Creating and Using a Database with Perl

Sometimes it is necessary to sort an associative array within a Perl script. Sorting by the key values of an associative array is done like this:

for (sort keys %phone_db) {
        print "$_ = $phone_db{$_}\n";
}

Each iteration of this loop will set the $_ scalar to a key value from the associative array provided in alphabetical order. This method works very nicely for sorting associative arrays by their keys. Sorting by an associative array's values is slightly more difficult:

sub sort_by_value {
        ( $phone_db{$a} cmp $phone_db{$b} ) || \
( $a cmp $b );
}
for (sort sort_by_value keys %phone_db) {
        print "$_ = $phone_db{$_}\n";
}

Keep in mind that these two methods for sorting an associative array do not actually rearrange the array in any fashion. They simply provide a way to pull every key and value pair from an associative array in a particular sorted order.

An example of how databases in Perl can be used is provided in Listing 1, a short script designed to keep a database of hits on a World Wide Web site. The script reads the NCSA HTTPD access log file, stores the information in the database and creates an HTML page that displays all the statistics for the site.

Listing 1. Example Web Site Hits Database Script

This implementation is not complete—it keeps track only of which documents were accessed and their sizes. A more complete implementation could also store information about the hosts that accessed the web server, for instance. Some method for “expiring” entries in the database after a particular time interval would be a handy feature as well.

The script begins by reading the existing database file and placing all the data into associative arrays indexed by the document file name. Next, the script reads the access log file from standard input and places the data into the associative arrays that store the statistics. Finally, the script creates an HTML page using tables to display the statistics.

The topics provided in this article are by no means a definitive reference guide for using the built-in database support included with Perl, but they can be used as a starting point for further experimentation and exploration.