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

FINAL FANTASY Ⅸ (Games)
FINAL FANTASY Ⅸ 1.0.4 Device: iOS Universal Category: Games Price: $16.99, Version: 1.0.4 (iTunes) Description: ==========●Special sale price for the FINAL FANTASY IX release! ●20% off from February 10 to February 21, 2016... | Read more »
Tennis Club Story (Games)
Tennis Club Story 1.03 Device: iOS Universal Category: Games Price: $4.99, Version: 1.03 (iTunes) Description: Aim for the ace position of tennis club prestige in this simulation! Your leadership decides if players make it to the big... | Read more »
Check out the new Pirate Attack update i...
Love pirates and board games? Well, you'll love the new Pirate Attack themed update that just launched in Game of Dice. It adds a bunch of new content themed around pirates, like an all new event map based on a pirate ship which revamps the toll... | Read more »
Splash Cars guide - How to paint the tow...
Splash Cars is an arcade driving game that feels like a hybrid between Dawn of the Plow and Splatoon. In it, you'll need to drive a car around to repaint areas of a town that have lost all of their color. Check out these tips to help you perform... | Read more »
The best video player on mobile
We all know the stock video player on iOS is not particularly convenient, primarily because it asks us to hook a device up to iTunes to sync video in a world that has things like Netflix. [Read more] | Read more »
Four apps to help improve your Super Bow...
Super Bowl Sunday is upon us, and whether you’re a Panthers or a Broncos fan you’re no doubt gearing up for it. [Read more] | Read more »
LooperSonic (Music)
LooperSonic 1.0 Device: iOS Universal Category: Music Price: $4.99, Version: 1.0 (iTunes) Description: LooperSonic is a multi-track audio looper and recorder that will take your loops to the next level. Use it like a loop pedal to... | Read more »
Space Grunts guide - How to survive
Space Grunts is a fast-paced roguelike from popular iOS developer, Orange Pixel. While it taps into many of the typical roguelike sensibilities, you might still find yourself caught out by a few things. We delved further to find you some helpful... | Read more »
Dreii guide - How to play well with othe...
Dreii is a rather stylish and wonderful puzzle game that’s reminiscent of cooperative games like Journey. If that sounds immensely appealing, then you should immediately get cracking and give it a whirl. We can offer you some tips and tricks on... | Read more »
Kill the Plumber World guide - How to ou...
You already know how to hop around like Mario, but do you know how to defeat him? Those are your marching orders in Kill the Plumber, and it's not always as easy as it looks. Here are some tips to get you started. This is not a seasoned platform... | Read more »

Price Scanner via MacPrices.net

Apple refurbished 15-inch Retina MacBook Pros...
Apple has Certified Refurbished 2015 15″ Retina MacBook Pros available for up to $380 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 15″ 2... Read more
BookBook For iPad Pro Coming Soon
The iPad Pro is a device unlike any other, and with Apple Pencil, it’s the ideal portable sketchpad: all that’s missing is the modern easel and portfolio to go. TwelveSouth’s BookBook for iPad Pro... Read more
12-inch 1.2GHz Silver Retina MacBook on sale...
B&H Photo has the 12″ 1.2GHz Silver Retina MacBook on sale for $1399 including free shipping plus NY sales tax only. Their price is $200 off MSRP, and it’s the lowest price for this model from... Read more
iPads on sale at Target: $100 off iPad Air 2,...
Target has WiFi iPad Air 2s and iPad mini 4s on sale for up to $100 off MSRP on their online store for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for... Read more
Target offers Apple Watch for $100 off MSRP
Target has Apple Watches on sale for $100 for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary: - Apple... Read more
Apple refurbished 2014 13-inch Retina MacBook...
Apple has Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and shipping is free... Read more
Macs available for up to $300 off MSRP, $20 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free, and... Read more
Watch Super Bowl 50 Live On Your iPad For Fre...
Watch Super Bowl 50 LIVE on the CBS Sports app for iPad and Apple TV. Get the app and then tune in Sunday, February 7, 2016 at 6:30 PM ET to catch every moment of the big game. The CBS Sports app is... Read more
Two-thirds Of All Smart Watches Shipped In 20...
Apple dominated the smart watch market in 2015, accounting for over 12 million units and two-thirds of all shipments according to Canalys market research analysts’ estimates. Samsung returned to... Read more
12-inch 1.2GHz Retina MacBooks on sale for up...
B&H Photo has 12″ 1.2GHz Retina MacBooks on sale for $180 off MSRP. Shipping is free, and B&H charges NY tax only: - 12″ 1.2GHz Gray Retina MacBook: $1499 $100 off MSRP - 12″ 1.2GHz Silver... Read more

Jobs Board

*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* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Subject Matter Expert - Experis (Uni...
This position is for an Apple Subject Matter Expert to assist in developing the architecture, support and services for integration of Apple devices into the domain. Read more
*Apple* Macintosh OSX - Net2Source Inc. (Uni...
…: * Work Authorization : * Contact Number(Best time to reach you) : Skills : Apple Macintosh OSX Location : New York, New York. Duartion : 6+ Months The associate would Read more
Computer Operations Technician ll - *Apple*...
# Web Announcement** Apple Technical Liaison**The George Mason University, Information Technology Services (ITS), Technology Support Services, Desktop Support Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.