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IntroductionAn Open Letter to Future Perl Programmers
I want to tell you about some of the things I've learned during the time that I've been teaching and working with Perl. That's why this book is here.
First of all, someone once told me, "If you want to learn a programming language, you have to do two things. You need to read a lot of code and you need to write a lot of code." We've tried to make that process inevitable with this book. You will not get to the other end of this book successfully until you've read and written a number of programs.
It follows that you can't learn Perl by simply reading a book. You can look at the information, understand it, but you can't KNOW it until you try it with your own hands. (If you want to grok it, you have to DO it.) It doesn't matter if the book title says "Instant" or "Made Easy"; you will not learn the language from a book, you learn it through an effort of your mind.
If you'll lend me your mind for a little while, I'll try to drive your effort along a path that exposes you to the power of the Perl programming language. Your part of the bargain is to complete the exercises and answer the questions along the way, even if the topic doesn't seem related to the burning problem you want to solve. If you pay attention to the scenery on the way, you'll end up a more capable programmer.To Get Started
So what do you need to get started with this book? You will need a system with Perl installed. If you need help with this step, you might find information relevant to your platform in Appendix B. Otherwise, you'll want to check the book's companion website, which you can find at phptr/phptrinteractive/.
The next thing you'll need is a command processor (or a shell). UNIX programmers can use any of the traditional shells. Win32 programmers can use your native OS command processor (COMMAND.COM or CMD.EXE) or an aftermarket shell (such as 4DOS, Korn shell, or Bash). For the type of work we'll be doing here, there's very little point-and-click. If you don't find enough information in Chapter 1, "Getting Started," look for additional help with setup and choosing a shell at the companion website.
Another thing you will need is the Perl documentation. You can get it from the same place you got Perl itself. This documentation will make your work with this book much easier. Everything you need to know to solve the exercises is in this book, but you may find that you can come up with more creative or bulletproof solutions if you learn to search the documentation. Before you tear into the exercises, make sure that you have the documentation properly installed in a convenient fashion.At First, A Brisk Tour
In the first two chapters, we take you rapidly through a number of techniques that you will use in your subsequent Perl programs. Don't worry too much if you can't understand why we do things a certain way in the first couple of chapters. Everything you see there will be explored in detail later in the book. But when you get to Chapter 3, "Scalar Variables," you'll already know enough Perl to do some interesting exercises.
Every book has at least one formal structure. That is the beginning-to-end or sequential structure. Sometimes a book will have another structure. It will be available through an index that describes the book in an associative fashion. A dictionary or encyclopedia is like this. The book you hold in your hand will also have this structure. But there is another. A "narrative structure" is also present.
Through the narrative that associates your study with each of four fictional characters, you will watch (and help build) each of four different projects in Perl. Later on in this introduction, I'll tell you more about the characters themselves and their final objectives.Confidentially, you should know...
One more word to the wise: If you currently plan to run your Perl programs in a UNIX environment, don't just let your eyes glaze over when we talk about what it takes to get the programs to run on a Win32 box. If you plan to become an accomplished Perl programmer, there is a good chance that you'll have to face this sooner or later. Those of you who are currently using a Microsoft operating system and imagine that you always will, don't ignore the parts of this journey that apply to UNIX systems. By knowing how it works in the UNIX world, you'll open a new vista for yourself, maybe even discover a new way of thinking about how your programs should be designed. Telling the Whole Truth
Sometimes, there is a distinction drawn between concepts such as function, operator, command, expression, keyword, and modifier. Each of these terms has a formal meaning in Perl, but you needn't worry about that while you're learning the language. If it looks like a function, smells like a function, and tastes like a function, you can call it that, even if some erudite Usenet snob tells you that it's really an operator. He may be right, but so are you.
So we will avoid worrying too much about the "inner secrets" until we need to know them. If it looks and smells like a two-dimensional array (or a function), then I'll let you call it that and I'll expect the same forgiveness from you.The People you'll meet
As you pursue your studies here, you'll run into some interesting characters.
Uncle Larry: Everybody called him Uncle Larry and almost no one could remember his full name. (One of his friends once said, "His real name? Why, I call him Uncle Larry and he responds. I speak of him by that name to others and they know who I mean. I guess that would make it his real name.") He claimed to be nothing more than a simple storyteller, but most of the people who knew him thought Larry was a prophet. He told stories, yes. But he also served as a mentor for most of the people he knew. He lent a hand when people had questions, and he always had a ready answer when someone found themselves at a dead end with a programming problem. (Larry develops a Storyteller's program that demonstrates pattern substitution and file I/O techniques.)
Major Ellie: Victim of an inexplicable temporal accident, Major Ellie found herself cast into the pre-spacefaring society of 21st century earth. (Yes, this is a pre-spacefaring society that we live in today.) Ellie knew that Arthur Clarke's prediction would one day be accurate. (Clarke said, "If mankind persists as long as even the shortest lived species of dinosaur, the word ship will, for all but a brief time at the dawn of history, mean a vessel that carries people between the stars.") Ellie thrust herself into the work of helping to drag mankind into space, and later to the stars. (Ellie works on a Solar System model that explores Perl data structures.)
Coach Randal: Randal knew something that many sports fans, and everyone who hates sports, didn't know. If you watched for a while, you'd usually see someone exceed their potential and do something extraordinary. That was what made it worthwhile. Never mind the money, the publicity, the personalities, and the egos. Sport was about the players, small or large, who found a way to dig deep into their souls and find their absolute best. This is what caused them to take less money, work the long days, live with the pressure and, sometimes, the heartbreak. Those golden moments of heroism made everything worthwhile. (Randal works on a couple of tools that explore the use of stacks and queues in Perl.)
Barber Tom: Nobody feels the pulse of the community more surely than a good barber. Sooner or later, everyone had to come and see him. When they felt comfortable with him, people told Tom their secrets and concerns. He knew this and believed that it obligated him to share the benefit of his insight whenever it was required. This he did cheerfully, even on those rare occasions when he was the only person who realized that his insight was a benefit. (Tom works on an interactive program that maintains a list of "To Do" items.)Conventions Used In This Book
The following typographical conventions are followed in this book:
A command you should enter into the computer
$ echo $LOGNAME johnmc
An example of interaction with the computer. Notice that the parts you would type are bold, the output from the computer is in the plain style.
The names of UNIX utility programs will look like this. The parenthesized number is a manual page section reference. If you read the man page on your system for the utility, you'll see that it's in the section referred to by this number. Some versions of UNIX use a different method of referring to the various sections. This is the traditional approach.
Our primary interest here is that the name is that of a UNIX utility and that there is a man page for the utility.
for(), foreach(), or while()
The name of a Perl function.
In this book, we present the names of loop constructs with trailing parentheses. This is simply a visual reminder that the reference is to a construct that does expect a parenthesized clause.
Line number references are italicized to help make it easier to follow the discussions that follow a code example.
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