Using Hash Variables

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This section describes how a hash variable can be assigned with a list value. Hash elements are referred by key subscription notation as scalar expressions, like $identifier{'key'}. An unassigned hash variable contains the (undef) value.

Hash variables can be used in a Perl script under these rules:

• An hash variable should be assigned with a list value constructor using an assignment operation. Every 2 elements in the list value will be formed into a pair of key and value into the hash variable. If there is an extra key at the end of the list without a value, this key will have no value assigned. For example: %siteRanks = ("herongyang.com", 4, "perl.org", 6, "google.com", 9).
• To increase legibility, the => operator can be used to link a value to the key in the list value constructor. For example: %siteRanks = ("herongyang.com" => 4, "perl.org" => 6, "google.com" => 9).
• Each value in a hash variable can be referenced by a scalar expression with the key subscription notation, $identifier{"key"}. The "key" is the string value of the key linked to the value in the hash. For example: $siteRanks{"herongyang.com"} is a scalar expression referring to the value linked to the key "herongyang.com" in the hash variable.
• If a new value is assigned to a new key in the hash variable, a new pair of key and value will be added into the hash variable.
• If you are trying to access a value with a key that has no value assigned, you will get the undef value.

Here are some examples of hash variable assignment operations with list value constructors:

   %messages = 'hello'; 
   %primes = 3,5,7,11;
   %vacations = 'Mon','Tue','Wed';
   %vacations = ('Mon','Tue','Wed');
   %vacations = (Mon,Tue,Wed);
   %menu = 'Mon',2,'Apple';
   %dates = 'Jan',31,'Feb',28,'Mar',31;
   %prices = ('Milk',1.99,'Coke',0.99,'Bread',1.49);

Here is a Perl tutorial script on how to use array variables:

#- HashVariable.pl
#- Copyright (c) 1995 by Dr. Herong Yang, http://www.herongyang.com/
#

   %siteRanks;                         # Undefined
   print(%siteRanks, "\n");

   %siteRanks = ("herongyang.com", 4, "perl.org", 6, "google.com", 9);
   print(%siteRanks, "\n");            # Defined now

   %siteRanks = ("herongyang.com"=>4, "perl.org"=>6, "google.com"=>9);
   print(%siteRanks, "\n");            # Using => operator

   $myRank = $siteRanks{"herongyang.com"}; 
   print($myRank, "\n");               # Refers to a value by key

   $siteRanks{"google.com"} = 10;      # Updates a value by key
   print($siteRanks{"google.com"}, "\n");

   $siteRanks{"yahoo.com"} = 8;        # Adding a new key-value pair
   print($siteRanks{"yahoo.com"}, "\n");

   print ("undef", "\n") if ($siteRanks{"xyz.com"} == undef);
                                       # Undefined key

Here is the output of the tutorial script:

herongyang.com4google.com9perl.org6
herongyang.com4google.com9perl.org6
4
10
8
undef

Sections in This Chapter

Scalar Values and List Values

Scalar Value Constructors

Scalar Value Interpretation

List Value Constructors

Variables - Scalar, Array and Hash

Using Scalar Variables

Using Array Variables

Using Hash Variables

"undef" Value and Undefined Variables