You want to process a large dataset you'd like to commit to a DBM file in a particular order.
Use the DB_File's B-tree bindings and supply a comparison function of your own devising:
use DB_File;
# specify the Perl sub to do key comparison using the
# exported $DB_BTREE hash reference
$DB_BTREE->{'compare'} = sub {
my ($key1, $key2) = @_ ;
"\L$key1" cmp "\L$key2" ;
};
tie(%hash, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE)
or die "can't tie $filename: $!";An annoyance of hashes, whether in memory or as DBM files, is that they do not maintain proper ordering. The CPAN module Tie::IxHash can make a regular hash in memory maintain its insertion order, but that doesn't help for DBM databases or arbitrary sorting criteria.
The DB_File module supports a nice solution to this using a B-tree implementation. One advantage of a B-tree over a regular DBM hash is its ordering. When the user defines a comparison function, all calls to keys, values, and each are automatically ordered. For example, Example 14-3 is a program that maintains a hash whose keys will always be sorted case-insensitively.
#!/usr/bin/perl
#
sortdemo - show auto dbm sorting
use strict;
use DB_File;
$DB_BTREE->{'compare'} = sub {
my ($key1, $key2) = @_ ;
"\L$key1" cmp "\L$key2" ;
};
my %hash;
my $filename = '/tmp/sorthash.db';
tie(%hash, "DB_File", $filename, O_RDWR|O_CREAT, 0666, $DB_BTREE)
or die "can't tie $filename: $!";
my $i = 0;
for my $word (qw(Can't you go camp down by Gibraltar)) {
$hash{$word} = ++$i;
}
while (my($word, $number) = each %hash) {
printf "%-12s %d\n", $word, $number;
}By default, the entries in a B-tree DB_File database are stored alphabetically. Here, though, we provide a case-insensitive comparison function, so using each to fetch all the keys would show:
by 6 camp 4 Can't 1 down 5 Gibraltar 7 go 3 you 2
This sorting property on hashes is so convenient that it's worth using even without a permanent database. If you pass undef where the filename is expected on the tie, DB_File will create a file in /tmp and then immediately unlink it, giving an anonymous database:
tie(%hash, "DB_File", undef, O_RDWR|O_CREAT, 0666, $DB_BTREE) or die "can't tie: $!";
Remember these two things if you supply a comparison for your BTREE database. One, the new compare function must be specified when you create the database. Two, you cannot change the ordering once the database has been created; you must use the same compare function every time you access the database.
Using BTREE databases under DB_File also permits duplicate or partial keys. See its documentation for examples.