Associative Arrays
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What is an Associative Array, or Hash?
Hashes are arrays that pair up a scalar key with a scalar
value. You use the key to get the value, in other words, the
key is associated with a value. Keys must be unique,
but values need not be unique.
Order of Keys in a Hash
There is no "obvious" order to the keys in a hash, but are
sorted to allow Perl to retrieve values quickly. They can be sorted by
other operations.
Creating Hashes
There are different ways of creating associative arrays. Some examples
are:
<![CDATA[
# $a{0} = 'a', $a{1} = 'b', $a{2} = 'c'
%a = (0, 'a', 1, 'b', 2, 'c');
# less or more readable depending on if you
# started learning w/ Perl version 4 or 5
%b = (larry => fine, moe => howard);
%c = (); # clear the hash
]]>
Examples of Hashes that Already Exist in Perl
Two examples are:
- %ENV - the operating system environment variables and their values
- %INC - the files used in "require" and "use"
statements and their paths
Accessing Elements
You access hash values, which are scalars, with the syntax:
<![CDATA[
$hash{'some_key'} = $some_value;
$a = $date{feb};
$address{town} = 'Beantown';
]]>
As usual, to reference a scalar, you prefix the name with a
$ sign.
Getting to Keys and Values of an Associative Array
To access values of these hashes, we use the keys()
function. Here is an example of how to print out all our sorted environment
variables and their values:
<![CDATA[
foreach $value (sort (keys (%ENV)))
{
print "$value = $ENV{$value}\n";
}
]]>
Here foreach, similar to the csh shell, will
set $value to all the possible values of the sorted keys
of %ENV. We then print out the key and the associated
value, $ENV{$value}.
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Last Modified: $Date: 1997/05/02 07:17:39 $