Using Hash Variables

Perl Tutorials - Herong's Tutorial Examples

∟Using Hash Variables

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 hash variables:

#- HashVariable.pl
#- Copyright (c) HerongYang.com. All Rights Reserved.
#
   %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

Table of Contents

About This Book

Perl on Linux Systems

Perl on macOS Computers

ActivePerl on Windows Systems

►Data Types: Values and Variables

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

Expressions, Operations and Simple Statements

User Defined Subroutines

Perl Built-in Debugger

Name Spaces and Perl Module Files

Symbolic (or Soft) References

Hard References - Addresses of Memory Objects

Objects (or References) and Classes (or Packages)

Typeglob and Importing Identifiers from Other Packages

String Built-in Functions and Performance

File Handles and Data Input/Output

Open Files in Binary Mode

Open Directories and Read File Names

File System Functions and Operations

Image and Picture Processing

Using DBM Database Files

Using MySQL Database Server

Socket Communication Over the Internet

XML::Simple Module - XML Parser and Generator

XML Communication Model

SOAP::Lite - SOAP Server-Client Communication Module

Perl Programs as IIS Server CGI Scripts

CGI (Common Gateway Interface)

XML-RPC - Remote Procedure Call with XML and HTTP

RPC::XML - Perl Implementation of XML-RPC

Integrating Perl with Apache Web Server

CGI.pm Module for Building Web Pages

LWP::UserAgent and Web Site Testing

Converting Perl Script to Executable Binary

CPAN (Comprehensive Perl Archive Network)

Archived Tutorials

References

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