Associative arrays

Ordinary list arrays allow us to access their element by number. The first element of array @food is $food[0]. The second element is $food[1], and so on. But Perl also allows us to create arrays which are accessed by string. These are called associative arrays.

To define an associative array we use the usual parenthesis notation, but the array itself is prefixed by a % sign. Suppose we want to create an array of people and their ages. It would look like this:

%ages = ("Michael Caine", 39,
         "Dirty Den", 34,
         "Angie", 27,
         "Willy", "21 in dog years",
         "The Queen Mother", 108);

Now we can find the age of people with the following expressions

$ages{"Michael Caine"};		# Returns 39
$ages{"Dirty Den"};		# Returns 34
$ages{"Angie"};			# Returns 27
$ages{"Willy"};			# Returns "21 in dog years"
$ages{"The Queen Mother"};	# Returns 108

Notice that like list arrays each % sign has changed to a $ to access an individual element because that element is a scalar. Unlike list arrays the index (in this case the person's name) is enclosed in curly braces, the idea being that associative arrays are fancier than list arrays.

An associative array can be converted back into a list array just by assigning it to a list array variable. A list array can be converted into an associative array by assigning it to an associative array variable. Ideally the list array will have an even number of elements:

@info = %ages;		# @info is a list array. It
			# now has 10 elements
$info[5];		# Returns the value 27 from
			# the list array @info
%moreages = @info;	# %moreages is an associative
			# array. It is the same as %ages

Operators

Associative arrays do not have any order to their elements (they are just like hash tables) but is it possible to access all the elements in turn using the keys function and the values function:

foreach $person (keys %ages)
{
	print "I know the age of $person\n";
}
foreach $age (values %ages)
{
	print "Somebody is $age\n";
}

When keys is called it returns a list of the keys (indices) of the associative array. When values is called it returns a list of the values of the array. These functions return their lists in the same order, but this order has nothing to do with the order in which the elements have been entered.

When keys and values are called in a scalar context they return the number of key/value pairs in the associative array.

There is also a function each which returns a two element list of a key and its value. Every time each is called it returns another key/value pair:

while (($person, $age) = each(%ages))
{
	print "$person is $age\n";
}

Environment variables

When you run a perl program, or any script in UNIX, there will be certain environment variables set. These will be things like USER which contains your username and DISPLAY which specifies which screen your graphics will go to. When you run a perl CGI script on the World Wide Web there are environment variables which hold other useful information. All these variables and their values are stored in the associative %ENV array in which the keys are the variable names. Try the following in a perl program:

print "You are called $ENV{'USER'} and you are ";
print "using display $ENV{'DISPLAY'}\n";