Learning Perl

Learning Perl

by Randal L. Schwartz, Tom Phoenix, brian d foy

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Overview

Learning Perl, popularly known as "the Llama," is the book most programmers rely on to get started with Perl. The bestselling Perl tutorial since it was first published in 1993, this new fifth edition covers recent changes to the language up to Perl 5.10.

This book reflects the combined experience of its authors, who have taught Perl at Stonehenge Consulting since 1991. Years of classroom testing and experience helped shape the book's pace and scope, and this edition is packed with exercises that let you practice the concepts while you follow the text. Topics include:

  • Perl data & variable types
  • Subroutines
  • File operations
  • Regular expressions
  • String manipulation
  • Lists & sorting
  • Process management
  • Smart matching
  • Using third party modules

Perl is the language for people who want to get work done. Originally targeted to sysadmins for heavy-duty text processing, Perl is now a full-featured programming language suitable for almost any task on almost any platform-from short fixes on the command line to web applications, bioinformatics, finance, and much more. Other books may teach you to program in Perl, but this book will turn you into a Perl programmer.

Product Details

ISBN-13: 9780596551858
Publisher: O'Reilly Media, Incorporated
Publication date: 06/27/2008
Sold by: Barnes & Noble
Format: NOOK Book
Pages: 352
Sales rank: 1,021,949
File size: 2 MB

About the Author

Randal Schwartz coauthored the "must-have" standards: Programming Perl, Learning Perl, Learning Perl for Win32 Systems, and Effective Perl Programming, and writes regular columns for WebTechniques, PerformanceComputing, SysAdmin, and Linux magazines. He's a frequent contributor to the Perl newsgroups, and has moderated comp.lang.perl.announce since its inception. Randal's desire to give back to the Perl community inspired him to help create and provide initial funding for The Perl Institute. He is a founding board member of the Perl Mongers (perl.org), the worldwide Perl grassroots advocacy organization. Since 1985, Randal has owned and operated Stonehenge Consulting Services, Inc.

Tom Phoenix has worked in the field of education since 1982 and has taught Perl classes for Stonehenge Consulting Services since 1996. He answers many of the questions on the comp.lang.perl.* newsgroups and contributes to the development and usefulness of Perl.

Read an Excerpt

Chapter 4: Subroutines

System and User Functions

We've already seen and used some of the builtin system functions, such as chomp, reverse, print, and so on. But, as other languages do, Perl has the ability to make subroutines, which are user-defined functions.1 These let us recycle one chunk of code many times in one program.2

The name of a subroutine is another Perl identifier (letters, digits, and underscores, but can't start with a digit) with a sometimes-optional ampersand (&) in front. There's a rule about when you can omit the ampersand and when you cannot; we'll see that rule by the end of the chapter. For now, we'll just use it every time that it's not forbidden, which is always a safe rule. And we'll tell you every place where it's forbidden, of course.

That subroutine name comes from a separate namespace, so Perl won't be confused if you have a subroutine called &fred and a scalar called $fred in the same program--although there's no reason to do that under normal circumstances.

Defining a Subroutine

To define your own subroutine, use the keyword sub, the name of the subroutine (without the ampersand), then the indented3 block of code (in curly braces) which makes up the body of the subroutine, something like this:

sub marine {
  $n += 1;  # Global variable $n
  print "Hello, sailor number $n!\n";
}

Subroutine definitions can be anywhere in your program text, but programmers who come from a background of languages like C or Pascal like to put them at the start of the file. Others may prefer to put them at the end of the file, so that the main part of the program appears at the beginning. It's up to you. In any case, you don't normally need any kind of forward declaration.4

Subroutine definitions are global; without some powerful trickiness, there are no private subroutines.5 If you have two subroutine definitions with the same name, the later one overwrites the earlier one.6 That's generally considered bad form, or the sign of a confused maintenance programmer.

As you may have noticed in the previous example, you may use any global variables within the subroutine body. In fact, all of the variables we've seen so far are globals; that is, they are accessible from every part of your program. This horrifies linguistic purists, but the Perl development team formed an angry mob with torches and ran them out of town years ago. We'll see how to make private variables in the section "Private Variables in Subroutines" later in this chapter.

Invoking a Subroutine

Invoke a subroutine from within any expression by using the subroutine name (with the ampersand):7

&marine;  # says Hello, sailor number 1!
&marine;  # says Hello, sailor number 2!
&marine;  # says Hello, sailor number 3!
&marine;  # says Hello, sailor number 4!

Sometimes, we refer to the invocation as calling the subroutine.

Return Values

The subroutine is always invoked as part of an expression, even if the result of the expression isn't being used. When we invoked &marine earlier, we were calculating the value of the expression containing the invocation, but then throwing away the result.

Many times, we'll call a subroutine and actually do something with the result. This means that we'll be paying attention to the return value of the subroutine. All Perl subroutines have a return value--there's no distinction between those that return values and those that don't. Not all Perl subroutines have a useful return value, however.

Since all Perl subroutines can be called in a way that needs a return value, it'd be a bit wasteful to have to declare special syntax to "return" a particular value for the majority of the cases. So Larry made it simple. Every subroutine is chugging along, calculating values as part of its series of actions. Whatever calculation is last performed in a subroutine is automatically also the return value.

For example, let's define this subroutine:

sub sum_of_fred_and_barney {
  print "Hey, you called the sum_of_fred_and_barney subroutine!\n";
  $fred + $barney;  # That's the return value
}

The last expression evaluated in the body of this subroutine is the sum of $fred and $barney, so the sum of $fred and $barney will be the return value. Here's that in action:

$fred = 3;
$barney = 4;
$c = &sum_of_fred_and_barney; # $c gets 7
print "\$c is $c.\n";
$d = 3 * &sum_of_fred_and_barney; # $d gets 21
print "\$d is $d.\n";

That code will produce this output:

Hey, you called the sum_of_fred_and_barney subroutine!
$c is 7.
Hey, you called the sum_of_fred_and_barney subroutine!
$d is 21.

That print statement is just a debugging aid, so that we can see that we called the subroutine. You'd take it out when the program is finished. But suppose you added another line to the end of the code, like this:

sub sum_of_fred_and_barney {
  print "Hey, you called the sum_of_fred_and_barney subroutine!\n";
  $fred + $barney;  # That's not really the return value!
  print "Hey, I'm returning a value now!\n"; # Oops!
}

In this example, the last expression evaluated is not the addition; it's the print statement. Its return value will normally be 1, meaning "printing was successful,"8 but that's not the return value we actually wanted. So be careful when adding additional code to a subroutine to ensure that the last expression evaluated will be the desired return value.

So, what happened to the sum of $fred and $barney in that subroutine? We didn't put it anywhere, so Perl discarded it. If you had requested warnings, Perl (noticing that there's nothing useful about adding two variables and discarding the result) would likely warn you about something like "a useless use of addition in a void context." The term void context is just a fancy of saying that the answer isn't being stored in a variable or used by another function.

"The last expression evaluated" really means the last expression evaluated, rather than the last line of text. For example, this subroutine returns the larger value of $fred or $barney:

sub larger_of_fred_or_barney {
  if ($fred > $barney) {
    $fred;
  } else {
    $barney;
  }
}

The last expression evaluated is the single $fred or $barney, which becomes the return value. We won't know whether the return value will be $fred or $barney until we see what those variables hold at runtime.

A subroutine can also return a list of values when evaluated in a list context.9 Suppose you wanted to get a range of numbers (as from the range operator, ..), except that you want to be able to count down as well as up. The range operator only counts upwards, but that's easily fixed:

sub list_from_fred_to_barney {
  if ($fred < $barney) {
    # Count upwards from $fred to $barney
    $fred..$barney;
  } else {
    # Count downwards from $fred to $barney
    reverse $barney..$fred;
  }
}
$fred = 11;
$barney = 6;
@c = &list_from_fred_to_barney; # @c gets (11, 10, 9, 8, 7, 6)

In this case, the range operator gives us the list from 6 to 11, then reverse reverses the list, so that it goes from $fred (11) to $barney (6), just as we wanted.

These are all rather trivial examples. It gets better when we can pass values that are different for each invocation into a subroutine instead of relying on global variables. In fact, that's coming right up.

Arguments

That subroutine called larger_of_fred_or_barney would be much more useful if it didn't force us to use the global variables $fred and $barney. That's because, if we wanted to get the larger value from $wilma and $betty, we currently have to copy those into $fred and $barney before we can use larger_of_fred_or_barney. And if we had something useful in those variables, we'd have to first copy those to other variables, say $save_fred and $save_barney. And then, when we're done with the subroutine, we'd have to copy those back to $fred and $barney again.

Luckily, Perl has subroutine arguments. To pass an argument list to the subroutine, simply place the list expression, in parentheses, after the subroutine invocation, like this:

$n = &max(10, 15);  # This sub call has two parameters

That list is passed to the subroutine; that is, it's made available for the subroutine to use however it needs to. Of course, this list has to be stored into a variable, so the parameter list (another name for the argument list) is automatically assigned to a special array variable named @_ for the duration of the subroutine. The subroutine can access this variable to determine both the number of arguments and the value of those arguments.

So, that means that the first subroutine parameter is stored in $_[0], the second one is stored in $_[1], and so on. But--and here's an important note--these variables have nothing whatsoever to do with the $_ variable, any more than $dino[3] (an element of the @dino array) has to do with $dino (a completely distinct scalar variable). It's just that the parameter list must be stored into some array variable for the subroutine to use it, and Perl uses the array @_ for this purpose.

Now, you could write the subroutine &max to look a little like the subroutine &larger_of_fred_or_barney, but instead of using $a you could use the first subroutine parameter ($_[0]), and instead of using $b, you could use the second subroutine parameter ($_[1]). And so you could end up with code something like this:

sub max {
  # Compare this to &larger_of_fred_or_barney
  if ($_[0] > $_[1]) { 
    $_[0];
  } else {
    $_[1];
  }
}

Well, as we said, you could do that. But it's pretty ugly with all of those subscripts, and hard to read, write, check, and debug, too. We'll see a better way in a moment.

There's another problem with this subroutine. The name &max is nice and short, but it doesn't remind us that this subroutine works properly only if called with exactly two parameters:

$n = &max(10, 15, 27);  # Oops!

Excess parameters are ignored--since the subroutine never looks at $_[2], Perl doesn't care whether there's something in there or not. And insufficient parameters are also ignored--you simply get undef if you look beyond the end of the @_ array, as with any other array. We'll see how to make a better &max, which works with any number of parameters, later in this chapter.

The @_ variable is local to the subroutine;10 if there's a global value in @_, it is saved away before the subroutine is invoked and restored to its previous value upon return from the subroutine.[11] This also means that a subroutine can pass arguments to another subroutine without fear of losing its own @_ variable--the nested subroutine invocation gets its own @_ in the same way. Even if the subroutine calls itself recursively, each invocation gets a new @_, so @_ is always the parameter list for the current subroutine invocation.

Private Variables in Subroutines

But if Perl can give us a new @_ for every invocation, can't it give us variables for our own use as well? Of course it can.

By default, all variables in Perl are global variables; that is, they are accessable from every part of the program. But you can create private variables called lexical variables at any time with the my operator:

sub max {
  my($a, $b);       # new, private variables for this block
  ($a, $b) = @_;    # give names to the parameters
  if ($a > $b) { $a } else { $b }
}

These variables are private (or scoped) to the enclosing block; any other $a or $b is totally unaffected by these two. And that goes the other way, too--no other code can access or modify these private variables, by accident or design.12 So, we could drop this subroutine into any Perl program in the world and know that we wouldn't mess up that program's $a and $b (if any).13

It's also worth pointing out that, inside the if's blocks, there's no semicolon needed after the return value expression. Although Perl allows for the last semicolon in a block to be omitted, in practice that's omitted only when the code is so simple that the block is written in a single line, like the previous ones.

The subroutine in the previous example could be made even simpler. Did you notice that the list ($a, $b) was written twice? That my operator can also be applied to a list of variables enclosed in parentheses, so it's more customary to combine those first two statements in the subroutine...

Table of Contents


Preface     xi
Introduction     1
Questions and Answers     1
What Does "Perl" Stand For?     4
How Can I Get Perl?     8
How Do I Make a Perl Program?     12
A Whirlwind Tour of Perl     16
Exercises     17
Scalar Data     19
Numbers     19
Strings     22
Perl's Built-in Warnings     26
Scalar Variables     27
Output with print     29
The if Control Structure     33
Getting User Input     34
The chomp Operator     35
The while Control Structure     36
The undef Value     36
The defined Function     37
Exercises     38
Lists and Arrays     39
Accessing Elements of an Array     40
Special Array Indices     41
List Literals     41
List Assignment     43
Interpolating Arrays into Strings     46
The foreach Control Structure     47
Scalar and List Context     49
[left angle bracket]STDIN[right angle bracket] in List Context     52
Exercises     54
Subroutines     55
Defining a Subroutine     55
Invoking a Subroutine     56
Return Values     56
Arguments     58
Private Variables in Subroutines     60
Variable-Length Parameter Lists     60
Notes on Lexical (my) Variables     63
The use strict Pragma     64
The return Operator     65
Nonscalar Return Values     67
Persistent, Private Variables     68
Exercises     69
Input and Output     71
Input from Standard Input     71
Input from the Diamond Operator     73
The Invocation Arguments     75
Output to Standard Output     76
Formatted Output with printf     79
Filehandles     81
Opening a Filehandle     83
Fatal Errors with die     86
Using Filehandles     88
Reopening a Standard Filehandle     90
Output with say     90
Exercises     91
Hashes     93
What Is a Hash?     93
Hash Element Access     96
Hash Functions     100
Typical Use of a Hash      103
The %ENV hash     104
Exercises     105
In the World of Regular Expressions     107
What Are Regular Expressions?     107
Using Simple Patterns     108
Character Classes     113
Exercises     115
Matching with Regular Expressions     117
Matches with m//     117
Option Modifiers     118
Anchors     120
The Binding Operator, =

Foreword

Attention, class! Attention! Thank you.

Greetings, aspiring magicians. I hope your summer vacations were enjoyable, if too short. Allow me to be the first to welcome you to the College of Wizardry and, more particularly, to this introductory class in the Magic of Perl. I am not your regular instructor, but Professor Schwartz was unavoidably delayed, and has asked me, as the creator of Perl, to step in today and give a few introductory remarks.

Let's see now. Where to begin? How many of you are taking this course as freshmen? I see. Hmmm, I've seen worse in my days. Occasionally. Very occasionally.

Eh? That was a joke. Really! Ah well. No sense of humor, these freshmen.

Well now, what shall I talk about? There are, of course, any number of things I could talk about. I could take the egotistical approach and talk about myself, elucidating all those quirks of genetics and upbringing that brought me to the place of creating Perl, as well as making a fool of myself in general. That might be entertaining, at least to me.

Or I could talk instead about Professor Schwartz, without whose ongoing efforts the world of Pert would be much impoverished, up to and including the fact that this course of instruction wouldn't exist.

That might be enlightening, though I have the feeling you'll know more of Professor Schwartz by the end of this course than I do.

Or, putting aside all this personal puffery, I could simply talk about Pert itself, which is, after all, the subject of this course.

Or is it? Hmmm...

When the curriculum committee discussed this course, it reached the conclusion that this class isn't so much about Perl asit is about you! This shouldn't be too surprising, because Pert is itself also about you-at least in the abstract. Perl was created for someone like you, by someone like you, with the collaboration of many other someones like you. The Magic of Perl was sewn together, stitch by stitch and swatch by swatch, around the rather peculiar shape of your psyche. if you think Perl is a bit odd, perhaps that's why.

Some computer scientists (the reductionists, in particular) would like to deny it, but people have funny-shaped minds. Mental geography is not linear, and cannot be mapped onto a flat surface without severe distortion. But for the last score years or so, computer reductionists have been first bowing down at the Temple of Orthogonality, then rising up to preach their ideas of ascetic rectitude to any who would listen.

Their fervent but misguided desire was simply to squash your mind to fit their mindset, to smush your patterns of thought into some sort of hyperdimensional flatland. It's a joyless existence, being smushed.

Nevertheless, your native common sense has shown through in spots. You and your conceptual ancestors have transcended the dreary landscape to compose many lovely computer incantations. (Some of which, at times, actually did what you wanted them to.) The most blessed of these incantations were canonized as Standards, because they managed to tap into something mystical and magical, performing the miracle of Doing What You Expect.

What nobody noticed in all the excitement was that the computer reductionists were still busily trying to smush your minds flat, albeit on a slightly higher plane of existence. The decree, therefore, went out (I'm sure you've heard of it) that computer incantations were only allowed to perform one miracle apiece. "Do one thing and do it well" was the rallying cry, and with one stroke, shell programmers were condemned to a life of muttering and counting beads on strings (which in these latter days have come to be known as pipelines).

This was when I made my small contribution to saving the world. I was rolling some of those very beads around in my fingers one day and pondering the hopelessness (and haplessness) of my existence, when it occurred to me that it might be interesting to melt down some of those mystical beads and see what would happen to their Magic if I made a single, slightly larger bead out of them. So I fired up the old Bunsen burner, picked out some of my favorite beads, and let them melt together however they would. And lo! the new Magic was more powerful than the sum of its parts and parcels.

That's odd, thought 1. Why should it be that the Sedulous Bead of Regular Expressions, when bonded together with the Shellacious Bead of Gnostic Interpolation, and the Awkward Bead of Simple Data Typology, should produce more Magic, pound for pound, than they do when strung out on strings? I said to myself, could it be that the beads can exchange power with each other because they no longer have to commune with each other through that skinny little string? Could the pipeline be holding back the flow of information, much as wine doth resist flowing through the neck of Doctor von Neumann's famous bottle?

This demanded (of me) more scrutiny (of it).

So I melted that larger bead together with a few more of my favorite beads, and the same thing happened, only more so. It was practically a combinatorial explosion of potential incantations: the Basic Bead of Output Formats and the Lispery Bead of Dynamic Scoping bonded themselves with the C-rationalized Bead of Operators Galore, and together they put forth a brilliant pulse of power that spread to thousands of machines throughout the entire civilized world. That message cost the Net hundreds if not thousands of dollars to send everywhere. Obviously I was either onto something, or on something.

I then gathered my courage about me and showed my new magical bead to some of you, and you then began to give me your favorite beads to add in as well. The Magic grew yet more powerful, as yet more synergy was imbued in the silly thing. It was as if the Computational Elementals summoned by each bead were cooperating on your behalf to solve your problems for you. Why the sudden peace on earth and good will toward mentality? Perhaps it was because the beads were your favorite beads? Perhaps it was because I'm just a good bead picker?

Perhaps I just got lucky.

Whatever, the magical bead eventually grew into this rather odd-looking Amulet you see before you today. See it glitter, almost like a pearl.

That was another joke. Really! I assure you! Ah well. I was a freshman once too... The Amulet isn't exactly beautiful though; in fact, up close it still looks like a bunch of beads melted together. Well, all right, I admit it. it's downright ugly. But never mind that. It's the Magic that counts. Speaking of Magic, took who just walked in the door! My good buddy Merlyn, er, I should say, Professor Schwartz, is here just in the nick of time to begin telling you how to perform miracles with this little Amulet, if you're willing to learn the proper mysterious incantations. And you're in good hands; I must admit that there's no one better at muttering mysterious incantations than Professor Schwartz. Eh, Merlyn?

Anyway, to sum up. What you'll need most is courage. It is not an easy path that you've set your foot upon. You're learning a new language: a language full of strange runes and ancient chants, some easy and some difficult, many of which sound familiar, and some of which don't. You may be tempted to become discouraged and quit. But think you upon this: consider how long it took you to learn your own native tongue. Was it worth it? I think so. And have you finished learning it? I think not. Then do not expect to learn all the mysteries of Perl in a moment, as though you were consuming a mere peanut, or an olive. Rather, think of it as though you were consuming, say, a banana. Consider how this works. You do not wait to enjoy the banana until after you have eaten the whole thing. No, of course not. You enjoy each bite as you take it. And each bite motivates you to take the next bite, and the next.

So then, speaking now of the fruit of Merlyn's labors, I would urge you to enjoy this, um, course. The fruit course, of course. Ahem, that was a joke too. Ah well.

Here then, Professor, I present to you your new class. They seem to have no sense of humor whatsoever, but I expect you'll manage somehow.

Class, I present to you Professor Randal L. Schwartz, Doctor of Syntax, Wizard at Large, and of course, just Another Perl Hacker. He has my blessings, just as you have my blessings. May you Learn Perl. May you do Good Magic with Perl. And above all, may you have Lots of Fun with Perl. So be it!

So do it!

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