will set key Comedian to Sienfeld locally
$bar{Comedian} = "Sienfeld";

# Now, we write out the new hash
%hash = %bar;

CHANGING VALUES

I won't go into how to change value in a hash. When you are tying to a hash in the hash of hashes, you make your calls as usual. You make your calls as usual when you are tied to the entire HoH's, except when you are setting new values (anything that would call STORE).

Due to tie() not being very friendly while tying to HoH's, you can not make a call such as

$hash{FOO}->{BAR} = "zog";

when tied to a HoH's. So, you must make this call like:

$hash{FOO} = {BAR => "zog"};

You can see how/when to do this in the EXAMPLE section. When you want to delete a key in a hash, use undef like:

$hash{FOO} = {BAR => undef};

or, when tying to single hash:

$hash{BAR} = undef;

%hash = ();

This will CLEAR the hash, as well as remove ALL data from the file you are tied to. Be sure you want to do this when you call it.

INSTALLATION

You install Tie::TwoLevelHash, as you would install any perl module library, by running these commands:

perl Makefile.PL
make
make test
make install
make clean

AUTHOR

Copyright 1998, Kevin Meltzer. All rights reserved. It may be used and modified freely, but I do request that this copyright notice remain attached to the file. You may modify this module as you wish, but if you redistribute a modified version, please attach a note listing the modifications you have made.

Address bug reports and comments to: kmeltz@cris.com

The author makes no warranties, promises, or gaurentees of this software. As with all software, use at your own risk.

VERSION

Version $Revision: 1.2 $ $Date: 1998/10/30 13:52:04 $

CHANGES

$Log: TwoLevelHash.pm,v $ Revision 1.2 1998/10/30 13:52:04 kmeltz Fixed FETCH so it will return correctly when doing %foo = %bar; when using a tie to a HoH's. Still not working right for hash in the HoH's, so the GetHash method stays.

Revision 1.1 1998/10/27 15:43:47 kmeltz Changed croaks to carps for Hash Invalid warning. May need to continue script, so let script die and module return undef. Changed CLEAR to not erase TLH file when clearing hash in HoH's, or resetting it. Added exported method GetHash. This allows for user to import hash values into their script, and change values before setting them to TLH file. Changed a bunch in the POD.

AVAILABILITY

The latest version of Tie::TwoLevelHash should always be available from:

$CPAN/modules/by-authors/id/K/KM/KMELTZ/

Visit http://www.perl.com/CPAN/ to find a CPAN site near you.

SEE ALSO

perl(1), perlfunc(1), perltie(1)