Filling Your Hash

Individual hash elements are created by assigning values to them, much as with array elements. For example, you can create individual hash elements like the following:

$Authors{'Dune'}='Frank Herbert';

In this example, you assign to the hash %Authors. The key for this element is the word Dune, and the data is the name Frank Herbert. This assignment creates a relationship in the hash between Dune and Frank Herbert. $Authors{'Dune'} can be treated like any other scalar; it can be passed to functions, modified by operators, printed, or reassigned. When you're changing a hash element, always remember that you're modifying the value stored in the hash element, not the hash itself.

Why does the example use $Authors{} instead of %Authors{}? Like arrays, when hashes are represented as a whole, they have their own marker in front of the variable name (%). When you access an individual element of a hash, a scalar value, you precede the variable name with a dollar sign ($) indicating a single value is being referenced, and you use the braces to indicate the value. To Perl, $Authors{'Dune'} represents a single scalar value—in this case, Frank Herbert.

Hashes with one key aren't particularly useful. To put several values into a hash, you could use a series of assignments, as shown in the following:

$food{'apple'}='fruit';
$food{'pear'}='fruit';
$food{'carrot'}='vegetable';

To make this operation shorter, you can initialize the hash with a list. The list should consist of pairings of keys and values, as shown here:

%food=('apple', 'fruit', 'pear', 'fruit', 'carrot', 'vegetable'),

This example looks similar to array initializations discussed in Hour 4, "Stacking Building Blocks: Lists and Arrays." In fact, as you'll learn later in this hour, hashes can be treated as a special kind of array in many contexts.

When you're initializing a hash, keeping track of which items are keys and which items are values in a large list can be confusing. Perl has a special operator called a comma-arrow operator, =>. Using the => operator and taking advantage of the fact that Perl ignores whitespace, you can write hash initializations like the following:

%food=( 'apple' => 'fruit',
     'pear'   => 'fruit',
     'carrot' => 'vegetable',
    );

Perl programmers, holding laziness as a virtue, have two additional shortcuts for hash initializations. The left side of the => operator is expected to be a simple string and does not need to be quoted. Also, a single-word hash key inside the curly braces is automatically quoted. So the initializations shown previously become the following:

$Books{Dune}='Frank Herbert';
%food=( apple => 'fruit',  pear => 'fruit',  carrot => 'vegetable' );