Refresher: Lists and Hashes

Technically, you should already know the basics behind these two Perl data structures, as covered in The Perl You Need to Know Part 1. Let's review, as they say:

A list, also known as an array in many other programming languages, is an ordered sequence of data items. Perl lists are denoted with the @ character prefix, and are defined thusly:

@colors=("red","green","blue");

Simple enough, we now have a list named colors which contains three items, sometimes known as elements, each a string value. Because a list is ordered, the sequence of the items matters: the first item is "red", the second is "green", and so on. Although colors itself is a list, any single item in the list is a Perl scalar value, which we can pluck out as follows:

$firstItem=$colors[0];

Notice that we precede colors with the scalar prefix, $, since we are accessing only one item in the list, not the whole list itself. The subscript 0 refers to the first item in the list, since list indices began counting at zero, which was "red". Very elementary stuff, indeed!

You can add an item to a list either using a direct assignment to an unused index number, or, simply "push" an item onto the end of the list using the push function:

$colors[3]="magenta";
push(@colors,"magenta");

A hash takes the list concept and, though it pains one to say this, "kicks it up a notch" (so says a certain TV chef). The hash, more obtusely known as an associative array, lets you associate two pieces of data with each other, rather than a simple linear list of items. Hash items are known as "key-value pairs", because each hash item consists of a key (one piece of data) which is associated with a value (another type of data). For example, you can imagine such a pair where the key is "color" and "blue" is the value. Hashes are represented using the % prefix, such as:

%item=("color"=>"blue");

The above creates a hash named item with a key-value pair of color->blue. As with lists, you access a single hash value as a scalar, using the key value as the subscript in your reference:

	
$itemColor=$item{"color"};

In this assignment, the variable $itemColor would receive the value "blue". You can add any number of additional keys to this hash -- unlike a list, however, the order of the keys is irrelevent.

$item{"size"}="large";
$item{"price"}="22.99";

You can retrieve a list of all the keys held in a hash using the keys function:

	
@itemKeys=keys (%item);

Above, the list @itemKeys would contain the items "color", "size", and "price" ... though not necessarily in that order. In fact, you don't know what order they will be in, unless you have explicitly sorted the keys which we'll look at shortly.

Now that we're comfortable with the concepts of lists and hashes, it's time for some juggling!

The Perl You Need to Know Part 10: Untangling Lists and Hashes
The Perl You Need to Know
All Sorts of Sorts: Lists