Creating and Using a Database with Perl

Perl programmers, like programmers of any other language, typically need to store large amounts of data. For this data to be manageable, it needs to be stored in a conveniently accessible format. It never hurts to make the stored data easy to write, as well.

Even though Perl is an exceptional language for text processing, in many circumstances, a more structured database-like format offers quicker access. In addition, it may also be necessary for a Perl script to read or write a database that is also accessed through a C program.

To accomplish this, the Perl distribution includes packages that give a Perl programmer access to a variety of different database formats available in a Unix environment. These formats include: the Berkeley DB format, the Free Software Foundation's GDBM format and the NDBM format.

The associative array (or “hash”) is one of the most powerful data structures available to a Perl programmer. To those familiar with traditional arrays (in C, Pascal or Perl), an associative array can be thought of as an array indexed using an arbitrary string instead of an integer subscript. Basically, an associative array is a data structure that allows the programmer to associate one string—a key—with another—its value.

Here is an example of an associative array that can be used to convert the abbreviated name of a day of the week to its full name.

%days = (
        "Sun", "Sunday",
        "Mon", "Monday",
        "Tue", "Tuesday",
        "Wed", "Wednesday",
        "Thu", "Thursday",
        "Fri", "Friday",
        "Sat", "Saturday"
);

The % in front of the variable name days is used to tell Perl the variable is an associative array. As shown, associative arrays are initialized by using pairs of values that relate to each other.

To access the data stored in an associative array, you can use a syntax similar to the following:

$long_name = $days{"Sun"};

This expression will set the scalar variable long_name to the value associated with the key “Sun”, the string “Sunday” in this example.

You can see already that associative arrays can be a powerful tool for organizing data used inside of a Perl script. This technique can easily be extended to something more useful by creating values made up of more than one field. Take, for instance, this simple address book database where multiple fields in the associative array's value are separated by colons:

$phone_db = (
        "Bill Jones", "123 West Avenue:New York, NY:12345",
        "Jane Smith", "6789 1st Street:Chicago, IL:56789"
);

New entries in this database can be added with an expression like:

$phone_db{"Bill Smith"} = join(":", $street, $city, $zip_code);

($street, $city, $zip_code) = split(/:/, $phone_db{"Bill Smith"});

while (($name, $record) = each %phone_db) {
        print "$name:$record\n";
}

To solve this problem, Perl supports “binding” associative arrays to the various types of database formats mentioned above. This allows a Perl programmer to create, access and update databases in the popular Unix database formats as easily as performing operations on an associative array.

Perl version 5 includes a set of “packages” that manipulate the various database formats. These packages are:

To use any of these database packages, a Perl programmer must include the package at the beginning of the script using the following statement:

use DB_File;

use Fcntl;