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iakovlev.org

Perl 5

      U N L E A S H E D

Kamran Husain , Robert F. Breedlove

Часть 1

Что такое Perl?


Содержание


Перл-интерпретируемый язык,оптимизированный для сканирования текстовых файлов, выделения информации из них и их распечатки. Он также хорош для многих системных задач. Он легок в использовании,эффективен,элегантен,минимален. Perl написал LarryWall (lwall@sems.com), не без помощи конечно.

Зачем нужен Perl?

Юниксовые админы и разработчики подчас зависят от различных языков для решения своих задач. Например,для работы с файлами можно использовать шелл и утилиты sh,grep,awk,sed. Админ конечно может использовать и си, но это долго.

Было бы неплохо,если бы все вышеуказанные средства можно было обьединить с помощью одного языка. Вот тут и приходит на помощь перл.

Perl обьединяет в себе преимущества C, sed, awksh. Синтаксис Perl похож на си. Перл может иметь дело не только с текстовыми,но и с бинарными данными.

Краткая история Perl

It is helpful to your understanding of Perl to know a little bit about why Perl was created and how it evolved.

Larry Wall developed Perl in 1986. He was a systems programmer on a project that was developing multilevel, secure wide area networks. Larry was in charge of an installation consisting of three Vaxes and three Suns on the West Coast of the United States connected over an encrypted serial line (1200 baud!) to a similar configuration on the East Coast of the United States. Larry's primary job was system support "guru." During this stint, he developed several useful UNIX tools such as rn, patch, and warp.

Perl was developed in response to a management requirement for a configuration management and control system for all six Vaxes and all six Suns. As with most management requests, Larry had a month to develop this tool!

Larry considered the problem of a bicoastal configuration management tool, without writing it from scratch. The tool would have to be capable of viewing problem reports on both coasts with approvals and control. His answer was B-news.

Larry installed B-news on three machines and added two control commands. Configuration management was done using RCS, and approvals and submissions were done using news and rn.

However, managers always need one thing more. Larry's manager asked him to produce reports. B-news was maintained in separate files on a master machine, with lots of cross references between files. Larry's first thought was to use awk to produce the reports. Unfortunately, awk fell a bit short. It couldn't handle opening and closing multiple files based on information in the files. Larry didn't want to code a special purpose tool just for this job, so a new language was born.

The language wasn't originally called Perl. Larry, his coworkers, friends, and family considered just about every three- and four-letter word in existence. One of the earliest names was "Gloria" (his wife's name), but this was replaced due to the confusion it caused in his household. The name became "Pearl," which was changed into the present day "Perl," partly due to the existence of a graphics language called "pearl," but mostly because five letters was a bit much to type all the time. You'll find a reference to the former five-letter version in the entry for the acronym Practical Extraction and Report Language.

The early version of Perl lacked many of the features of today's version. The language included the following :

  • Pattern matching
  • File handles
  • Scalars
  • Formats
  • A crippled implementation of pattern matching (from rn)

The manual page was only 15 pages long. But Perl was faster than sed and awk and began to be used on other aspects of the project.

Larry moved on to support research and development and took Perl with him. Perl was becoming a good tool for system administration. Larry borrowed Henry Spencer's regular expression package and modified it for Perl. Then Larry added most of the goodies he and other people wanted and released it on the Internet.

The current version (5+) of the language is a complete rewrite from the previous versions. It provides the following additional benefits:

Usability enhancementsIt is now possible to write much more readable Perl code. (How any C-like language can be called readable is still beyond me!)
Simplified grammarThe new yacc grammar is one half the size of the old one. Many of the arbitrary grammar rules have been regularized. The number of reserved words has been cut by two-thirds. Despite this, nearly all old Perl scripts will continue to work the same.
Lexical scopingPerl variables may now be declared within a lexical scope.
Arbitrarily nested data structuresAny scalar value, including any array element, may now contain a reference to any other variable or subroutine.
Modularity and reusabilityThe Perl library is now defined in terms of modules that can be shared easily among various packages.
Object-oriented programmingA package can function as a class. Dynamic multiple inheritance and virtual methods are supported in a straightforward manner and with very little new syntax. File handles may now be treated as objects.
Embeddability and ExtensibilityPerl may now be embedded easily in your C or C++ application and can either call or be called by your routines through a documented interface.
POSIX compliantA major new module is the POSIX module, which provides access to all available POSIX routines and definitions via object classes, where appropriate.
Package constructors and destructorsThe new BEGIN and END blocks provide a means to capture control as a package is being compiled and after the program exits.
Multiple simultaneous
BM implementations
A Perl program may now access DBM, NDBM, SDBM, GDBM, and Berkeley DB files from the same script, simultaneously.
Subroutine definitions may be autoloaded The AUTOLOAD mechanism enables you to define any arbitrary semantics for undefined subroutine calls.
Regular expression enhancementsYou can now specify non-greedy quantifiers and performing grouping without creating a back reference.
You can write regular expressions with embedded white space and comments for readability. A consistent extensibility mechanism has been added that is upwardly compatible with all old, regular expressions.

The Benefits of Using Perl

Perl has many advantages as a general-purpose scripting language. These benefits include its generous licensing (it's free), its interpreted nature, the fact that Perl is available for most platforms, and more. The following sections detail some of the benefits of this excellent language.

Cost and Licensing

First, Perl is generally available on most server platforms, including the following:

  • Most UNIX variants
  • MS-DOS
  • Windows NT
  • Windows 95
  • OS/2
  • Macintosh

Perl also has the distinct advantage of being "low cost." It is distributed free of charge or, at most, for a small copying charge. Actually, Perl is distributed under the GNU "copyleft," which means that if you can execute Perl on your system, you should have access to the source of Perl for no additional charge. (Actually, a small copying charge might be imposed.) Perl may also be distributed under the "artistic license," which some people find less threatening than the copyleft.

Availability

Perl is readily available from many sources, including any comp.sources.unix archive or CPAN site. If you don't have Perl on your server or development machine, it is easy to obtain either as source code or precompiled binaries for many platforms. For those not on the Internet, Perl is available via anonymous Uucp from both uunet and osu-cis. Perl is often distributed with CD collections of utilities for UNIX platforms. (See appendix B, "Perl Module Archives," for information on Perl archives.)

Interpreted Language

Perl is interpreted. This can be either an advantage or disadvantage, depending on your needs. For example, Perl has a short development cycle compared to compiled languages, but it will never execute as fast as a compiled language. I discuss the disadvantages in the section called, "What Are the Negatives of Using Perl?," but there are some definite advantages.

One advantage of an interpreted language for tool or application development is that you can perform incremental, iterative development and testing without having to go through a create/compile/test/debug/fix cycle. By eliminating the compile portion of the cycle, interpreted languages can speed the development cycle drastically. It can also be helpful if you are evolving your application by implementing it with minimal capabilities and adding advanced capabilities later.

Because it is interpreted and relatively C-like, you can also use Perl as a prototyping language. This can be especially useful with complex or technically difficult projects such as network communication. You can use Perl's shortened development cycle to evaluate your design and then, once it is proven, rewrite the code in the language of your choice. By the way, C and C++ are good choices because Perl is a lot like C and supports much the same functionality.

Practical

Perl is written to be practical. This means that it is

  • Complete
  • Easy to use
  • Efficient

These design goals mean that Perl programs can generally accomplish a goal that would otherwise take several other languages, require complex programming, and take longer to process.

But for many of us, practicality goes beyond this. It means that you can get things done in Perl. In fact, there are usually several ways that Perl can accomplish the same task. It also means that the programmer can concentrate on getting the task done rather than dealing with the "beauty" of the language in which he or she is working.

Complete

As mentioned before, Perl combines some of the best features of several languages. Here's a list of these languages:

grep/awk
General pattern-matching languages for selecting elements from a file.
C
A general-purpose compiled programming language. (Perl is written in C.)
sh
A control language generally used for running programs and scripts written in other languages.
sed
A stream editor for processing text streams (STDIN/STDOUT).

These languages typically have been the tools used by UNIX administrators to accomplish tasks. In fact, they are often touted as the reason that UNIX is an excellent development platform. They are still excellent tools for the purposes for which they were written.

However, if you have to deal with several languages, you also have to deal with learning these languages. For instance, a task to process a single text file might require the administrator to write a shell script to run an awk program to select lines that are subsequently processed by sed.

A single Perl script can often do the work of several other utilities:

With Perl, the administrator or developer can accomplish his goals in a single, easy-to-use language that performs the same tasks as these languages.

With version 5.0 of Perl, the language also supports an object-oriented approach to pro-gramming. This means that packages/modules can be distributed as objects and used without knowledge of the underlying code. These packages can also be extended as they can be in other object-oriented languages. The key is that programmers only use the object-oriented features of Perl if they need them for the particular program they are writing.

Easy to Use

Above all, Perl is a language in which you can do things. There are usually several ways to accomplish the same task. Although some techniques are more efficient with system resources than others, users can generally select the technique that is easier for them to use (and maintain/enhance in the future) and go with it.

The ease of use and completeness make Perl appropriate for quick-and-dirty, one-time utilities as well as structured, complex applications.

Efficient

Perl is a straight-line language, which means that simple programs do not have to deal with complex formatting or function/procedure or object/method structures to accomplish their task. As a simple example, let's pay homage to programming texts (including this one) with the "Hello World!" program. Here it is in C:

void main()
{
    printf("Hello World!");
}

And here it is in Perl:

print 'Hello World!'

Get in, get out, and get the job done.

Language Capabilities

Perl is optimized for text processing and, therefore, is very efficient at many tasks required of system administrators and application developers. Many of the files used in UNIX systems administration are plain text files. Selecting records, processing the selected records, and reporting exceptions are the heart of many tasks performed in UNIX administration.

In the current versions of Perl, the language also includes much additional functionality, making it appropriate for tasks such as processing socket calls, embedding in programs written in C, and maintaining POSIX-compliant systems.

Integration with C

Perl can access C libraries to take advantage of much of the code written for this popular language. Utilities included with Perl distributions enable you to convert the headers for these C libraries into their Perl equivalents.

Perl 5.0 can be integrated easily into C and C++ applications. Perl can call or be called by routines written in C or C++. The Perl interface is through a set of perl_call_* functions. The call to C libraries is through the XS language interface.

Specialized Extensions to Perl

There are many specialized extensions to Perl, primarily for handling specific databases such as Oracle, Ingres, Informix. These combine the strengths of the Perl language with the access to the host database.

At the time of this writing, ftp.demon.co.uk (158.152.1.69) is the official repository for database <foo>perls (see the following list), which can be found in /pub/perl/db/perl4/. It's mirrored at ftp.cis.ufl.edu (198.17.47.33) in /pub/perl/scripts/db/.

btreeperl NDBM extensions
ctreeperl C-Tree extensions
duaperl X.500 directory user agent
ingperl Ingres
isqlperl Informix
interperl Interbase
oraperl Oracle 6 and 7
pgperl Postgres
sybperl Sybase 4
uniperl UNIFY 5.0

See appendix B, "Perl Module Archives," for more information on these repositories.

Socket Capability

Perl has the capability to read/write TCP/IP sockets. This gives it the capability to communicate with servers of all types that rely on socket communication. It also enables you to write utility and "robot" programs in the Perl language. For example, Perl's socket capability can be used to write a robot program to automate the checking of a World Wide Web (WWW) site to verify the validity of links on your Web pages. This can be especially useful in keeping a site up-to-date, given the volatility of the Internet in its relative infancy.

Perl Is Relatively Easy to Learn

Unlike many programming languages, Perl is designed to be practical rather than beautiful. By this I mean that Perl was designed from the start to be easy to use, efficient, and complete rather than tiny, elegant, and minimal.

Programming in Perl is relatively easy, especially if you have experience in C or another C-like language. Like many scripting languages, Perl reads its programs from the first line to the last line. It doesn't require complex structures to be able to create a program. It does, however, support subroutines or functions and, in version 5.0, can be object oriented.

Perl Has Built-In Debugging Facilities

The Perl interpreter has a built-in debugger that can help reduce the time it takes to debug applications. The debugger is activated through the use of the -d switch on the command line. In addition, the -w switch provides a complete set of warnings that can be invaluable in debugging Perl scripts.

Perl Help Is Readily Available

Because Perl is very popular as a scripting language, there is a lot of help out there. Newsgroup discussions are a good place to start when you require help on Perl programming. There are newsgroups devoted entirely to Perl and newsgroups devoted to Web page creation in which the majority of the discussion is about Perl. Here are some of them:

NewsgroupComment
comp.lang.perl... This set of newsgroups covers information about Perl in general. Much of the discussion in the specific groups covers using Perl for utility purposes and also as a CGI scripting language.
comp.lang.perl.announce Provides information about new modules for Perl programming.
comp.lang.perl This is the main newsgroup about Perl.
comp.lang.perl.modules Provides discussions of Perl modules.
Comp.lang.perl.tk Provides discussions of Tk used with Perl.

There are, of course, Web pages related to Perl. Check the newsgroups for announcements about these pages. Here are just a couple that I have found as of this writing:

URLComment
http://www.perl.com/ This is the Perl language homepage. It provides links to Perl resources.
http://www.eecs.nwu.edu/perl/perl.html NWU's Perl page.
http://www.yahoo.com/Computers/Languages/Perl/ Yahoo's Perl index.
http://www.virtualschool.edu/mon/Perl.html The "middle of nowhere" Perl archive (Netscape 2.0 pages).
http://www.teleport.com/~rootbeer/perl.html References with a special emphasis on using Perl for Web-related programming and on learning Perl.

See appendix B for more complete information on Perl-related Web pages.

Several lists of frequently asked questions (FAQ) are posted to the Perl newsgroups. One of the best to start with is the Perl Meta-FAQ, produced by Neil Bowers (neilb@khoros.unm.edu). As you would expect, this is an FAQ about FAQs. It's available at this writing from the following sources:

HTMLhttp://www.khoros.unm.edu/staff/neilb/perl/metaFAQ/metaFAQ.html
PostScriptftp://ftp.khoros.unm.edu/pub/perl/metaFAQ.ps
ASCIIftp://ftp.khoros.unm.edu/pub/perl/metaFAQ.txt

Perl Examples Are Readily Available

Again, because Perl is so popular as a utility language, there are lots of examples of Perl modules out there. One of the best sources is by file transfer protocol (FTP) from one of the Comprehensive Perl Archive Network (CPAN) sites around the world (see appendix B).

What Are the Negatives of Using Perl?

Perl has few negatives as a scripting language for system administration tasks and as a language for module development. But there are a few.

Interpreted Language

Perl is interpreted. Therefore, it will not be as fast as compiled languages such as C or C++. Given the speed of modern CPUs, in all but very large or time-critical applications, this will not make a significant difference. And in fact, the interpreted nature of the language can reduce development time significantly by eliminating the time needed to compile and debug versions of the program (see the previous section "The Benefits of Using Perl").

Perceived as Public Domain

Perl isn't strictly in the public domain (see the license agreement for details). But it's close enough. Many large companies have policies against using public domain or copylefted software. In many cases, this bias is more of a mind-set than a negative, but it can be a detriment to using Perl (see the following section, "Informal Support").

Because Perl is in the public domain, there is no corporation that your company can apply leverage against to get something done. But you do have access to the Perl source to make specific needed changes to your environment, if required.

Informal Support

The support for Perl is on an informal basis through the volunteer efforts of users worldwide. Does this mean it is bad? No, not necessarily. In fact, the "support" given through the Internet newsgroups is probably as good as any given by a major corporation. But you can't depend on your question being answered, at least in a timely manner. And you don't have a corporation on which you can apply pressure to support your specific environment. On the other hand, you do have access to the source code for Perl and can look into problems yourself.

Protecting Proprietary Code

Perl isn't compiled (although there is an effort to make it so). Thus, if you distribute your solutions, you distribute code. This can be a deterrent to producing (at least your final application) in Perl. (See the previous discussion of the benefits of Perl, in the section "Interpreted Languages," as a prototyping language.)

Concerns About Reliability

Perl, in its version 5+ incarnation, is undergoing some major changes. Things might not work or might break later. This can be a concern for the future of applications written for a specific version and relying on a specific feature. On the positive side, there are a lot of people testing each release through use. Many of these bugs are quickly detected and ironed out.

Maintainability of Scripts

Perl has somewhat of a reputation for being unreadable. This can be a problem for system maintenance. However, Perl is probably no more unreadable than any C-like language. (C itself, in my opinion, is a very un-pretty-I won't say ugly-language; Perl suffers from that heritage.)

Like with any other language, the maintainability of Perl relies heavily on the willingness of the programmer to structure and comment/document the code. Because many "quick-and-dirty" utilities are written in Perl to get a specific job done and then expanded to be more generally usable, much of the available source code isn't all that pretty. (Sounds a little like the evolution of Perl itself, doesn't it?)

GNU Copyleft License Agreement

The GNU license under which Perl is distributed is really quite innocuous. But, it might be a problem depending upon the type of application you are developing. If you intend to do any of the following, Perl is probably not the best language to use:

  • Sell the application as a packaged product
  • Distribute an application that includes trade secrets
  • Keep your programming techniques secret

What Can Perl Do?

Perl is most commonly used to develop system administration tools. But it has also gained enormous popularity on the Internet. Perl can be, and is, used to develop many Internet applications and their supporting utility applications. The following sections describe some applications of Perl in systems administration and on the Internet.

UNIX System Maintenance

As mentioned before, Perl can perform the work of several other tools, and usually in less time. It is particularly adept at processing the text files typically used as configuration files.

CGI Scripts

Perl is one of the most popular languages for creating CGI applications. There are literally thousands of examples of dynamic CGI programming in Perl. Perl can be used to create dynamic Web pages that can change depending on factors such as which visitor is viewing them.

One of the most common uses of Perl on the Internet is to process form input. Perl is especially adept at this chore because most of that input is textual-Perl's strength.

Mail Processing

Another popular use of Perl is for the automated processing of Internet e-mail. Perl scripts have been used to filter mail based on address or content. Perl scripts have also been written to automate mailing lists. One of the most popular of these programs is Majordomo.

I personally have written a Perl script to automate my "What's New?" Web page. This script processes mail messages and adds them to my "What's New?" page. It also removes the entries from the page after they have been there for a certain length of time.

Automating Web Site Maintenance

Perl can be used to automate the maintenance of Web sites. Because Web pages are little more than text files in a specific format, Perl is particularly adept at processing them. Perl's socket capability can also be used to contact other sites and request information using HTTP. There has even been a Web server written in Perl.

In order to check the links on a site, a Perl program must parse the sites pages starting with the main page, extract the URLs, and determine whether these URLs are still active.

Automating File Retrieval

There are several FTP clients written in Perl. Perl can be used to automate file retrieval via FTP. Again, this combines the socket capability of Perl with its text-processing capability.

Is Perl for You?

Only you can answer that question. The next chapters will give you a grounding in the Perl language that may help you decide whether you wish to use Perl for Internet programming. If you choose not to make it your main Web programming language, then because of its versatility, ease of use, and popularity, you may find that it becomes your utility language for the Web, if nothing else.

Summary

Perl is a practical, easy-to-use, efficient programming language. Add it to your toolbox and use it especially when you have tasks that involve text processing.

Like any programming language, Perl is not the only language you should have in your toolbox, but, when chosen for the appropriate tasks, Perl can give you the ability to solve the problem quickly.

If you're looking for a language which is beautiful, elegant, or minimal, Perl isn't for you. If, on the other hand, you're looking for a tool to get things done, few languages can compare with Perl.

A Brief Introduction to Perl


CONTENTS


This chapter offers a very brief introduction to Perl programming and syntax. If this is the first time you are working with Perl, do not despair at the barrage of information in this chapter. As you progress through the book, any new or elaborate syntax will be explained. This chapter is intended as an introduction to Perl, not a complete tutorial-you'll learn more about the advanced features of Perl in the subsequent chapters. If you are already familiar with Perl, you might want to glance through this chapter to get a quick overview of the syntax and reserved words.

Note
Please refer to the inside front cover for a quick reference of all the special variables in Perl.

Running Perl

Perl is a program just like any other program on your system, only it's more powerful than most other programs! To run Perl, you can simply type perl at the prompt and then type your code. In almost all cases, you'll want to keep your Perl code in files just like shell scripts. A Perl program is referred to as a script.

Normally, the Perl program on your machine will be located in the /usr/bin, /usr/bin/perl5, or /usr/local/bin/perl5 directory. Use a find command to see whether you can locate Perl on your system. If you are certain that you do not have Perl on your system, turn to Chapter 24, "Building and Installing the Perl 5 Interpreter," for information on how to install Perl on your machine. Perl scripts are of the following form:

#!/usr/bin/perl
... insert code here ...
# comments are text after the # mark.
                  # comments can begin anywhere on a line.

Here's a simple Perl script:

#/usr/bin/perl
print "\n Whoa! That was good!\n";

If the path to the Perl program on your system is different, you'll have to use that pathname instead of /usr/bin/perl. You also can specify programs on the command line with the -e switch to Perl. For example, entering the following command at the prompt will print Howdy!.

$ perl -e 'print "Howdy !\n";'

In all but the shortest of Perl programs, you'll use a file to store your Perl code as a script. Using a script file offers you the ease of not having to type all the commands interactively and thus not being able to correct typing errors easily. Also, a script file provides a written record of what commands to use to accomplish a certain task.

To fire off a command on all lines in the input, use -n option. Thus, the line

$perl -n 's/old/new/g' test.txt

runs the command to substitute all strings old with new on each line from the file test.txt. If you use the -p option, it prints each line as it is read in. The -v option prints the version number of Perl you are running. This book is written for Perl 5.002.

Now, let's begin the introduction to the Perl language.

Variables in Perl

Perl has three basic types of variables: scalars, arrays, and associative arrays. A scalar variable is anything that can hold one number (either as a floating point number or as an integer) or a string. An array stores many scalars in a sequence, where each scalar can be indexed using a number starting with 0 on up. An associative array is like an array in that it stores strings in sequence but uses another string as an index to address individual items instead of a number. I cover how to use these three types of variables in this chapter.

The syntax for a scalar variable is $variable_name. A variable name is set up and addressed in the same way as Bourne shell variables. To assign values to a scalar, you use statements like these:

$name = "Kamran";
$number= 100;
$phone_Number = '555-1232';

A variable in Perl is evaluated at runtime to derive a value that is one of the following: a string, a number, or a pointer to scalar. (To see the use of pointers and references, refer to Chapter 3, "References.")

To print out the value of a variable, you use a print statement. Therefore, to print the value of $name, you would make the following call:

print $name;

The value of $name is printed to the screen. Perl scripts "expect" input from a standard input (the keyboard) and to write to the standard output. Of course, you can also use the print statement to print the values of special variables that are built into Perl.

Special Variables

Table 2.1 lists the special variables in Perl. The first column contains the variable, and the second contains a verbose name that you can use to make the code readable. The third column in the table describes the contents of each variable.

You can use the verbose names (in column 2) by including the following line in the beginning of your code:

use English;

This statement will let you use the English.pm module in your code. (I cover the use of modules in Chapter 4, "Introduction to Perl Modules.") Not all Perl variables have an equivalent name in the English.pm module. The entry "n/a" in the second column indicates that there is not an English name for the variable.

Table 2.1. Special variables in Perl.
VariableEnglish Name Description
$_ $ARGThe default input and output pattern searching space
$1-$9 n/aThe subpattern from the last set of parentheses in a pattern match
$& $MATch The last pattern matched (RO)
$` $PREMATch The string preceding a pattern match (RO)
$POSTMATch The string following a pattern match (RO)
$+ $LAST_PAREN_MATch The last bracket matched in a pattern (RO)
$* $MULTILINE_MATchING Set to 1 to enable multi-line matching; set to 0 by default
$. $INPUT_LINE_NUMBER The current input line number; reset on close() call only
$/ $INPUT_RECORD_SEPARATOR The newline by default
$| $AUTO_FLUSH If set to 1, forces a flush on every write or print; 0 by default
$, $OUTPUT_FIELD_SEPARATOR Specifies what is printed between fields
$\ $INPUT_RECORD_SEPARATOR The output record separator for the print operator
$" $LIST_SEPARATOR The separator for elements within a list
$; $SUBSCRIPT_SEPARATOR The character for multidimensional array emulation
$# $FORMAT Output format for printed numbers
$% $FORMAT_PAGE_NUMBER The current page number
$= $FORMAT_LINES_PER_PAGE The number of lines per page
$- $FORMAT_LINES_LEFT The number of lines still left to draw on the page
$~ $FORMAT_NAME The name of the current format being used
$^ $FORMAT_TOP_NAME The name of the current top-of-page format
$: $FORMAT_LINE_BREAK_chARACTERS The set of characters after which a string can be broken up to fill with continuation characters
$^L $FORMAT_FORMFEED The default form feed operator
$^A $AccUMULATOR The current format line accumulator for format() lines
$? $chILD_ERROR The status from the last tilde command
$! $ERRNO The last errno value
$@ $EVAL_ERROR The Perl error message from the last eval statement
$$ $PROCESS_ID The process number of this Perl script
$< $REAL_USER_ID The real UID of this process
$> $EFFECTIVE_USER_ID The effective UID of this process
$( $REAL_GROUP_ID The real group GID of this process
$) $EFFECTIVE_GROUP_ID The effective GID of this process
$0 $PROGRAM_NAME The name of the program in $ARGV[0]
$[ n/aIndex of the first element in the array
$] $PERL_VERSION The Perl version string
$^D $DEBUGGING The current value of the debugging flag
$^F $SYSTEM_FD_MAX The maximum file descriptors in the system (RO)
$^I $INPLACE_EDIT The in-place edit extension
$^P $PERLDB The value of the internal debugger flag
$^T $BASETIME The time at which the debugged script started running
$^W $WARNING The value of the -w switch
$^X $EXECUTABLE_NAME The name of the program in $ARGV[0]
$ARGV n/aThe name of the current file while reading from the <> in a while loop
$VERSION n/aThe version number of the Perl interpreter
%ENV n/aThe hash of the environment variables for the process
%Inc n/aThe hash of filenames that have been included in the current file
%SIG n/aThe hash of all signal handlers for the current process
@ARGV n/aThe command-line arguments for the script
@EXPORT n/aThe names of all exported functions in a module
@F n/aThe command-line options used for the current program
@Inc n/aThe pathnames of places to look in for all included files
@ISA n/aThe names of all modules to search for when looking for a module

Don't worry if you do not recognize some of these strange characters. I will be covering them all in the course of this book.

Now let's see how you can use these built-in variables as well as your own variables in code.

Code Blocks

Variables and assignment statements exist in code blocks. Each code block is a section of code between two curly braces. Recognizing code blocks matters when you are concerned about the scope of influence of code on the value of a variable. (More on scope in a moment.) Code blocks are simply assignment statements enclosed between curly braces. Normally, you see code blocks in loop constructs and conditionals. It's syntactically correct to use statements like this in Perl programs:

{
print something;
print more of something;
more statements;
}

This coding style is rare and is usually done only if the programmer explicitly wants to keep some special variables within the curly braces. Usually, most of the application's code will be in one type of block, either a subroutine, loop, or conditional, with only the lines not in such blocks being those that are global to the rest of the components of the program.

Here are some examples of code blocks available in Perl:

{
# a simple code block with statements in here.
}

while(condition) {
    ... execute code here while condition is true;
}

until(condition) {  # opposite of while statement.
    ... execute code here while condition is false;
}

do {
    ... do this at least once ...
    ... stop if condition is false ...
} while(condition);

do {
    ... do this at least once ...
    ... stop if condition is true ...
} until(condition);

if (condition1) {
    condition1_code true;
} else {
...    no condition1 up to conditionN  is true;
}

if (condition1) {
...    condition1_code true;
} elsif (condition2) {
    condition1_code true;
....
} elsif (conditionN) {
    conditionN_code true;
} else {
...    no condition from 1 up to N  is true;
}

unless (condition1) { # opposite of "if" statement.
...    do this if condition is false;
}

The condition in these blocks of code is anything from a Perl variable to an expression that returns either a true or false value. A true value is a non-zero value or a non-empty string.

Code blocks can be declared within code blocks to create levels of code blocks. Variables declared in one code block are usually global to the rest of the program. To keep the scope of the variable limited to the code block in which it is declared, use the my $variableName syntax. If you declare with local $variableName syntax, the $variableName will be available to all lower levels but not outside the code block.

Figure 2.1 illustrates how the scoping rules work in Perl. The main block declares two variables, $x and $y. There are two blocks of code between curly braces, block A and block B. The variable $x is not available to either of these blocks, but $y will be available.

Scoping rules in Perl:

Because block A is declared in the main block, the code in it will be able to access $y but not $x because $x is declared as "my". The variable $f will not be available to other blocks of code even if they are declared within block A. The variable $g is not declared as "local" or "my", so it's not visible to the main module nor to block B.

The code in block B declares two variables, $k and $m. The variable $k can be assigned the value of $g, provided that the code in block A is called before the code in block B. If the code in block B is called before the code in block A, the variable $g will not be declared, and a value of 'undef' will be assigned to $k. Also, $m cannot use the value of $f because $f is declared in block A as a "my" variable. The values of $y and $g are available to code in block B.

Finally, another code block (call it C) could be assigned in block B. Block C is not shown in the figure. All variables in this new block C that are declared as neither "my" nor "local" would be available to blocks A and B and the main program. Code in block C would not be able to access variables $f, $k, and $m because they are declared as "my". The variable $g would not be available to code in block B or C because it is local to block A.

Keep in mind that variables in code blocks are also declared at the first time they are assigned a value. This creation includes arrays and strings. Variables are then evaluated by the parser when they appear in code, and even in strings. There are times when you do not want the variable to be evaluated. This is the time when you should be aware of quoting rules in Perl.

Quoting Rules

Three different types of quotes can be used in Perl. Double quotes (") are used to enclose strings. Any scalars in double-quoted strings are evaluated by Perl. To force Perl not to evaluate anything in a quote, you'll have to use single quotes ('). Anything that looks like code and is not quoted is interpreted as code by the Perl interpreter, which attempts to evaluate the code as an expression or a set of executable code statements. Finally, to run some values in a shell program and get its return value back, use the back quote (`) symbol. See the Perl script in Listing 2.1 for an example.


Listing 2.1. Quoting in a Perl script.
1 #!/usr/bin/perl
2 $folks="100";
3 print "\$folks = $folks \n";
4 print '\$folks = $folks \n';
5 print "\n\n BEEP! \a  \LSOME BLANK \ELINES HERE \n\n";
6 $date = `date +%D`;
7 print "Today is [$date] \n";
8 chop $date;
9 print "Date after chopping off carriage return: [".$date."]\n";

The output from the code in Listing 2.1 is as follows:

$folks = 100
\$folks = $folks \n

BEEP!  some blank LINES HERE

Today is [03/29/96
]
Date after chopping off carriage return: [03/29/96]

Let's go over the code shown in Listing 2.1. First of all, note that the actual listing did not have line numbers. The line numbers in this and subsequent scripts are used to identify specific lines of code.

Line 1 is the mandatory first line of the Perl script. Change the path shown in Listing 2.1 to where your Perl interpreter is located if the script does not run. Be sure to make a similar change to the rest of the source listings in this book.

Line 2 assigns a string value to the $folks variable. Note that you did not have to declare the variable $folks because it was created when used for the first time.

Line 3 prints the value of $folks in between double quotes. The $ sign in $folks has to be escaped with a backslash to prevent Perl from evaluating the value of $folks instead of printing the following line:

$folks = 100

In line 4, Perl does not evaluate anything between the single quotes. Therefore, the entire contents of the line are left untouched and printed here:

\$folks = $folks \n

Perl has several special characters to format text data for you. Line 5 prints multiple blank lines with the \n character and beeps at the terminal. Notice how the words SOME BLANK are printed in lowercase letters. This is because they are encased between the \L and \E special characters, which force all characters to be lowercase. Some of these special characters are listed in Table 2.2.

Table 2.2. Special characters in Perl.
CharacterMeaning
\\ Backslash.
\0ooo Octal number in ooo (for example, \0213).
\a Beep.
\b Backspace.
\c Inserts the next character literally (for example, \$ puts $).
\cC Inserts control character C.
\l Next character is lowercase.
\L \E All characters between \L and \E are lowercase.
\n New line (line feed).
\r Carriage return (MS-DOS).
\t Tab.
\u Next character is uppercase.
\U \E All characters between \U and \E are uppercase.
\x## Hex number in ## (for example, \x1d).

In line 6, the script uses the back quotes (`) to execute a command and return the results in the $date variable. The string in between the two back quotes is what you would type at the command line, with one exception: if you use Perl variables in the command line for the back quotes, Perl evaluates these variables before passing them off to the shell for execution. For example, line 6 could be rewritten as this:

$parm = "+%D";
$date = `$date $parm`;

The returned value in $date is printed out in line 7. Note that there is an extra carriage return in the text for data. To remove it, use the chop command as shown in line 8.

Then in line 9 the $date output is shown to print correctly. Note how the period (.) is used to concatenate three strings together for the output.

It's easy to construct strings in Perl with the period (.) operator. Given two strings, $first and $last, you can construct the string $fullname like this to get "Jim Smith":

$first = "Jim";
$last = "Smith";
$fullname = $first . " " . $last;

Numbers in Perl are stored as floating-point numbers; even variables used as integers are really stored as floating point numbers. There are a set of operations you can do with numbers. These operations are listed in Table 2.3. The table also lists Boolean operators.

Table 2.3. Numeric operations with Perl.
OperationDescription
$r = $x + $y Adds $x to $y and assigns the result to $r
$r = $x - $y Subtracts $y from $x and assigns the result to $r
$r = $x * $y Multiplies $y and $x and assigns the result to $r
$r = $x / $y Divides $x by $y and assigns the result to $r
$r = $x % $y Modulo; divides $x by $y and assigns the remainder to $r
$r = $x ** $y Raises $x to the power of $y and assigns the result to $r
$r = $x << $n Shifts bits in $x left $n times and assigns to $r
$r = $x >> $n Shifts bits in $x right $n times and assigns to $r
$r = ++$x Increments $x and assigns $x to $r
$r = $x++ Assigns $x to $r and then increments $x
$r += $x; Adds $x to $r and then assigns to $r
$r = --$x Decrements $x and assigns $x to $r
$r = $x-- Assigns $x to $r and then decrements $x
$r -= $x; Subtracts $x from $r and then assigns to $r
$r /= $x; Divides $r by $x and then assigns to $r
$r *= $x; Multiplies $r by $x and then assigns to $r
$r = $x <=> $y $r is 1 if $x > $y; 0 if $x == $y; -1 if $x < $y
$r = $x || $y $r is the logical OR of variables $x and $y
$r = $x && $y $r is the logical AND of variables $x and $y
$r = ! $x $r is the opposite Boolean value of $x

You can compare values of variables to check results of operations. Table 2.4 lists the comparison operators for numbers and strings.

Table 2.4. Comparison operations with Perl.
OperationDescription
$x == $y True if $x is equal to $y
$x != $y True if $x is not equal to $y
$x < $y True if $x is less than $y
$x <= $y True if $x is less than or equal to $y
$x > $y True if $x is greater than $y
$x >= $y True if $x is greater than or equal to $y
$x eq $y True if string $x is equal to string $y
$x ne $y True if string $x is not equal to string $y
$x lt $y True if string $x is less than string $y
$x le $y True if string $x is less than or equal to string $y
$x gt $y True if string $x is greater than string $y
$x ge $y True if string $x is greater than or equal to string $y
$x x $y Repeats $x, $y times
$x . $y Returns the concatenated value of $x and $y
$x cmp $y Returns 1 if $x gt $y; 0 if $x eq $y; -1 if $x lt $y
$w ? $x : $y Returns $x if $w is true; $y if $w is false

Arrays and Associative Arrays

Perl has arrays to let you group items using a single variable name. Perl offers two types of arrays: those whose items are indexed by number (arrays) and those whose items are indexed by a string (associative arrays). An index into an array is referred to as the subscript of the array.

Tip
An associative array is referred to as "hash" because of the way it's stored internally in Perl.

Arrays are referred to with the @ symbol. Individual items in an array are derived with a $ and the subscript. Therefore, the first item in an array @count would be $count[0], the second item would be $count[1], and so on. See Listing 2.2 for usage of arrays.


Listing 2.2. Using arrays.
 1 #!/usr/bin/perl
 2 #
 3 # An example to show how arrays work in Perl
 4 #
 5 @amounts = (10,24,39);
 6 @parts = ('computer', 'rat', "kbd");
 7
 8 $a = 1; $b = 2; $c = '3';
 9 @count = ($a, $b, $c);
10
11 @empty = ();
12
13 @spare = @parts;
14
15 print '@amounts = ';
16 print "@amounts \n";
17
18 print '@parts = ';
19 print "@parts \n";
20
21 print '@count = ';
22 print "@count \n";
23
24 print '@empty = ';
25 print "@empty \n";
26
27 print '@spare = ';
28 print "@spare \n";
29
30
31 #
32 # Accessing individual items in an array
33 #
34 print '$amounts[0] = ';
35 print "$amounts[0] \n";
36 print '$amounts[1] = ';
37 print "$amounts[1] \n";
38 print '$amounts[2] = ';
39 print "$amounts[2] \n";
40 print '$amounts[3] = ';
41 print "$amounts[3] \n";
42
43 print "Items in \@amounts  = $#amounts \n";
44 $size = @amounts; print "Size of Amount  = $size\n";
45 print "Item 0 in \@amounts = $amounts[$[]\n";
46

Here's the output from Listing 2.2:

@amounts = 10 24 39
@parts = computer rat kbd
@count = 1 2 3
@empty =
@spare = computer rat kbd
$amounts[0] = 10
$amounts[1] = 24
$amounts[2] = 39
$amounts[3] =
Items in @amounts  = 2
Size of Amount  = 3
Item 0 in @amounts = 10

In line 5, three integer values are assigned to the @amounts array. In line 6, three strings are assigned to the @parts array. In line 8, the script assigns both string and numeric values to variables and then assigns the values of the variables to the @count array. An empty array is created in line 11. In line 12, the @spare array is assigned the same values as those in @parts.

Lines 15 through 28 print out the first five lines of the output. In lines 34 to 41, the script addresses individual items of the @amounts array. Note that $amounts[3] does not exist; therefore, it is printed as an empty item.

The @#array syntax is used in line 43 to print the last index in an array, so the script prints 2. The size of the amounts array is ($#amounts + 1). If an array is assigned to a scalar, as shown in line 44, the size of the array is assigned to the scalar.

Line 45 shows the use of a special Perl variable, $[, which is the base subscript (0) of an array.

What Are Associative Arrays?

An associative array is really an array with two items per index. The first item at each index is called a key and the other item is called a value. You index into an associative array using keys to get values. An associative array name is preceded with a percent (%) sign and indexed items are enclosed within curly braces ({}). See Listing 2.3 for some sample uses of associative arrays.


Listing 2.3. Using associative arrays.
 1 #!/usr/bin/perl
 2 #
 3 # Associative Arrays.
 4 #
 5
 6 %subscripts = (
 7      'bmp', 'Bitmap',
 8      "cpp", "C++ Source",
 9      "txt", 'Text file' );
10
11 $bm = 'asc';
12 $subscripts{$bm} = 'Ascii File';
13
14 print "\n =========== Raw dump of hash  ========= \n";
15 print %subscripts;
16
17 print "\n =========== using foreach  ========= \n";
18 foreach $key (keys (%subscripts)) {
19     $value = $subscripts{$key};
20     print "Key = $key, Value = $value \n";
21     }
22
23 print "\n === using foreach with sort ========= \n";
24 foreach $key (sort keys (%subscripts)) {
25     $value = $subscripts{$key};
26     print "Key = $key, Value = $value \n";
27     }
28
29 print "\n =========== using each()  ========= \n";
30 while (($key,$value) = each(%subscripts)) {
31     print "Key = $key, Value = $value \n";
32     }
33

Here's the output from Listing 2.3:

=========== Raw dump of hash  =========
txtText filecppC++ SourceascAscii FilebmpBitmap
=========== using foreach  =========
Key = txt, Value = Text file
Key = cpp, Value = C++ Source
Key = asc, Value = Ascii File
Key = bmp, Value = Bitmap

=== using foreach with sort =========
Key = asc, Value = Ascii File
Key = bmp, Value = Bitmap
Key = cpp, Value = C++ Source
Key = txt, Value = Text file

=========== using each()  =========
Key = txt, Value = Text file
Key = cpp, Value = C++ Source
Key = asc, Value = Ascii File
Key = bmp, Value = Bitmap

An associative array called %subscripts is created in line 6 to line 9. Three items of (key,value) pairs are added to %subscripts as a list. At line 11, a new item is added to the %subscript array by assigning $bm to a key and then using $bm as the index. We could have just as easily added the string 'Ascii File' with this hard-coded statement:

$subscripts{'asc'} = 'Ascii File';

Items in an associative array are referred to as items stored in a hash, because this is the way items are stored internally. Look at the output from line 15, which dumps out the associative array items.

In line 17, the script uses a foreach statement to loop over the keys in the %subscripts array. The keys() function returns a list of keys for a given hash. The value of the item at $subscripts{$key} is assigned to $value at line 19. You could combine lines 18 and 19 into one statement like this without loss of meaning:

print "Key = $key, Value = $subscripts{$key} \n";

Using the keys alone did not list the contents of the %subscripts hash in the order we want. To sort the output, you should sort the keys into the hash. This is shown in line 24. The sort() function takes a list of items and returns a text-sorted version. The foreach function takes the output from the sort() function applied to the value returned by the keys() function. To sort in decreasing order, you can apply the reverse function to the returned value of sort() to get this line:

for $i (reverse sort (keys %@array)) {

It's more efficient to use the each() function when working with associative arrays because only one lookup is required per item to get both the key and its value. See Line 30 where the ($key,$value) pairs are assigned to the returned values by the each() command. The variable $key is assigned to the first item, and the variable $value is assigned to the second item that is returned from the each() function call.

The code in line 30 is important and deserves some explaining. First of all, the while() loop is used here. The format for a while loop is defined as this:

while( conditionIsTrue) {
    codeInLOOP
}

codeOutOfLOOP

If the condition in the while loop is a nonzero number, a nonempty string, or a nonempty list, the code in the area codeInLOOP is executed. Otherwise, the next statement outside the loop (that is, after the curly brace) is executed.

Second, look at how the list ($key,$value) is mapped onto the list returned by the each() function. The first item of the returned list is assigned to $key, the next item to $value. This is part of the array-slicing operations available in Perl.

Array Operations

When working with arrays in Perl, you are really working with lists. You can add or remove items from the front or back of the list. Items in the middle of the list can be indexed using subscripts or keys. Sublists can be created by extracting items from lists, and lists can be concatenated to create one or more new lists.

Let's view some examples of how they fit together. See Listing 2.4, which uses some of these concepts.


Listing 2.4. Array operations.
 1 #!/usr/bin/perl
 2 #
 3 # Array operations
 4 #
 5
 6 $a = 'RFI';
 7 $b = 'UPS';
 8 $c = 'SPIKE';
 9
10 @words = ('DC','AC','EMI','SURGE');
11
12 $count = @words;  # Get the count
13
14 #
15 # Using the for operator on a list
16 #
17 print "\n \@words = ";
18 for $i (@words) {
19     print "[$i] ";
20     }
21
22 print "\n";
23 #
24 # Using the for loop for indexing
25 #
26 for ($i=0;$i<$count;$i++) {
27     print "\n Words[$i] : $words[$i];";
28     }
29 #
30 # print 40 equal signs
31 #
32 print "\n";
33 print "=" x 40;
34 print "\n";
35 #
36 # Extracting items into scalars
37 #
38 ($x,$y) = @words;
39 print "x = $x, y = $y \n";
40 ($w,$x,$y,$z) = @words;
41 print "w = $x, x = $x, y = $y, z = $z\n";
42
43 ($anew[0], $anew[3], $anew[9], $anew[5]) = @words;
44
45 $temp = @anew;
46
47 #
48 # print 40 equal signs
49 #
50 print "=" x 40;
51 print "\n";
52
53 print "Number of elements in anew = ". $temp, "\n";
54 print "Last index in anew = ". $#anew, "\n";
55 print "The newly created Anew arrary is: ";
56 $j = 0;
57 for $i (@anew) {
58     print "\n \$anew[$j] = is $i ";
59     $j++;
60     }
61 print "\n";
62
63

Here's the output from Listing 2.4:

 @words = [DC] [AC] [EMI] [SURGE]

 Words[0] : DC;
 Words[1] : AC;
 Words[2] : EMI;
 Words[3] : SURGE;
========================================
x = DC, y = AC
w = AC, x = AC, y = EMI z = SURGE
========================================
Number of elements in anew = 10
Last index in anew = 9
The newly created Anew arrary is:
 $anew[0] = is DC
 $anew[1] = is
 $anew[2] = is
 $anew[3] = is AC
 $anew[4] = is
 $anew[5] = is SURGE
 $anew[6] = is
 $anew[7] = is
 $anew[8] = is
 $anew[9] = is EMI

Lines 6, 7, and 8 assign values to scalars $a, $b, and $c, respectively. In line 10, four values are assigned to the @words array. At line 12, you get a count of the number of elements in the array.

The for() loop statement is used to cycle through each element in the list. Perl takes each item in the @words array, assigns it to $i, and then executes the statements in the block of code between the curly braces. You could rewrite line 17 as the following and get the same result:

for $i ('DC','AC','EMI','SURGE') {

In the example in Listing 2.4, the value of each item is printed with square brackets around it. Line 22 simply prints a new line.

Now look at line 26, where the for loop is defined. The syntax in the for loop will be very familiar to C programmers:

for (startingCondition; endingCondition; at_end_of_every_loop) {
        execute_statements_in_this_block;
    }

In line 26, $i is set to zero when the for loop is started. Before Perl executes the next statement within the block, it checks to see whether $i is less than $count. If $i is less than $count, the print statement is executed. If $i is greater than or equal to $count, the next statement following the ending curly brace is executed. After executing the last statement in a for loop code block (see line 28), Perl increments the value of $i with the statement for the end of loop: $i++. So $i is incremented. Perl goes back to the top of the loop to test for the ending condition to see what to do next.

In lines 32 through 34, an output-delimiting line is printed with 40 equal signs. The x operator in line 33 causes = to be repeated by the number following it. Another way to print a somewhat fancier line would be to use the following in lines 32 through 34:

32 print "[\n";
33 print "-=" x 20;
34 print "]\n";

Next, in line 38 the first two items in @words are assigned to variables $x and $y, respectively. The rest of the items in @words are not used. In line 40, four items from @words are assigned to four variables. The mapping of items from @words to variables is done on a one-to-one basis, based on the type of parameter on the left side of the equal sign.

Had I used the following line in place of line 40, I would get the value of $words[0] in $x and the rest of @words in @sublist:

($x,@sublist) = @words;

In line 43 a new array, @anew, is created and assigned values from the @words array, but not on a one-to-one basis. In fact, you'll see that the @anew array is not even the same size as @words. Perl automatically resizes the @anew array to be at least as large the largest index. In this case, because $anew[9] is being assigned a value, @anew will be at least 10 items long to cover items from 0 to 9.

In lines 53 and 54, the script prints out the value of the number of elements in the array and the highest valid index in the array. Lines 57 through 60 print out the value of each item in the anew area. Notice that items in the @anew array are not assigned any values.

You can create other lists from lists, as well. See the example in Listing 2.5.


Listing 2.5. Creating sublists.
 1 #!/usr/bin/perl
 2 #
 3 # Array operations
 4 #
 5
 6 $a = 'RFI';
 7 $b = 'UPS';
 8 $c = 'SPIKE';
 9
10 @words = ('DC','AC','EMI','SURGE');
11
12 $count = @words;  # Get the count
13 #
14 # Using the for operator on a list
15 #
16 print "\n \@words = ";
17 for $i (@words) {
18     print "[$i] ";
19     }
20
21 print "\n";
22 print "=" x 40;
23 print "\n";
24
25 #
26 # Concatenate lists together
27 #
28 @more = ($c,@words,$a,$b);
29 print "\n  Putting a list together: ";
30 $j = 0;
31 for $i (@more) {
32     print "\n \$more[$j] = is $i ";
33     $j++;
34     }
35 print "\n";
36
37 @more = (@words,($a,$b,$c));
38 $j = 0;
39 for $i (@more) {
40     print "\n \$more[$j] = is $i ";
41     $j++;
42     }
43 print "\n";
44
45
46 $fourth = ($a x 4);
47 print " $fourth\n";

Here's the output from Listing 2.5:

 @words = [DC] [AC] [EMI] [SURGE]
========================================

  Putting a list together:
 $more[0] = is SPIKE
 $more[1] = is DC
 $more[2] = is AC
 $more[3] = is EMI
 $more[4] = is SURGE
 $more[5] = is RFI
 $more[6] = is UPS

 $more[0] = is DC
 $more[1] = is AC
 $more[2] = is EMI
 $more[3] = is SURGE
 $more[4] = is RFI
 $more[5] = is UPS
 $more[6] = is SPIKE

 RFIRFIRFIRFI

In Listing 2.5, one list is created from another list. In Line 10, the script creates and fills the @words array. In Lines 16 through 19, the script prints the array. Lines 21 through 23 are repeated again (which we will convert into a subroutine soon).

At line 28, the @more array is created by placing together the value of $c, all the items in the entire @words array, followed by the values $a and $b. The size of the @more array will therefore be 6. The items in the @more array are printed in lines 31 through 35.

The code at line 37 creates another @more array with a different ordering. The previously created @more array is freed back to the memory pool. The newly ordered @more list is printed from lines 40 through 43.

The script then uses the x operator in line 46 to create another item by concatenating four copies of $a into the variable $fourth.

I have covered how to add items to arrays but not how to remove them. To remove an item from an array, use the delete command on an array item. For example, to delete $more[2], you would use the command:

delete $more[2];

If you are like me, you probably do want to type the same lines of code again and again. For example, the code in lines 21 through 23 of Listing 2.5 could be made into a function that looks like this:

sub printLine {
  print "\n";
  print "=" x 40;
  print "\n";
}

Now when you want print the lines, call the subroutine with this line of code:

&printLine;

I cover other aspects of subroutines in the section "Subroutines" of this chapter, and a bit more in Chapter 3.

Now let's get back to some of the things you can do with arrays using the functions supplied with Perl. See Listing 2.6 for a script that uses the array functions I discuss here.


Listing 2.6. Using array functions.
 1 #!/usr/bin/perl
 2 #
 3 # Functions for Arrays
 4 #
 5 sub printLine {
 6 print "\n"; print "=" x 60; print "\n";
 7 }
 8
 9 $quote= 'Listen to me slowly';
10
11 #
12 # USING THE SPLIT function
13 #
14 @words = split(' ',$quote);
15
16 #
17 # Using the for operator on a list
18 #
19 &printLine;
20 print "The quote from Sam Goldwyn: $quote ";
21 &printLine;
22 print "The words \@words = ";
23 for $i (@words) {
24     print "[$i] ";
25     }
26
27 #
28 # chOP
29 #
30 &printLine;
31 chop(@words);
32 print "The chopped words \@words = ";
33 for $i (@words) {
34     print "[$i] ";
35     }
36 print "\n .. restore";
37 #
38 # Restore!
39 #
40 @words = split(' ',$quote);
41
42 #
43 # Using PUSH
44 #
45 @temp = push(@words,"please");
46 &printLine;
47 print "After pushing \@words = ";
48 for $i (@words) {
49     print "[$i] ";
50     }
51
52 #
53 # USING POP
54 #
55 $temp = pop(@words);  # Take the 'please' off
56 $temp = pop(@words);  # Take the 'slowly' off
57 &printLine;
58 print "Popping twice \@words = ";
59 for $i (@words) {
60     print "[$i] ";
61     }
62 #
63 # SHIFT from the front of the array.
64 #
65 $temp = shift @words;
66 &printLine;
67 print "Shift $temp off, \@words= ";
68 for $i (@words) {
69     print "[$i] ";
70     }
71 #
72 # Restore words
73 #
74 @words = ();
75 @words = split(' ',$quote);
76 &printLine;
77 print "Restore words";
78 #
79 # SPLICE FUncTION
80 #
81 @two = splice(@words,1,2);
82 print "\n Words after splice = ";
83 for $i (@words) {
84     print " [$i]";
85     }
86 print "\n Returned from splice = ";
87 for $i (@two) {
88     print " [$i]";
89     }
90 &printLine;
91
92 #
93 # Using the join function
94 #
95 $joined = join(":",@words,@two);
96 print "\n Returned from join = $joined ";
97 &printLine;

The split() function is used in line 14 to split the items in the string $quote into the @words array.

Next, the script uses chop() on a list. This function removes a character from a string. When applied to an array, chop() removes a character from each item on the list. See lines 31 through 35.

You can add or delete items from an array using the pop(@Array) or push(@Array) functions. The pop() function removes the last item from a list and returns it as a scalar. Look at the push(ARRAY,LIST); call to add items to a list. The push() function takes an array as the first parameter and treats the rest of the parameters as items to place at the end of the array. At line 45, the push() function pushes the word please into the back of the @words array. In lines 55 and 56, two words are popped off the @words list. The size of the array @words changes with each command.

Let's look at how the shift() function is used in line 67. The shift(ARRAY) function returns the first element of an array. The size of the array is decreased by 1. You can use shift() in one of three ways:

shift (@mine); # return first item of @mine
shift @mine; # return first item of @mine
shift; # return first item in @ARGV

The special variable @ARGV is the argument vector for your Perl program. The number of elements in @ARGV is easily found by assigning a scalar to $ARGC that is equal to @#ARGV before any operations are applied to @ARGV.

Then, after restoring @words to its original value, the script uses the splice() function to remove items from the @words array. The splice() function is a very important function and is really the key behind the pop(), push(), and shift() functions. Here's the syntax for the splice() function:

splice(@array,$offset,$length,$list)

The splice() function returns the items removed in the form of a list. It replaces the $length items in @array starting from $offset with the contents of $list. If you leave out the $list parameter and just use splice(@array,$offset,$length), nothing is inserted in the original array. Any removed items are returned from splice(). If you leave out the $length parameter to splice() and use it as splice(@array,$offset), the value of $length is used to determine the number of the @array to use starting from the offset.

File Handles and Operators

Now that I have covered basic array and numeric operations, let's cover some of the input/output operations where files are concerned. A Perl program has three file handles when it starts up: STDIN (for standard input), STDOUT (for standard output), and STDERR (for standard error message output). Note the use of capitals and the lack of a dollar ($) sign to signify that these are file handles. For a C/C++ programmer, the three handles are akin to stdin, stdout, and stderr.

To open a file for I/O you have to use the open statement. Here's the syntax for the open call:

open(HANDLE, $filename);

HANDLE is then used for all the operations on a file. To close a file, you use the function close HANDLE;.

For writing text to a file given a handle, you can use the print() statement to write to the file:

print HANDLE $output;

The HANDLE defaults to STDIN if no handle is specified. To read one line from the file given a HANDLE, you use the <> operators:

$line = <HANDLE>

In this code, $line will be assigned all the input until a carriage return or eof. When writing interactive scripts, you normally use the chop() function to remove the end-of-line character. To read from the standard input into a variable $response, you use these statements in sequence:

$response = <STDIN>;
chop $response; # remove offensive carriage return.

You can perform binary read and write operations on a file using the read() and write() functions. Here's the syntax for each type of function:

read(HANDLE,$buffer,$length[,$offset]);
write(HANDLE,$buffer,$length[,$offset]);

The read function is used to read from HANDLE into $buffer, up to $length bytes from the $offset in bytes from the start of the file. The $offset is optional, and read() defaults reading to the current location in the file if $offset is left out. The location in the file to read from is advanced $length bytes. To check if you have reached the end of the file, use the command:

eof(HANDLE);

A nonzero value returned signifies the end of the file; a zero returned indicates that there is more to read in the file.

The write function is used to write the contents of $buffer to HANDLE. The number of bytes to write is set in $length. The location to write at the handle is set in the variable $offset as the number of bytes from the start of the file. The $offset is optional, and write() defaults writing to the current location in the file if $offset is left out. The location in the file written to is advanced $length bytes.

You can move to a position in the file using the seek() function:

seek(HANDLE,$offset,$base)

The $offset is from the location specified in $base. The seek function behaves exactly like the C function call in that if $base is 0, the $offset is from the start of the file. If $base is set to 1, the program uses the current location of the file pointer. If $base is $2, the program uses an offset from the end of the file where the value of $offset is negative.

There can be errors associated with opening files. It's a good idea to see what the errors are before proceeding further in a program. To print error messages before a script crashes, the die function is used. A call to open a file called test.data would like this:

open(TESTFILE,"test.data") || die "\n $0 Cannot open $! \n";

This line literally reads Open test.data for input or die if you cannot open it. The $0 is the Perl special variable for the process name, and the special variable $! is set to a string corresponding to the value of the system variable, errno.

The syntax in the string used for the filename also signifies the type of operation you intend to perform with the file. Table 2.5 shows some of the ways you can open a file.

Table 2.5. File open types.
FileAction
test.data Opens test.data for reading. The file must exist.
>test.data Opens test.data for writing. Creates the file if it does not exist and destroys any previous file called test.data.
>>test.data Opens test.data for writing. Creates the file if it does not exist and appends to any existing file called test.data.
+>test.data Opens test.data for reading and writing. Creates the file if it does not exist.
| cmd Opens a pipe to write to. (Chapter 14, "Signals, Pipes, FIFOs, and Perl," covers pipes.)
cm | Opens a pipe to read from.

When working with multiple files, you can have more than one unique handle to write to or read from. Use the select HANDLE; call to set the default file handle to use with print statements. For example, suppose you have two file handles, LARRY and CURLY; here's how to switch between handles:

select LARRY;
print "Whatsssa matter?\n"; # write to LARRY
select CURLY;
print "Whoop, whoop, whoop!"; # write to CURLY
select LARRY;
print "I oughta.... "; # write to LARRY again

Of course, by explicitly stating the handle name you could get the same result with these three lines of code:

print LARRY "Whatsssa matter?\n"; # write to LARRY
print CURLY "Whoop, whoop, whoop!"; # write to CURLY
print LARRY "I oughta.... "; # write to LARRY again

This is a very brief introduction to using file handles in Perl. I cover the use of file handles throughout the rest of this book, so don't worry if this pace of information is too quick. You'll see plenty of examples throughout the book.

You can also check for the status of a file given a filename. The available tests are listed in the source test file shown in Listing 2.7.


Listing 2.7. Testing file parameters.
 1 #!/usr/bin/perl
 2
 3 $name = "test.txt";
 4 print "\nTesting flags for $name \n";
 5 print "\n========== Effective User ID tests ";
 6 print "\n is readable" if ( -r $name);
 7 print "\n is writable" if ( -w $name);
 8 print "\n is executable" if ( -x $name);
 9 print "\n is owned " if ( -o $name);
10 print "\n========== Real User ID tests ";
11 print "\n is readable" if ( -R $name);
12 print "\n is writable" if ( -W $name);
13 print "\n is executable" if ( -X $name);
14 print "\n is owned by " if ( -O $name);
15
16 print "\n========== Reality Checks ";
17 print "\n exists " if ( -e $name);
18 print "\n has zero size " if ( -z $name);
19 print "\n has some bytes in it " if ( -s $name);
20
21 print "\n is a file " if (-f $name);
22 print "\n is a directory " if (-d $name);
23 print "\n is a link " if (-l $name);
24 print "\n is a socket " if (-S $name);
25 print "\n is a pipe " if (-p $name);
26
27 print "\n is a block device " if (-b $name);
28 print "\n is a character device " if (-c $name);
29
30 print "\n has setuid bit set " if (-u $name);
31 print "\n has sticky bit set " if (-k $name);
32 print "\n has gid bit set " if (-g $name);
33
34 print "\n is open to terminal " if (-t $name);
35 print "\n is a Binary file " if (-B $name);
36 print "\n is a Text file " if (-T $name);
37
38 printf "\n";

Working with Patterns

Perl has a very powerful regular expression parser as well as a powerful string search-and-replace function. To search for a substring, you use the following syntax (normally within an if block):

if ($a =~ /"menu"/) {
    print "\n Found menu in $a! \n";
}

The value in $a is the number of matched strings. To search in a case-insensitive manner, use an i at the end of the search statement, like this:

if ($a =~ /"mEnU"/i) {
    print "\n Found menu in $a! \n";
}

You can even search for items in an array. For example, if $a was an array @a, the returned value from the search operation is an array with all the matched strings. If you do not specify the @a =~ portion, Perl uses the $_ default name space to search on.

To search and replace strings, use the following syntax:

$expr =~ s/"old"/"new"/gie

The g, i, and e are optional parameters. If g is not specified, only the first match to the old string will be replaced with new. The i flag specifies a case-insensitive search, and e forces Perl to use the new string as a Perl expression. Therefore, in the following example, the value of $a will be "HIGHWAY":

$a = "DRIVEWAY";
$a =~ s/"DRIVE"/"HIGH"/
print $a;

Perl has a grep() function that is very similar the grep function in UNIX. Perl's grep function takes a regular expression and a list. The return value from grep can be handled one of two ways: if assigned to a scalar, it's the number of matches found, or if assigned to a list, it's a sublist of all the items found via grep.

Please check the man pages for using grep. Some of the main types of predefined patterns are shown in the following list:

Code Pattern
*Zero or more of the previous pattern
+One or more of the previous pattern
.Any character
?Zero or one of the previous pattern
\0 Null
\000 Octal
\cX ASCII control character
\d Digits [0-9]
\D Anything but digits
\f Formfeed
\n Newline
\r Carriage return
\s Space or tab or return or newline
\S Anything but \s
\t Tab
\w [0-9a-zA-Z]
\W Anything but \w
\X00 Hex

Perl uses a special variable called $_. This is the default variable to use in Perl if you do not explicitly specify a variable name and Perl expects a variable. For example, in the grep() function, if you omit LIST, grep() will use the string in the variable $_. The $_ variable is Perl's default string in which to search, assign input, or read for data for a number.

Subroutines

Perl 5 supports subroutines and functions with the sub command. You can use pointers to subroutines, too. Here's the syntax for subroutines:

sub Name {

}

The ending curly brace does not require a semicolon to terminate it. If you are using a reference to a subroutine, it can be declared without a Name, as shown here:

$ptr = sub {

};

Note the use of the semicolon to terminate the end of the subroutine. To call this function, you use the following line:

&\$ptr(argument list);

Parameters to subroutines are passed in the @_ array. To get the individual items in the array, you can use $_[0], $_[1], and so on. You can define your own local variables with the local keyword. Here's an example:

sub sample {
local ($a, $b, @c, $x) = @_
    &lowerFunc();
}

In this subroutine, you'll find that $a = $_[0], $b = $_[1], and @c point to the rest of the arguments as one list with $x empty. Generally, an array is the last assignment in such an assignment because it chews up all your parameters.

The local variables will all be available for use in the lowerFunc() function. To hide $a, $b, @c, and $x from lowerFunc, use the my keyword like this:

my ($a, $b, @c, $x) = @_

Remember, $x is empty. Now, the code in lowerFunc() is not be able to access $a, $b, @c, or $x.

Parameters in Perl can be in form, from the looks of it. Since Perl 5.002, you can define
prototypes for subroutine arguments with the following syntax:

sub   Name (parameters) {

}

If the parameters are not what the function expects, Perl bails out with an error. The parameter format is as follows: $ for a scalar, @ for an array, % for a hash, & for a reference to a subroutine, and * for anything. Therefore, if you want your function to accept only three scalars, you would declare it as this:

sub func1($$$) {
    my ($x,$y,$z) = @_;
    code here
}

To pass the value of an array by reference (by pointer), you would use a backslash (\). If you pass two arrays without the backslash specifier, the contents of the two arrays will be concatenated into one long array in @_. The function prototype to pass three arrays, a hash, and the rest in an array, would look like this:

sub func2(\@\@\@\%@)

The returned value from a subroutine is always the value of the last expression executed in the statement. The value can be a scalar, array, hash, or reference to an array.

A Final Note

The Perl distribution comes with two programs: a2p to convert awk programs to Perl, and s2p to convert sed programs to Perl. It's often convenient to write a sed script or an awk program to do a certain task. To see how to do the same thing in Perl, run the a2p or s2p program. For example, to convert mine.awk to mine.pl, you use the following command:

$ a2p mine.awk > mine.pl

Summary

This chapter has been a whirlwind introduction to Perl. I must admit that this chapter does not cover every aspect of Perl programming basics. As you progress through the book, you'll learn more ways to do things than are described here. Even if you are new to Perl, you should not have any problems understanding how to use Perl because the programming paradigms in Perl are not that different from any other programming language.

For more information, consult the following books:

  • Teach Yourself Perl 5 in 21 Days, Dave Till, 0-672-30894-0, Sams Publishing, 1995.
  • Learning Perl, Randall Schwartz, 1-56592-042-2, O'Reilly & Associates, 1993.
  • Programming Perl, Larry Wall and Randall Schwartz, 0-937175-64-1, O'Reilly & Associates, 1990.

Chapter 3

References


CONTENTS


This chapter describes the use of Perl references and the concept of pointers. It also shows you how to use references to create fairly complex data structures and pass pointers, as well as how to use pointers to subroutines and to pass parameters.

Introduction to References

A reference is simply a pointer to something; it is very similar to the concept of a pointer in C or PASCAL. That something could be a Perl variable, array, hash, or even a subroutine. A reference in your program is simply an address to a value. How you use the value of that reference is really up to you as the programmer and what the language lets you get away with. In Perl, you can use the terms pointer and reference interchangeably without any loss of meaning.

There are two types of references in Perl 5 with which you can work: symbolic and hard.

A symbolic reference simply contains the name of a variable. Symbolic references are useful for creating variable names and addressing them at runtime. Basically, a symbolic reference is like the name of a file or a soft link on a UNIX system. Hard references are more like hard links in the file system; that is, a hard link is merely another path to the same file. In Perl, a hard reference is another name for a data item.

Hard references in Perl also keep track of the number of references to items in an application. When the reference count becomes zero, Perl automatically frees the item being referenced. If that item happens to be a Perl object, the object is "destructed," that is, freed to the memory pool. Perl is object-oriented in itself because everything in a Perl application is an object, including the main package. When the main package terminates, all other objects within the main object are also terminated. Packages and modules in Perl further the ease of use of objects in Perl. Perl modules are covered in Chapter 4, "Introduction to Perl Modules."

When you use a symbolic reference that does not exist, Perl creates the variable for you and uses it. For variables that already exist, the value of the variable is substituted instead of the $variable token. This substitution lets you construct variable names from variable names.

Consider the following example:

$lang = "java";
$java = "coffee";

print "${lang}\n";
print "hot${lang}\n";
print "$$lang \n"

The third print line is important. $$lang is first reduced to $java, then the Perl interpreter will recognize that $java can also be reparsed, and the value of $java, "coffee", is used.

Symbolic references are created via the ${} construct, so ${lang} translates to java, and hot${java} translates to hotjava. If you want to address a variable name hotjava, you could use the statement: ${hot${lang}}. This would be interpreted as, "take the value in $lang, and append it to the word hot. Now take the constructed string (hotjava) and use it as a name because there is a ${} around it."

In other words, the value of the scalar produced by $$lang is taken to be the name of a new variable, and the variable at $java is used. Here's the output from this example:

java
hotjava
coffee

Thus, the difference between a hard reference ($lang) and a symbolic reference ($$lang) is how the variable name is derived. In a hard reference, you are referring to a variable's value directly. With a symbolic reference, you are using another level of indirection by constructing or deriving a symbol name from an existing variable.

References are easy to use in Perl as long as they are used as scalars. To use hard references as anything but scalars, you have to explicitly dereference the variable and tell it how to be used.

Using References

A scalar value in this chapter refers to a variable, such as $pointer, that contains one data item. This item is a scalar and any scalar may hold a hard reference. Arrays and hashes contain scalars; therefore, they can hold many references. Thus, with judicious use of arrays and hashes, you can easily build complex data structures of different combinations of arrays of arrays, arrays of hashes, hashes of functions, and so on.

There are several ways to construct references, and you can have references to just about anything-arrays, scalar variables, subroutines, file handles, and, yes (to the delight of C programmers), even to other references.

To use the value of $pointer as the pointer to an array, you reference the items in the array as @$pointer. The notation @$pointer roughly translates to "take the value in $pointer, and then use this value as the address to an array." Similarly, you use %$pointer for hashes. That is, "take the value of $pointer and interpret is as an address to a hash."

The Backslash Operator

Using the backslash operator is analogous to using the ampersand (&) operator in C to pass the address of an operator. This method is usually used to create a second, new reference to the variable in question. Here's how to create a reference to a scalar variable:

$variable = 22;
$pointer = \$variable;

$ice = "jello"
$iceptr = \$ice;

Now $pointer points to the location containing the value of $variable. The pointer $iceptr points to jello. Even if the original reference ($variable) goes away, you can still access the value from the $pointer reference. It's a hard reference at work here, so you have to get rid of both $pointer and $variable to free up the space in which the value of jello is allocated. Similarly, $variable contains the number 22 and because $pointer refers to $variable, dereferencing the $pointer with the statement $$pointer returns a value of 22. In a subroutine, both $variable and $pointer have to be declared as "local" or "my" variables. If they are both not declared as such, at least one of these variables will persist as a global variable long after the subroutine in which they are declared returns. As long as either of these variables exists, the space for storing the numbers will also exist.

The variable $pointer contains the address of the $variable, not the value itself. To get the value, you have to dereference $pointer with two dollar signs, $$. Listing 3.1 illustrates how this works.


Listing 3.1. References to scalars.
1 #!/usr/bin/perl
2
3 $value = 10;
4
5 $pointer = \$value;
6
7 printf "\n Pointer Address $pointer of  $value \n";
8
9 printf "\n What Pointer *($pointer) points to $$pointer\n";

$value in this script is set to 10. $pointer is set to point to the address of $value. The two printf statements show how the value of the variable is being referenced. If you run this script, you'll see something very close to this output:

Pointer Address SCALAR(0x806c520) of 10

What Pointer *(SCALAR(0x806c520)) points to 10

The address shown in the output from your script definitely will be different from the one shown here. However, you can see that $pointer gave the address, and $$pointer gave the value of the scalar pointed to by $variable.

The word SCALAR followed by a long hexadecimal number in the address value tells you that the address points to a scalar variable. The number following SCALAR is the address where the information of the scalar variable is being kept.

References and Arrays

This is perhaps the most important thing you must remember about Perl: all Perl @ARRAYs and %HASHes are always one-dimensional. As such, the arrays and hashes hold only scalar values and do not directly contain other arrays or complex data structures. If it's a member of an array, it's either a data item or a reference to a data item.

You can also use the backslash operator on arrays and hashes, just as you would for scalar variables. For arrays, you use something like the Perl script in Listing 3.2.


Listing 3.2. Using array references.
 1 #!/usr/bin/perl
 2 #
 3 # Using Array references
 4 #
 5 $pointer = \@ARGV;
 6 printf "\n Pointer Address of ARGV = $pointer\n";
 7 $i = scalar(@$pointer);
 8 printf "\n Number of arguments : $i \n";
 9 $i = 0;
10 foreach (@$pointer) { # Access the entire array.
11            printf "$i : $$pointer[$i++]; \n";
12            }

Let's examine the lines that pertain to references in this shell script, which prints out the contents of the input argument array @ARGV. Line 5 is where the reference $pointer is set to point to the array @ARGV. Line 6 simply prints the address of ARGV out for you. You probably will never have to use the address of ARGV, but had you been using another array, this would be a quick way to get to the address of the first element of the array.

Now $pointer will return the address of the first element of the array. This reference to an array should sound familiar to C programmers, where a reference to a one-dimensional array is really just a pointer to the first element of the array.

In line 7, the function scalar() (not to be confused with the type of variable scalar) is called to get the count of the elements in an array. The parameter passed in could be @ARGV, but in the case of the reference in $pointer, you have to specify the type of parameter expected by the scalar() function. Are you confused yet? There is a scalar() function; a scalar variable holds one value; and a hard reference is a scalar unless it's dereferenced to behave like a non-scalar.

Note
Remember that a reference to something will always be used as scalar. There is no implicit dereferencing in Perl. You specify how you want the scalar value of a reference to be used. Once you have a scalar reference, you can dereference it to be used as a pointer to an array, hash, function, or whatever structure you want.

The type of $pointer in this case is a pointer to the array whose number of elements you have to return. The call is made to the function with @$pointer as the passed parameter. $pointer really gives the address of the first entry in the array, and @ forces the passing of the address of the first element for use as an array reference.

The same reference to the array in line 10 is the same as in line 7. In line 11 all the elements of the array are listed out using the $$pointer[$i] item. How would the Perl compiler interpret the same statement to dereference $pointer to get an item in an array? Well, $pointer points to the first element in the array. Then you go to the ($i - 1)th item in the array (via the use of $pointer[$i++]) and also increment the value of $i. Finally, the value at $$pointer[$i] is returned as a scalar. Because the autoincrement operator is low on the priority list, $i is incremented last of all.

The program is appropriately called testmeout. Here is sample input and output for the code in Listing 3.2.

$ testmeout 1 2 3 4

 Pointer Address of ARGV = ARRAY(0x806c378)

 Number of arguments : 4
0 : 1;
1 : 2;
2 : 3;
3 : 4;

The number following ARRAY in the pointer address of ARGV in this example is the address of ARGV. Not that that address does you any good, but just realize that references to arrays and scalars are displayed with the type to which they happen to be pointing.

The backslash operator can be used with associative arrays too. The idea is the same: you are substituting the $pointer for all references to the name of the associative array. You use %$pointer instead of @$pointer to refer to an array. By specifying the percent sign (%) you are forcing Perl to use the value of $pointer as a pointer to a hash.

For pointers to functions, the address is printed with the word CODE. For a hash, it is printed as HASH. Listing 3.3 provides an example of using hashes.


Listing 3.3. Using references to associative arrays.
 1 #!/usr/bin/perl
 2
 3 #
 4 # Using References to Associative Arrays
 5 #
 6
 7 %month = (
 8             '01', 'Jan',
 9             '02', 'Feb',
10             '03', 'Mar',
11             '04', 'Apr',
12             '05', 'May',
13             '06', 'Jun',
14             '07', 'Jul',
15             '08', 'Aug',
16             '09', 'Sep',
17             '10', 'Oct',
18             '11', 'Nov',
19             '12', 'Dec',
20             );
21
22 $pointer = \%month;
23
24 printf "\n Address of hash = $pointer\n ";
25
26 #
27 # The following lines would be used to print out the
28 # contents of the associative array if %month was used.
29 #
30 # foreach $i (sort keys %month) {
31 # printf "\n $i $$pointer{$i} ";
32 # }
33
34 #
35 # The reference to the associative array via $pointer
36 #
37 foreach $i (sort keys %$pointer) {
38            printf "$i is $$pointer{$i} \n";
39 }

The associative array is referenced via the code in line 22 that contains $pointer = \%month;. This will create a hard reference, $pointer, to the hash called %month. Now you can also refer to the %month associative array by using the value in the $pointer variable. Using the %month variable, you would refer to an element in the hash using the syntax $month{$index}. In order to use the $pointer value, you would simply replace the month with $pointer in the name of the variable. This is very similar to the procedure used with pointers to ordinary arrays. The elements of the %month associative array are referenced with the $$pointer{$index} construct. Of course, because the array is really a hash, the $index is the key into the hash and not a number.

Here is the output from running this test script.

$ mth

 Address of hash = HASH(0x806c52c)

 01 is Jan
 02 is Feb
 03 is Mar
 04 is Apr
 05 is May
 06 is Jun
 07 is Jul
 08 is Aug
 09 is Sep
 10 is Oct
 11 is Nov
 12 is Dec

Associative arrays do not have to be constructed using the comma operator. You can use the => operator instead. In later Perl modules and sample code, you'll see the use of the => operator, which is the same as the comma operator. Using the => operator makes the code a bit easier to read aloud. Examine the output of Listing 3.3 with the print statements in the program to see how the output was generated.

Now let's look at how pointers to arrays and hashes can be dereferenced to get individual items. See the code in Listing 3.4 to see how you can use the => operator.


Listing 3.4. Alternative use of the => operator.
 1 #!/usr/bin/perl
 2
 3 #
 4 # Using Array references
 5 #
 6
 7 %weekday = (
 8             '01' => 'Mon',
 9             '02' => 'Tue',
10             '03' => 'Wed',
11             '04' => 'Thu',
12             '05' => 'Fri',
13             '06' => 'Sat',
14             '07' => 'Sun',
15             );
16
17 $pointer = \%weekday;
18
19 $i = '05';
20
21 printf "\n ================== start test ================= \n";
22 #
23 # These next two lines should show an output
24 #
25             printf '$$pointer{$i} is ';
26             printf "$$pointer{$i} \n";
27             printf '${$pointer}{$i} is ';
28             printf "${$pointer}{$i} \n";
29
30             printf '$pointer->{$i} is ';
31             printf "$pointer->{$i}\n";
32
33 #
34 # These next two lines should not show anything
35 #
36             printf '${$pointer{$i}} is ';
37             printf "${$pointer{$i}} \n";
38             printf '${$pointer->{$i}} is ';
39             printf "${$pointer->{$i}}";
40
41 printf "\n ================== end of test ================= \n";

Here is the output from the Perl script shown in listing 3.4.

 ================== start test =================
$$pointer{$i} is Fri
${$pointer}{$i} is Fri
$pointer->{$i} is Fri
${$pointer{$i}} is
${$pointer->{$i}} is
 ================== end of test =================

In this output, you can see that the first two lines gave you the expected output. The first reference is used in the same way as regular arrays. The second line uses ${pointer} and indexes using {$i}, and the leftmost $ dereferences (gets) the value at the location reached after the indexing.

Then there are the two lines that did not work. In the third line of the output, $pointer{$i} tries to reference an array using the first element instead of its address. The fourth line, ${$pointer->{$i}}, has an extra level of indirection leading to a scalar being used as a pointer and therefore prints nothing.

The -> operator should be very familiar to C++ or C programmers. Using a reference like $variable->{$k} is synonymous with the use of $$variable{$k}. The -> simply means "use the value of the left side of -> as an address and dereference it as a pointer to an array." So, in line 30, you use $pointer-> in place of $pointer to refer to an array. The {$i} is used to index into the array directly, because the $pointer-> is already defined as pointing to an array. In the case of $$pointer{$i}, two preceding dollar signs ($$) are required: one to dereference the value in $pointer, and the other to use the value at the i-th index in the array as a scalar.

We will cover the use of the -> operator in a moment when we use it to index into elements of arrays. Let's first look at how we can use simple array concepts to construct multidimensional arrays.

Using Multidimensional Arrays

The way to create a reference to an array is with the statement @array = list. You can create a reference to a complex anonymous array by using square brackets. Consider the following statement, which sets the parameters for a three-dimensional drawing program:

$line = ['solid', 'black', ['1','2','3'] , ['4', '5', '6']];

This statement constructs an array of four elements. The array is referred to by the scalar $line. The first two elements are scalars indicating the type and color of the line to draw. The next two elements of the array referred to by $line are references to anonymous arrays; they contain the starting and ending points of the line.

To get to the elements of the inner array elements, you can use the following multidimensional syntax:

$arrayReference->[$index] for a single dimensional array, and
$arrayReference->[$index1][$index2] for a two dimensional array, and
$arrayReference->[$index1][$index2][$index3] for a three dimensional array.

Let's see how creating arrays within arrays works in practice. Refer to Listing 3.5 to print out the information pointed to by the $list reference.


Listing 3.5. Using multidimensional array references.
 1 #!/usr/bin/perl
 2
 3 #
 4 # Using Multidimensional Array references
 5 #
 6
 7 $line = ['solid', 'black', ['1','2','3'] , ['4', '5', '6']];
 8
 9 print "\$line->[0] = $line->[0] \n";
10 print "\$line->[1] = $line->[1] \n";
11 print "\$line->[2][0] = $line->[2][0] \n";
12 print "\$line->[2][1] = $line->[2][1] \n";
13 print "\$line->[2][2] = $line->[2][2] \n";
14 print "\$line->[3][0] = $line->[3][0] \n";
15 print "\$line->[3][1] = $line->[3][1] \n";
16 print "\$line->[3][2] = $line->[3][2] \n";
17
18 print "\n"; # The obligatory output beautifier.

Here is the output of the program that shows how to use two-dimensional arrays.

$line->[0] = solid
$line->[1] = black
$line->[2][0] = 1
$line->[2][1] = 2
$line->[2][2] = 3
$line->[3][0] = 4
$line->[3][1] = 5
$line->[3][2] = 6

You can modify the script in Listing 3.5 to work with three-dimensional (or even n-dimensional) arrays, as shown in Listing 3.6.


Listing 3.6. Extending to multiple dimensions.
 1 #!/usr/bin/perl
 2
 3 #
 4 # Using Multidimensional Array references again
 5 #
 6
 7 $line = ['solid', 'black', ['1','2','3', ['4', '5', '6']]];
 8
 9 print "\$line->[0] = $line->[0] \n";
10 print "\$line->[1] = $line->[1] \n";
11 print "\$line->[2][0] = $line->[2][0] \n";
12 print "\$line->[2][1] = $line->[2][1] \n";
13 print "\$line->[2][2] = $line->[2][2] \n";
14
15 print "\$line->[2][3][0] = $line->[2][3][0] \n";
16 print "\$line->[2][3][1] = $line->[2][3][1] \n";
17 print "\$line->[2][3][2] = $line->[2][3][2] \n";
18
19 print "\n";

In this example, the array is three deep; therefore, a reference like $line->[2][3][0] has to be used. For a C programmer, this is akin to the statement Array_pointer[2][3][0], where pointer is pointing to what's declared as an array with three indexes.

In the previous examples, only hard-coded numbers were used as the indexes. There is nothing preventing you from using variables instead. As with array constructors, you can mix and match hashes and arrays to create as complex a structure as you want.

Creating complex structures is the next step. Listing 3.7 illustrates how these two types of arrays can be combined. It uses the point numbers and coordinates to define a cube.


Listing 3.7. Using multidimensional arrays.
 1 #!/usr/bin/perl
 2
 3 #
 4 # Using Multidimensional Array and Hash references
 5 #
 6
 7 %cube = (
 8             '0', ['0', '0', '0'],
 9             '1', ['0', '0', '1'],
10             '2', ['0', '1', '0'],
11             '3', ['0', '1', '1'],
12             '4', ['1', '0', '0'],
13             '5', ['1', '0', '1'],
14             '6', ['1', '1', '0'],
15             '7', ['1', '1', '1']
16             );
17
18 $pointer = \%cube;
19
20 print "\n Da Cube \n";
21 foreach $i (sort keys %$pointer) {
22             $list = $$pointer{$i};
23             $x = $list->[0];
24             $y = $list->[1];
25             $z = $list->[2];
26             printf " Point $i =  $x,$y,$z \n";
27
28 }

In this listing, %cube contains point numbers and coordinates in a hash. Each coordinate itself is an array of three numbers. The $list variable is used to get a reference to each coordinate definition with the following statement:

$list = $$pointer{$i};

After you get the list, you can reference off of it to get to each element in the list with this statement:

$x = $list->[0];
$y = $list->[1];

Note that the same result of assigning values to $x, $y, and $z could be achieved by these two lines of code:

($x,$y,$z) = @$list;
$x = $list->[0];

This works because you are dereferencing what $list points to and using it as an array, which in turn is assigned to the list ($x,$y,$z). $x is still assigned with the -> operator.

When working with hashes or arrays, dereferencing by -> is like a dollar-sign ($) dereference. When accessing individual array elements, you are often faced with writing statements like these two:

$$names[0] = "Kamran";
$names->[0] = "Kamran";

Both lines are equivalent. The substring "$names" in the first line has been replaced with the
-> operator to create the second line. The same procedure can be applied for hash operations:

$$lastnames{"Kamran"} = "Husain";
$lastnames->{"Kamran"} = "Husain";

Arrays in Perl can be created with a fixed size set to the value of the highest index that is used. They do not have to remain at this size, though, and can grow on demand. Referencing them for the first time creates the array and space for the item that is being indexed in the array. Referencing the array again at different indexes creates those elements at the indexed references if they do not already exist. Array references can be created automatically when first referenced in the left side of an equation. Using a reference such as $array[$i] creates an array into which you can index with $i. Such is the case with scalars and even multidimensional arrays.

References to Subroutines

Just as you can reference individual items such as arrays and scalar variables, you can also point to subroutines. In C, this would be akin to pointing to a function. To construct such a reference, you use a statement like this:

$pointer_to_sub = sub { ... declaration of sub ... } ;

Note the use of the semicolon at the end of the sub() declaration. The subroutine pointed to by $pointer_to_sub points to the same function reference even if the statement is placed in a loop. This feature in Perl lets you declare several anonymous sub() functions in a loop without worrying about the fact that you are chewing up memory by declaring the same function over and over as you go about in a loop. As you come around the loop and reassign a scalar to the sub, Perl simply assigns to the same subroutine declared with the first use of the sub() statement.

To call a referenced subroutine, use this syntax:

&$pointer_to_sub( parameters );

This code works because you are dereferencing the $pointer_to_sub and using it with the ampersand (&) as a pointer to a function. The parameters portion may or may not be empty, depending on how your function is defined. The code within a sub is simply a declaration created with this statement. The code within the sub is not executed immediately; however, it is compiled and set for each use. Consider the script shown in Listing 3.8.


Listing 3.8. Using references to subroutines.
 1 #!/usr/bin/perl
 2
 3 sub print_coor{
 4             my ($x,$y,$z) = @_;
 5             print "$x $y $z \n";
 6             return $x;};
 7
 8 $k = 1;
 9 $j = 2;
10 $m = 4;
11 $this  = print_coor($k,$j,$m);
12
13 $that  = print_coor(4,5,6);

When you execute this listing, you get the following output:

$ test
1 2 4
4 5 6

This output tells you that assignments of $x, $y, and $z were done when the first declaration of print_coor was encountered as a call. Each reference to $this and $that now points to a completely different subroutine, the arguments to which were passed at runtime.

Using Subroutine Templates

Subroutines are not limited to returning only data types. They can return references to other subroutines, too. The returned subroutines run in the context of the calling routine but are set up in the original routine that created them. This type of behavior is caused by the way closure is handled in Perl. Closure means that if you define a function in one context, it runs in that particular context in which it was first defined. (A book on object-oriented programming would provide more information on closure.)

To see how closure works, look at Listing 3.9, which you can use to set up different types of error messages. Such subroutines are useful in creating templates of all error messages.


Listing 3.9. Using closure.
 1 #!/usr/bin/perl
 2
 3 sub errorMsg {
 4          my $lvl = shift;
 5                 #
 6                 # define the subroutine to run when called.
 7                 #
 8          return sub {
 9
10                         my $msg = shift;  # Define the error type now.
11                         print "Err Level $lvl:$msg\n"; }; # print later.
12          }
13
14 $severe  = errorMsg("Severe");
15 $fatal = errorMsg("Fatal");
16 $annoy = errorMsg("Annoying");
17
18 &$severe("Divide by zero");
19 &$fatal("Did you forget to use a semi-colon?");
20 &$annoy("Uninitialized variable in use");

The subroutine errorMsg declared here uses a local variable called lvl. After this declaration, errorMsg uses $lvl in the subroutine it returns back to the caller. Therefore, the value of $lvl is set in the context when the subroutine errorMsg is first called, even though the keyword my is used. Therefore, the following three calls set up three different $lvl variable values, each in their own context:

$severe  = errorMsg("Severe");
$fatal   = errorMsg("Fatal");
$annoy   = errorMsg("Annoying");

Now, when the reference to a subroutine is returned by the call to the errorMsg function in each of the lines above, the value of $lvl within the errorMsg function is retained for each context in which $lvl was declared. Thus, the $msg value from the referenced call is used, but the value of $lvl is the value that was first set in the actual creation of the function.

Sound confusing? It is. This is primarily the reason why you do not see this type of code in most Perl programs.

Implementing State Machines

Using arrays and pointers to subroutines, you can come up with some nifty applications. Consider using an array of pointers to subroutines to implement a state machine. Listing 3.10 provides an example of a simple, asynchronous state machine.


Listing 3.10. A simple, asynchronous state machine.
 1 #!/usr/bin/perl
 2 # --------------------------------------------------------------
 3 # Define each state as subroutine. Then create a
 4 # reference to each subroutine. We have four states here.
 5 # --------------------------------------------------------------
 6 $s0 = sub {
 7            local $a = $_[0];
 8            print "State 0 processing $a \n";
 9            if ($a eq '0')  { return(0); }
10            if ($a eq '1')  { return(1); }
11            if ($a eq '2')  { return(2); }
12            if ($a eq '3')  { return(3); }
13            return 0;
14            };
15 # --------------------------------------------------------------
16 $s1 = sub {
17            local $a = shift @_;
18            print "State 1 processing $a \n";
19            if ($a eq '0')  { return(0); }
20            if ($a eq '1')  { return(1); }
21            if ($a eq '2')  { return(2); }
22            if ($a eq '3')  { return(3); }
23            return 1;
24            };
25 # --------------------------------------------------------------
26 $s2 = sub {
27            local $a = $_[0];
28            print "State 2 processing $a \n";
29            if ($a eq '0')  { return(0); }
30            if ($a eq '1')  { return(1); }
31            if ($a eq '2')  { return(2); }
32            if ($a eq '3')  { return(3); }
33            return 2;
34            };
35 # --------------------------------------------------------------
36 $s3 = sub {
37            my  $a = shift @_;
38            print "State 3 processing $a \n";
39            if ($a eq '0')  { return(0); }
40            if ($a eq '1')  { return(1); }
41            if ($a eq '2')  { return(2); }
42            if ($a eq '3')  { return(3); }
43            return 3;
44            };
45 # --------------------------------------------------------------
46 # Create an array of pointers to subroutines. The index
47 # into this array is the current state.
48 # --------------------------------------------------------------
49 @stateTable = ($s0, $s1, $s2, $s3);
50 # --------------------------------------------------------------
51 # Initialize the state to 0.
52 # --------------------------------------------------------------
53 $this = 0;
54 # --------------------------------------------------------------
55 # Implement the state machine.
56 #   set current state to 0
57 #   forever
58 #        get response
59 #        set current state to next state based on response.
60 # --------------------------------------------------------------
61 while (1)
62            {
63            print "\n This state is : $this -> what next? ";
64            $reply = <STDIN>;
65            chop($reply);
66            #
67            # Stop the machine here
68            #
69            if ($reply eq 'q') { exit(0); }
70            print " Reply = $reply \n";
71            #
72            # Get the present state function.
73            #
74            $state = $stateTable[$this];
75            #
76            # Get the next state from this state.
77            #
78            $next = &$state($reply);
79            printf "Next state = $next from this state $this\n";
80            #
81            # Now advance present state to next state
82            #
83            $this = $next;
84     }

Let's see how each function implements the state transitions. All input into each state consists of removing the initial state as the first parameter into the subroutine. In Perl, the @_ variable is the array of input parameters into a subroutine and is always defined in each subroutine. In line 37, the shift command forces the first item from the list of input parameters into $a. The value of $a is then used as the current state of the program.

There are four states in this state machine: S0, S1, S2, and S3. Each state accepts input in the form of a number. Each number is used to get the next state to go to. Note how $a is declared in each state function using the my and local types. So if $a has a value of 2 and receives an input of 3, the current state is 2, and the program will do a state transition from 2 to 3. After the function returns, the current state will be 3.

Lines 6 through 14 define a subroutine that defines the functionality of a state. State S0 transitions to states S1 on receiving a 1, S2 on receiving a 2, and S3 on receiving a 3. All other input will not cause a state transition. The other states, {S1,S2,S3}, behave in an analogous way.

The stateTable array is used to store pointers to each of the functions of the state machine. The four entries are set in line 49. The initial state is set to 0.

Lines 61 through 84 implement the code for transitioning through the state machine by accepting input from <STDIN> and calling the present state function to handle the input. Line 74 is where you get the pointer to the function handling all input for each state in the state machine, and line 78 is where the state-handling function is called. The next state value returned by the function is set to the present state ($this) in line 83.

Passing More Than One Array into a Subroutine

Having arrays is great for collecting relevant information. Now you'll see how to work with multiple arrays via subroutines. Passing one or more arrays into Perl subroutines is done by reference. However, you have to keep in mind a few subtle things about using the @_ symbol when processing these arrays in the subroutine.

The @_ symbol is an array of all the items in a subroutine. So, if you have a call to a subroutine as follows:

$a = 2;
@b = ("x","y","z");
@c = ("cat","mouse","chase");
&simpleSub($a,@b,@c);

the @_ array within the subroutine will be (2, "x", "y", "z", "cat", "mouse", "chase"). That is, the contents of all the elements will be glued together to form one long array.

Obviously, this ability to glue together arrays will be a problem to deal with if you want to do operations on two distinct arrays sequentially. For example, if you have a list of names and a list of phone numbers, you would want to take the first item from the names array and the first item from the number array and print an item. Then take the next name and the next number and print a combination, and so on. If you pass in the contents of the arrays to a function that simply uses @_, the subroutine will see one long array, the first half of which will be a list of strings (names) and the second half of which will be a list of numbers.

The subroutine would have to split the @_ in half into two distinct arrays before it can start processing. The problem gets more complicated if you were to pass three or four arrays such as those containing items like address and ZIP code. Now the subroutine will have to manipulate @_ even more to get the required number of arrays.

The simplest way to handle the passing of multiple arrays into a subroutine is to use references to arrays in the argument list to the subroutine. That is, you pass in a reference to each array that the subroutine will be using. The references will be ordered in the @_ array within the subroutine. The code in the subroutine can dereference each item in the @_ to the type of array being referenced. This procedure is known as passing by reference. The value of what is being referenced can be changed by the subroutine. When an explicit value is sent to a subroutine, (that is, you are passing by value), only the copy of what is sent on the stack is changed, not the actual value. In Perl, values are passed by reference unless you send in a constant number. For example, from the following code:

sub doit {
$_[0] *= 3.141;
}
$\="\n";
$x = 3;
print $x;
doit ($x);
print $x;
# The following line will cause an error since you will attempt to
# modify a read-only value:
# doit(3);

you will see the following values being printed:

3
9.423

The second number is the new value of $x after the call to the doit subroutine. Calling the doit subroutine with a constant value such as shown in the commented lines above will result in an exception with an error message indicating that your program attempted to modify a read-only value. The preceeding test confirms that Perl indeed passes values of variables by reference and not by value.

Note
The value of the $\ system variable is the output separator. In the preceding example, it is set to a newline. By setting the value of $\ to \n, the print statements did not have to prepend a \n to any string being printed. It's a matter of style, of course, and you do not have to use the $\ variable if you do not want to. The default value of this $\ variable is null. The $\ is useful in instances when you are writing special text records with the print statement that have to have a special record separator such as END\n and RECORDEND\n\n.

Listing 3.11 provides a sample subroutine that expects a list of names and a list of phone numbers.


Listing 3.11. Passing multiple arrays into a subroutine.
 1 #!/usr/bin/perl
 2
 3 @names = (mickey, goofy, daffy );
 4 @phones = (5551234, 5554321, 666 );
 5 $i = 0;
 6 sub listem {
 7             my (@a,@b) = @_;
 8             foreach (@a) {
 9             print "a[$i] = ". $a[$i] . " " . "\tb[$i] = " . $b[$i] ."\n";
10            $i++;
11            }
12             }
13
14 &listem(@names, @phones);

Here's the output from this program:

a[0] = mickey           b[0] =
a[1] = goofy  b[1] =
a[2] = daffy   b[2] =
a[3] = 5551234         b[3] =
a[4] = 5554321         b[4] =
a[5] = 666      b[5] =

The @b array is empty, and @a is just like the array @b. This is because the @_ array is a solitary array of all parameters into a subroutine. If you pass in 50 arrays, @_ is still going to be one array of all the elements of the 50 arrays concatenated together.

In the subroutine in this example, the assignment

my (@a, @b) = @_

gets loosely interpreted by your Perl interpreter as "let's see, @a is an array, so let's assign one array from @_ to @a and then assign everything else to @b." Never mind the fact that @_ is itself an array and will therefore get assigned to @a, leaving nothing to assign to @b.

In order to get around this @_-interpretation feature and to be able to pass arrays into subroutines, you would have to pass arrays in by reference. This is done by modifying the script to look like the one shown in Listing 3.12.


Listing 3.12. Passing multiple arrays by reference.
 1 #!/usr/bin/perl
 2
 3 @names = (mickey, goofy, daffy );
 4 @phones = (5551234, 5554321, 666 );
 5 $i = 0;
 6 sub listem {
 7             my ($a,$b) = @_;
 8             foreach (@$a) {
 9                print "a[$i] = " . @$a[$i] . " " . "\tb[$i] = " . @$b[$i] ."\n";
10                         $i++;
11                         }
12             }
13
14 &listem(\@names, \@phones);

Here are the major changes made to this script:

  • The local variables for the sub listem are now scalars, not array references. This way, $a is the first item on the @_ list, and $b is the second item.
  • The local parameters ($a and $b) are used as array references with the statements @$a and @$b, respectively.
  • The call to the subroutine passes the references to the arrays with the backslash, \@names and \@phones, thus passing only two items to the subroutine.

The output from this listing is what we expected:

a[0] = mickey b[0] = 5551234
a[1] = goofy  b[1] = 5554321
a[2] = daffy  b[2] = 666

Pass by Value or by Reference?

Scalar variables, when used in a subroutine argument list, are always passed by reference. You do not have a choice here. You can modify the values of these variables if you really want to. To access these variables, you can use the @_ array and index each individual element in it, using $_[$index], where $index as an integer goes from 0 on up.

Arrays and hashes are different beasts altogether. You can either pass them as references once, or you can pass references to each element in the array. For long arrays, the choice should be fairly obvious, pass the reference to the array only. In either case, you can use the reference(s) to modify what you want in the original array.

Also, the @_ mechanism concatenates all the input arrays to a subroutine into one long array. Sure, this feature is nice if you do want to process the incoming arrays as one long array. Normally, you want to keep the arrays separate when processing them in a subroutine, and passing by reference is the best way that you can do that.

References to File Handles

There are times when you have to write the same output to different output files. For instance, an application programmer might want output to go to a screen in one instance, the printer in another, and a file in yet another, or perhaps even all three at the same time. Rather than make separate statements per handle, it would be nice to write something like this:

spitOut(\*STDIN);
spitOut(\*LPHANDLE);
spitOut(\*LOGHANDLE);

Note how the file handle reference is sent with the \*FILEHANDLE syntax. This is because you're referring to the symbol table in the current package. In the subroutine handling the output to the file handle, you have code that looks something like this:

sub spitOut {
    my $fh = shift;
    print $fh "Gee Wilbur, I like this lettuce\n";
}

What Does the *variable Operator Do?

In UNIX (and other operating systems, too) the asterisk is a sort of wildcard operator. In Perl you can refer to other variables, arrays, subroutines, and so on by using the asterisk operator like this:

*iceCream;

The asterisk used this way is also known as a typeglob. The asterisk on the front can be thought of as a wildcard match for all the mangled names used internally by Perl. When evaluated, a typeglob of *name produces a scalar value that represents the first object found with that name.

A typeglob can be used the same way a reference can be used because the dereference syntax always indicates the kind of reference desired. Therefore, ${*iceCream} and ${\$iceCream} both mean the same scalar variable. Basically, *iceCream refers to the entry in the internal _main associative array of all symbol names for the _main package. Thus, *kamran really translates to $_main{'kamran'} if you are in the _main package context.

A package context implies the use of the associative array of symbol names, called a symbol table, by Perl for resolving variable names in a program. We will cover symbols and symbol tables in Chapter 4. What is confusing is that the terms module and package are used interchangeably in all Perl documentation and these two terms mean the very same thing. Basically, your Perl program runs in the _main package (think "module") and uses other modules to switch symbol tables. Code running in the context of a module has its own symbol table that is different from the symbol table in the main module.

Using Symbolic References

The use of brackets around symbolic references makes it easier to construct strings:

$road = ($w) ? "free":"high";
print "${road}way";

This line will print highway or freeway, depending on the value of $w. This type of syntax will be very familiar to folks writing makefiles or shell scripts. In fact, you can use this ${variable} construct outside of double quotes, like the examples shown here:

print ${road};
print ${road} . "way";
print ${ road } . "way";
$if = "road";
print "\n ${if} way \n";

Note that you can use reserved words in the ${ } brackets, too. However, using reserved words for anything other than their purpose is playing with fire. Be imaginative and make up your own variables.

One last point. Symbolic references cannot be used on variables declared with the my construct because these variables are not kept in any symbol table. Variables declared with the my construct are valid only for the block in which they're created. Variables declared with the local word are visible to all ensuing lower code blocks because they are in a symbol table.

Declaring with Curly Braces

The previous section brings up an interesting point about curly braces for use other than as hashes. In Perl, curly braces are normally reserved for delimiting blocks of code. Let's say you are returning the passed list by sorting it in reverse order. The passed list is in @_ of the called subroutine. Thus, these two statements are equivalent:

sub backward {
            { reverse sort @_ ; }
            };

sub backward {
            reverse sort @_ ;
            };

Curly braces, when preceded with the @ operator, allow you to set up small blocks of evaluated code. The code in Listing 3.13 evaluates an array.


Listing 3.13. Evaluating references to arrays.

1 #!/usr/bin/perl
2 sub average {
3            ($a,$b,$c) = @_;
4                        $x = $a + $b + $c;
5                        $x2 = $a*$a + $b*$b + $c*$c;
6          return ($x/3, $x2/3 ); }

7 $x = 1;
8 $y = 34;
9 $x = 47;

10 print "The midpt is @{[&average($x,$y,$z)]} \n";

You should see the printout of 27 and 1121.6666. In line 10, when @{} is seen in the double-quoted string, the contents of @{} are evaluated as a block of code. The block creates a reference to an anonymous array containing the results of the call to the subroutine average($x,$y,$z). The array is constructed because of the [] brackets around the call. Thus, the [] construct returns a reference to an array, which in turn is converted by @{} into a string and inserted into the double-quoted string.

Multidimensional Associative Arrays

Perl does not directly support multidimensional associative arrays. In most cases, you would not want to use multidimensional arrays, though they are sometimes useful for tracking synonymous variable names.

The syntax for using more than one index into an associative array is not the same as that for multidimensional arrays that use a numeric index. Therefore, you cannot use statements such as this:

$description{'pan'}{'handle'};

as you would with regular arrays. What you can use is the following:

$description{'pan' , 'handle'};

The latter statement lets you index into the %description array using two strings, so you can index the array as

$description{'pan' , 'cake'};
$description{'pan' , 'der'};
$description{'pan' , 'da'};

Your first index here for a row would be pan and each index into the row would be cake, der, da, and handle. It's a bit cumbersome to use, but it will work.

You are not limited to using commas to separate indexes into an associative array. By using the $; system variable you can use more than one index into an associative array and use a separator other than just a comma. The $; system variable is a subscript separator for all items used to index an associative array. The default value of $; is the Ctrl-\ character, but you can set it to anything you want.

When more than one index is used to reference an associative array, all items are concatenated together with the use of the $; variable. That is, the statement

$description{"texas", "pan","handle"} ;

is interpreted as

$description{"texas" . $; . "pan" . $; . "handle"} ;

By setting the value of $; to "::", you can use the index specifier. The following lines of code will illustrate how to do this:

$; = "::";
$description{"pan", "cake"} = "edible";
$description{"pan::da"} = "cute";

The "::" is now interchangeable with the comma separator. There is one catch to using the "::" as a separator: the "::" is also used as an object::member syntax as you will see in Chapter 5, "Object-Oriented Programming in Perl." So a statement like this with the $; set to "::"

$description{"pan::handle", "cake"}

will get translated to

$description{"pan::handle::cake"}

which is something you probably do not want! We will cover this syntax and how to work with objects in Chapter 5, so be patient.

Strict References

To force only hard references in a program and protect yourself from accidentally creating symbolic references, you can use a module called strict, which forces Perl to do strict type checking. To use this module, place the following statement at the top of your Perl script:

use strict 'refs';

From this point, only hard references are allowed for the rest of the script. You place this statement within curly braces, too, where the type checking would be limited to only within the code block for the curly braces.

To turn off the strict type checking at any time within a code block, use this statement:

no strict 'refs';

For More Information

Besides the obvious documents, such as the Perl man pages, look at the Perl source code. The t/op directory in the Perl source tree has some regression test routines that should definitely get you thinking. There are lots of documents and references at the Web sites www.perl.com/index.html, mox.perl.com/index.html, and www.metronet.com/perlinfo/doc/manual/html/perl.html.

Summary

There are two types of references you can deal with in Perl 5: hard or symbolic. Hard links work like the links in UNIX file systems. You can have more than one hard link to the same item. Perl keeps a reference count for you. This reference count is incremented or decremented as references to the item are created or destroyed. When the count goes to zero, the link and the object it is pointing to are both destroyed. Symbolic links are created via the ${} construct and are useful in providing multiple stages of references to objects.

You can have references to scalars, arrays, hashes, subroutines, and even other references. References themselves are scalars and have to be dereferenced to the context before being used. Use @$pointer for an array, %$pointer for a hash, &$pointer for a subroutine, and so on. Multidimensional arrays are possible by using references in arrays and hashes. You can also have references to other elements holding even more references to create very complicated structures. There is a scalar() function, a scalar variable holds one value, and a hard reference is a scalar unless it's dereferenced to behave like a non-scalar. Got that?

Parameters are passed into a subroutine through references. The @_ array is really one long array of all the passed parameters concatenated in one long array. To send separate arrays, use the references to the individual items.

The next chapter covers Perl objects and references to objects. I deliberately did not cover Perl objects in this chapter because they require some knowledge of objects, constructors, and packages.

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