TweetFollow Us on Twitter

A Bit More Perl

Volume Number: 18 (2002)
Issue Number: 9
Column Tag: Section 7

A Bit More Perl

control flow, data structure, resources...

by Rich Morin

As promised last month, this column will go a bit deeper into Perl, looking at its data structures and control flow operators, closing out with a discussion of Perl resources.

Perl only has a few data types, but their versatility allows them to serve in a large number of roles. In several years of Perl programming, I've never found myself reaching for a data type (or anything else, really :-) that Perl didn't have.

Scalars

The "scalar" is Perl's basic data element. A scalar can be an integer, a floating-point number, a text string (usually ASCII, but other encodings are allowed), or a "reference" to some other type of entity. With a bit of work, scalars can be mapped onto bit strings or even used to represent sets of alternative quantum states. As Perl's slogan says: There's More Than One Way To Do It.

Because strings are "first-class citizens" in Perl, they are used in most situations where a C programmer might use an array of characters. In fact, if I see an array of characters being used in a Perl program, my first assumption is that a C programmer has been unable or unwilling to learn about Perl's string manipulation facilities! Here are some examples of Perl scalars:

% cat x0
#!/usr/bin/env perl
$num_1 = 123;
$num_2 = 123.45;
$n1_r1 = \$num_1;
$n1_r2 = "num_1";
printf("num_1=%d, num_2=%f\n", $num_1, $num_2);
printf("n1_r1=%d, n1_r2=%f\n", $$n1_r1, $$n1_r2);
$str_1 = '123';
$str_2 = "The value of \$num_1 is $num_1.";
print "The value of \$str_1 is |$str_2|.\n";
% x0
num_1=123, num_2=123.450000
n1_r1=123, n1_r2=123.000000
The value of $str_1 is |The value of $num_1 is 123.|.

Note that Perl allows both "hard" and "symbolic" references. $n1_r1 (a hard reference) will run a bit faster than $n1_r2 (a symbolic reference). The naming and behavior are taken from "hard" and "symbolic" links, as used in BSD file systems.

Aggregates

Perl has only two aggregate data structures: arrays and hashes. As indicated above, however, they are quite powerful. Perl arrays can be subscripted (from either end!), used as queues, stacks, or deques (double-ended queues), and more. If you're looking for a way to store an ordered collection, an array will probably serve your needs.

Perl's hashes handle unordered collections of data, storing each value under a unique (scalar) key. They are quite similar to the tables one finds in relational database systems; in fact, hashes can be "tied" to database table to provide persistent storage.

% cat x1
#!/usr/bin/env perl
@A = (1, 'deux', 'III', 4);
print "$A[0], $A[1], $A[-2], $A[-1]\n";
%H = (uno => 1, dos => 2);
printf("sum = %d\n", $H{uno} + $H{dos});
% x1
1, deux, III, 4
sum = 3

Although Perl's arrays and hashes can only contain scalars, multi-level data structures (as well as trees, graphs, etc.) can be formed by using references. For convenience, Perl supports abbreviated ways to use these complex forms. Here are some multi-level structures:

$AAA[1][2][3] = 1;      # 3-dimensional array
$HHH{a}{b}{c} = 2;      # 3-dimensional hash
$AH[0]{a}     = 3;      # array of hashes
$HA{b}[1]     = 4;      # hash of arrays
$AHA[2]{c}[3] = 5;      # array of ...
$HAHA{d}[4]{e}[5]++;    # hash of ...

Control Flow

Although Perl supports the traditional C-style "for" loop, it isn't used much in practice. Instead, Perl programmers tend to use list-based looping, as:

foreach $item (@A) { ...
foreach $key (sort(keys(%H})) { ...

Explicit references can be useful in dealing with multi-level data structures:

% cat x2
#!/usr/bin/env perl
$HH{a}{b} = 'ab';
$HH{c}{d} = 'cd';
foreach $k1 (sort(keys(%HH))) {
    $r1 = $HH{$k1};
    foreach $k2 (sort(keys(%{$r1}))) {
        $tmp = $r1->{$k2};
        print "\$HH{$k1}{$k2}=$tmp\n";
    }
}
% x2
$HH{a}{b}=ab
$HH{c}{d}=cd

Perl also provides the until and while looping operators, along with a plethora of ways to do conditional execution:

% cat x3
#!/usr/bin/env perl
$foo = 'bar';
if     ($foo eq 'bar') { print '0 '; };
unless ($foo ne 'bar') { print '1 '; };
print '2 ' if     ($foo eq 'bar');
print "3 " unless ($foo ne bar);
$foo eq 'bar' and print '4 ';
$foo ne 'bar' or  print '5 ';
print "\n";
% x3
0 1 2 3 4 5 

Perl also provides GOTOs and loop modifiers, subroutines (anonymous and/or recursive, if need be), exception handling, and some even trickier facilities. It also has tightly integrated regular expressions, allowing (parts of) strings to be matched, extracted, and/or modified. In short, Perl is a powerful language, suited for everything from short one-offs to substantial applications.

Resources

Perl has a very active user community, providing a variety of forums for communication. Whether you prefer conferences, IRC channels, local user group meetings, mailing lists, newsletters, usenet newsgroups, or weblogs, Perl has it. To find these resources, start at www.perl.{org,com}, the primary sources of Perl information.

Perl has enormous amounts of online documentation, Typing man perl will yield a list of several dozen subsidiary man pages, along with some advice on how to approach them. Typing perldoc will lead you into a function index and an online FAQ. Spend some time scanning through these; it will pay off handsomely later on...

Finally, of course, there are literally dozens of books on Perl, ranging from introductory and overview texts to detailed coverage of specialized subtopics. The majority of Perl books are published by O'Reilly and Associates (www.oreilly.com), who also operates www.perl.com. O'Reilly's Perl books tend to be authoritative, diverse, readable, and practical; if you had to pick a single publisher of Perl books, you could stick to O'Reilly and survive quite nicely. And, if you could only buy one book on Perl, their Programming Perl would be the clear winner.

Fortunately, you don't have to restrict yourself to a single publisher, let alone a single book. My Perl collection, for instance, includes fine volumes by Addison-Wesley, Manning, and Wiley. So, look around a bit! That said, here is a "reading list" of Perl books for the typical MacTech reader. Be warned; the books get significantly more chewy as the list goes on:

  • Elements of Programming with Perl - Johnson (Manning)

  • Learning Perl - Schwartz & Christiansen (O'Reilly)

  • Programming Perl - Wall, et al (O'Reilly)

  • Effective Perl Programming - Hall (Addison-Wesley)

  • Object-Oriented Perl - Conway (Manning)

  • Mastering Algorithms with Perl - Orwant, et al (O'Reilly)

  • Advanced Perl Programming - Srinivasan (O'Reilly)

  • Mastering Regular Expressions - Friedl (O'Reilly)

Here are some reference books that you might want to add in:

  • Perl in a Nutshell - Siever, et al (O'Reilly)

  • Perl Cookbook - Christiansen & Torkington (O'Reilly)

If you're doing web programming in Perl, consider getting:

  • CGI Programming with Perl - Guelich, et al (O'Reilly)

  • Network Programming with Perl - Stein (Addison-Wesley)

  • Official Guide to Programming with CGI.pm - Stein (Wiley)

  • Perl & LWP - Burke (O'Reilly)

  • Programming Web Graphics with Perl & GNU Software - Wallace (O'Reilly)

  • Web Client Programming with Perl - Wong (O'Reilly)

  • Writing Apache Modules with Perl and C - Stein & MacEachern (O'Reilly)

Finally, if you're using any Perl add-ons, you should consider books such as:

  • Learning Perl/Tk - Walsh (O'Reilly)

  • Mastering Perl/Tk - Lidie & Walsh (O'Reilly)

  • Perl & XML - Ray & McIntosh (O'Reilly)

  • Programming the Perl DBI - Descartes and Bunce (O'Reilly)


Rich Morin has been using computers since 1970, Unix since 1983, and Mac-based Unix since 1986 (when he helped Apple create A/UX 1.0). When he isn't writing this column, Rich runs Prime Time Freeware (www.ptf.com), a publisher of books and CD-ROMs for the Free and Open Source software community. Feel free to write to Rich at rdm@ptf.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Spotify 1.0.53.758. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
beaTunes 4.6.14 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
Vienna 3.1.10 :d05d7a5d: - RSS and Atom...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
Day One 2.1.8 - Maintain a daily journal...
Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through... Read more
iDefrag 5.1.8 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
Arq 5.7.9 - Online backup to Google Driv...
Arq is super-easy online backup for Mac and Windows computers. Back up to your own cloud account (Amazon Cloud Drive, Google Drive, Dropbox, OneDrive, Google Cloud Storage, any S3-compatible server... Read more
VirtualBox 5.1.20 - x86 virtualization s...
VirtualBox is a family of powerful x86 virtualization products for enterprise as well as home use. Not only is VirtualBox an extremely feature rich, high performance product for enterprise customers... Read more
beaTunes 4.6.14 - Organize your music co...
beaTunes is a full-featured music player and organizational tool for music collections. How well organized is your music library? Are your artists always spelled the same way? Any R.E.M. vs REM?... Read more
iDefrag 5.1.8 - Disk defragmentation and...
iDefrag helps defragment and optimize your disk for improved performance. Features include: Supports HFS and HFS+ (Mac OS Extended). Supports case sensitive and journaled filesystems. Supports... Read more
Day One 2.1.8 - Maintain a daily journal...
Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through... Read more

Latest Forum Discussions

See All

Blizzard is looking to hire a mobile dev...
A new thread on the popular video game rumor forum, NeoGAF, uncovered an interesting job listing over at Blizzard Entertainment. It appears the studio behindStarCraft, World of WarCraft, Hearthstone,andOverwatch is looking to bring on a new hire... | Read more »
Legend of Zelda meets Cooking Mama in ne...
Dungeon Chef is what happens when you mix the RPG elements (and style) of a Legend of Zelda game, with cooking elements. Although, now that The Legend of Zelda: Breath of the Wild also has cookingelements, so maybe the gameplay is not so novel.... | Read more »
ChordFlow (Music)
ChordFlow 1.0.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0.0 (iTunes) Description: ChordFlow is a chord sequencer with a unique 4-track polyphonic arpeggiator, extensive chord library, MIDI out and Ableton Link... | Read more »
The Walking Dead: A New Frontier is out...
The newest season of Telltale Games'The Walking Dead is well underway. After the release of the third episode, "Above the Law" about a month ago, episode four, "Thicker Than Water" is hot and ready for more zombies and gut-wrenching emotional... | Read more »
Best games we played this week
Another week, another new wave of mobile games do dive into. We've dug through the list of apps that came out this week to tell you which apps are worth your sweet time. And while there weren't too many games this week, there were some big ones.... | Read more »
Vignettes (Games)
Vignettes 1.0.1 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.1 (iTunes) Description: Vignettes is a casual but unique exploration game without text or characters, where objects shapeshift as you spin them around... | Read more »
Get Me Outta Here is an 80s retro shoote...
Are you ready to fight some aliens? Because Crescent Moon Games has released the retro shooter Get Me Outta Here on iOS devices today. [Read more] | Read more »
Get a bunch of Apple productivity apps f...
If you're an Apple Mac owner, you're probably aware of the host of Apple productivity apps the company includes in all new Mac purchases. Apps like iMovie, Keynote, and of course, GarageBand. While you used to be able to also buy these apps... | Read more »
Terra Mystica (Games)
Terra Mystica 1.03 Device: iOS Universal Category: Games Price: $9.99, Version: 1.03 (iTunes) Description: Short Summary:≈≈≈≈≈≈≈≈≈≈≈≈≈ | Read more »
Ms. Spell (Games)
Ms. Spell 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Cast spells and battle monsters in this turn based game, that has you delving into ever the changing Dreadwood to retrieve the lost... | Read more »

Price Scanner via MacPrices.net

15-inch Touch Bar MacBook Pros, Apple refurbi...
Apple is offering Certified Refurbished 2016 15″ Touch Bar MacBook Pros for $360 to $420 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2.6GHz... Read more
13-inch MacBook Airs on sale for up to $150 o...
Overstock.com has 13″ MacBook Airs on sale for up to $150 off MSRP including free shipping: - 13″ 1.6GHz/128GB MacBook Air (sku MMGF2LL/A): $869.99 $130 off MSRP - 13″ 1.6GHz/256GB MacBook Air (sku... Read more
15-inch Touch Bar MacBook Pros on sale for $1...
B&H Photo has the new 2016 15″ Apple Touch Bar MacBook Pros in stock today and on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 15″ 2.7GHz Touch Bar... Read more
15-inch 2.7GHz Touch Bar MacBook Pros on sale...
Amazon has 2016 15″ 2.7GHz Apple Touch Bar MacBook Pros in stock today and on sale for $150-$200 off MSRP. Shipping is free: - 15″ 2.7GHz Touch Bar MacBook Pro Space Gray (sku MLH42LL/A): $2599 $200... Read more
Apple now offering Certified Refurbished 13-i...
Apple is now offering Certified Refurbished 2016 13″ Touch Bar MacBook Pros for $270-$300 off original MSRP. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.9GHz... Read more
MyGiHealth Digestive Symptom Tracker Version...
My Total Health, Inc. has announced the release of MyGiHealth 2.1, an important update to their digestive symptom tracker developed exclusively for iPhone, iPad and iPod touch devices. MyGiHealth is... Read more
Galaxy S8 Materials Costs Highest by Far Comp...
The new Samsung Galaxy S8 equipped with 64 gigabytes (GB) of NAND flash memory carries a bill of materials (BOM) cost that comes out to US$301.60, much higher than for previous versions of the... Read more
iCarMode 4.0 Car Dashboard App For iOS Integr...
Indie developer Diego Resnik has announced the release of iCarMode 4.0, an update to his productivity app developed for iOS devices. iCarMode has positioned itself as a true car dashboard app,... Read more
How to save $150+ on Apple’s 13-inch 2.0GHz n...
Apple Authorized Reseller B&H Photo has non-Touch Bar 13″ 2.0GHz MacBook Pros on sale for $150 off MSRP for a limited time. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.0GHz... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more

Jobs Board

*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Music Marketing Lead, iTunes & *Apple*...
# Music Marketing Lead, iTunes & Apple Music Job Number: 56868140 Culver City, California, United States Posted: Apr. 17, 2017 Weekly Hours: 40.00 **Job Summary** Read more
*Apple* Media Products - Commerce Engineerin...
Apple Media Products - Commerce Engineering Manager Job Number: 57037480 Santa Clara Valley, California, United States Posted: Apr. 18, 2017 Weekly Hours: 40.00 Job Read more
*Apple* Mac Computer Technician - GeekHampto...
…complex computer issues over the phone and in person? GeekHampton, Long Island's Apple Premium Service Provider, is looking for you! Come work with our crew Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.