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.

 
AAPL
$119.00
Apple Inc.
+1.40
MSFT
$47.75
Microsoft Corpora
+0.28
GOOG
$540.37
Google Inc.
-0.71

MacTech Search:
Community Search:

Software Updates via MacUpdate

HoudahSpot 3.9.6 - Advanced file search...
HoudahSpot is a powerful file search tool built upon MacOS X Spotlight. Spotlight unleashed Create detailed queries to locate the exact file you need Narrow down searches. Zero in on files Save... Read more
RapidWeaver 6.0.3 - Create template-base...
RapidWeaver is a next-generation Web design application to help you easily create professional-looking Web sites in minutes. No knowledge of complex code is required, RapidWeaver will take care of... Read more
iPhoto Library Manager 4.1.10 - Manage m...
iPhoto Library Manager lets you organize your photos into multiple iPhoto libraries. Separate your high school and college photos from your latest summer vacation pictures. Or keep some photo... Read more
iExplorer 3.5.1.9 - View and transfer al...
iExplorer is an iPhone browser for Mac lets you view the files on your iOS device. By using a drag and drop interface, you can quickly copy files and folders between your Mac and your iPhone or... Read more
MacUpdate Desktop 6.0.3 - Discover and i...
MacUpdate Desktop 6 brings seamless 1-click installs and version updates to your Mac. With a free MacUpdate account and MacUpdate Desktop 6, Mac users can now install almost any Mac app on macupdate.... Read more
SteerMouse 4.2.2 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. It also supports Apple Mighty Mouse very well. SteerMouse can assign various functions to buttons that Apple's software does not allow,... Read more
iMazing 1.1 - Complete iOS device manage...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
PopChar X 7.0 - Floating window shows av...
PopChar X helps you get the most out of your font collection. With its crystal-clear interface, PopChar X provides a frustration-free way to access any font's special characters. Expanded... Read more
Carbon Copy Cloner 4.0.3 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
ForeverSave 2.1.3 - Universal auto-save...
ForeverSave auto-saves all documents you're working on while simultaneously doing backup versioning in the background. Lost data can be quickly restored at any time. Losing data, caused by... Read more

Latest Forum Discussions

See All

Make Way for Fat Chicken, from the Maker...
Make Way for Fat Chicken, from the Makers of Scrap Squad Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Relevant Games has announced they will be releasing their reverse tower defense game, | Read more »
Tripnary Review
Tripnary Review By Jennifer Allen on November 26th, 2014 Our Rating: :: TRAVEL BUCKET LISTiPhone App - Designed for the iPhone, compatible with the iPad Want to create a travel bucket list? Tripnary is a fun way to do exactly that... | Read more »
Ossian Studios’ RPG, The Shadow Sun, is...
Ossian Studios’ RPG, The Shadow Sun, is Now Available for $4.99 Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Mmmm, Tasty – Having the Angry Birds for...
The very first Angry Birds debuted on iOS back in 2009. When you sit back and tally up the number of Angry Birds games out there and the impact they’ve had on pop culture as a whole, you just need to ask yourself: “How would the birds taste... | Read more »
Rescue Quest Review
Rescue Quest Review By Jennifer Allen on November 26th, 2014 Our Rating: :: PATH BASED MATCH-3Universal App - Designed for iPhone and iPad Guide a wizard to safety by matching gems. Rescue Quest might not be an entirely original... | Read more »
You Can Play the Final Chapter of Lone W...
You Can Play the Final Chapter of Lone Wolf: Dawn Over V’taag Right Now Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Swords of Anima (Games)
Swords of Anima 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: A new tactical turn-based RPG experience. Command the Savior Rex Squad in an epic journey of courage and deception. Can you... | Read more »
Audio Defence: Zombie Arena
Audio Defence: Zombie Arena By Lee Hamlet on November 26th, 2014 Our Rating: :: DRAGS ITS FEETUniversal App - Designed for iPhone and iPad From the makers of Papa Sangre comes a defense game that forces players to listen carefully... | Read more »
Tales from the Borderland​s Will be Comi...
Tales from the Borderland​s Will be Coming to iOS by the End of the Year Posted by Jessica Fisher on November 26th, 2014 [ permalink ] Telltale Games has announced | Read more »
Sunburn! Review
Sunburn! Review By Campbell Bird on November 26th, 2014 Our Rating: :: DON'T DIE ALONEUniversal App - Designed for iPhone and iPad Platform through the depths of space to make sure your entire crew dies together in this satisfying... | Read more »

Price Scanner via MacPrices.net

2014 1.4GHz Mac mini on sale for $449, save $...
 B&H Photo has the new 1.4GHz Mac mini on sale for $449.99 including free shipping plus NY tax only. Their price is $50 off MSRP, and it’s the lowest price available for this new model. Adorama... Read more
Early Black Friday pricing on 27-inch 5K iMac...
 B&H Photo continues to offer Black Friday sale prices on the 27″ 3.5GHz 5K iMac, in stock today and on sale for $2299 including free shipping plus NY sales tax only. Their price is $200 off MSRP... Read more
Early Black Friday sale prices on iPad Air 2,...
 MacMall is discounting iPad Air 2s by up to $75 off MSRP as part of their Black Friday sale. Shipping is free: - 16GB iPad Air WiFi: $459 $40 off - 64GB iPad Air WiFi: $559 $40 off - 128GB iPad Air... Read more
Early Black Friday MacBook Air sale prices, $...
 MacMall has posted early Black Friday MacBook Air sale prices. Save $101 on all models for a limited time: - 11″ 1.4GHz/128GB MacBook Air: $798 - 11″ 1.4GHz/256GB MacBook Air: $998 - 13″ 1.4GHz/... Read more
Why iPhone 6 Tablet/Laptop Cannibalization Is...
247wallst.com blogger Douglas A. McIntyre noted last week that according to research posted on the Applovin blog site the iPhone 6 is outselling the iPhone 6 Plus by a wide margin . Hardly a surprise... Read more
Worldwide Tablet Growth Expected to Slow to 7...
The global tablet market is expected to record massive deceleration in 2014 with year-over-year growth slowing to 7.2%, down from 52.5% in 2013, according to a new forecast from International Data... Read more
Touchscreen Glove Company Announces New Produ...
Surrey, United Kingdom based TouchAbility specializes in design and manufacture of a wide variety of products compatible with touchscreen devices including smartphones, tablets and computers. Their... Read more
OtterBox Alpha Glass Screen Protectors for iP...
To complement the bigger, sharper displays on the latest Apple devices, OtterBox has introduced Alpha Glass screen protectors to the iPhone 6 and iPhone 6 Plus. The fortified glass screen protectors... Read more
Early Black Friday Mac Pro sale, 6-Core 3.5GH...
 B&H Photo has the 6-Core 3.5GHz Mac Pro on sale today for $3499 including free shipping plus NY sales tax. Their price is $500 off MSRP, and it’s the lowest price available for this model from... Read more
Early Black Friday sale price: 15-inch 2.2GHz...
 B&H Photo has the 2014 15″ 2.2GHz Retina MacBook Pro on sale today for $1699.99. Shipping is free, and B&H charges NY sales tax only. Their price is $300 off MSRP, equalling Best Buy’s price... Read more

Jobs Board

*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.