Robert's Perl TutorialVersion 4.1.1 |
THIS DOCUMENT IS COPYRIGHTED.
Reproduction in whole or part is prohibited. Please email me at
robert@netcat.co.uk if you want to use this information anywhere.
The location of this document is
http://www.netcat.co.uk/rob/perl/win32perltut.html
A basic Perl course primarily for use on Win32 platforms. It assumes that the reader knows nothing of programming whatsoever, but needs a solid grounding for further work. After you finish this course you'll be ready to specialise in CGI, sysadmin or whatever you want to do with Perl.
You need to be able to differentiate between a PC and a toaster. No programming experience is necessary. You do need to understand the basics of PC operation. If you don't understand what directories and files are then you'll find this difficult. You might find it difficult even if you do :-)
You do need to exercise the brain cells, and you need time.
Note: You don't even need a Win32 PC if you are comfortable installing Perl under other operating systems like Linux, but not all the information here will be relevant.
You don't need a complier. Perl is an interpreted language, which means you run code directly, not compile it then run it.
Just work through from start to finish.
Generally, the explanation follows the code sample. Before you read the explanation, try and work out what the code does. Then check if you're right. In this way, you'll derive maximum value from the tutorial and exercise the old grey cells a little.
When you finish, please send me a critique. In fact, send one even if you don't finish. I appreciate all feedback! Please note -- I am not a source of free technical support. Do not email me your general Perl problems. If you want support, ask on Usenet or the ActiveState mailing lists. That said, I welcome problems related to the tutorial itself.
The humour is non-conventional. I think. Of more importance, the text is coloured strangely in places. My intention is to aid your comprehension, not attempt beautification. The meaning of the colours:
perl changeworld.pl parm1 datafile.txt
while (<DATFILE>) {
printf "%2s : $_",$.;
}
split.
All the code examples have been tested, and you can just cut'n'paste (brave statement). I haven't listed the output of each example. You need to run it and see for yourself. Consider this course interactive. Consider it any which way you like.
--
Robert Pepper
mailto:Robert@netcat.co.uk
http://www.netcat.co.uk/rob/perl/win32perltut.html
If you already understand what Perl is designed to do, know its features and limitations then you can skip this very small but highly informative section, over which I laboured long and hard for those that didn't know. If you are really sure, jump to the Setup Section.
Perl is a programming language. Perl stands for Practical Report and Extraction Language. You'll notice people refer to 'perl' and "Perl". "Perl" is the programming language as a whole whereas 'perl' is the name of the core executable. There is no language called "Perl5" -- that just means "Perl version 5". Versions of Perl prior to 5 are very old and very unsupported.
Some of Perl's many strengths are:
A company named ActiveState exists to provide Perl tools for the Win32 environment. ActiveState used to be ActiveWare, and before that it was sort of a part of Hip Communications. It now appears to be happy with its current name, having not changed it for over a year. Win32 means, at the time of writing, Windows 95, Windows 98 and Windows NT. It does not mean Windows 3.11, even with Win32s installed.
Prior to Perl version 5.005, there was one version of Perl for Win32, and another for all the other systems. The other version was known as the "native version".
The Win32 version was developed by ActiveState, called "Perl for Win32" and typically lagged slightly behind the native version. As of the 5.005 release, Perl for Win32 and the native version have merged -- the native version now supports Win32 directly and doesn't need any tweaking by ActiveState.
ActiveState have dropped "Perl for Win32" and renamed their distribution, which comes with an InstallShield installer, "ActivePerl".
Incidentally, a few months before 5.005 merge the native Perl version was changed so it would run on Win32 directly. This version was best known by the creator's name, "Gurusamy Sarathy". However, there were still quite a few differences between it and Perl for Win32, so many people ran both. The merge brought the best of both worlds together.
Probably. Perl runs on everything from Amigas to Macintoshes to Unix boxen. Perl also runs on Microsoft operating systems, namely Windows 95, Windows 98 and Windows NT 3.51 and later. There are versions of Perl that run on earlier versions of these operating systems but they are no longer developed or supported. See http://www.perl.com for full details.
Go surf. Notice how many websites have dynamic pages with .pl or similar
as the filename extension? That's Perl. It is the most popular language
for CGI programming for many reasons, most of which are mentioned above.
In fact, there are a great many more dynamic pages written with perl that
may not have a .pl extension. If you code in Active Server Pages, then you should try using ActiveState's PerlScript. Quite frankly, coding in PerlScript rather than VBScript or JScript is like driving a car as opposed to riding a bicycle. Perl powers a good deal of the Internet.
If you are a Unix sysadmin you'll know about sed, awk and shell scripts. Perl can do everything they can do and far more besides. Furthermore, Perl does it much more efficiently and portably. Don't take my word for it, ask around.
If you are an NT sysadmin, chances are you aren't used to programming. In which case, the advantages of Perl may not be clear. Do you need it? Is it worth it?
After you read this tutorial you will know more than enough to start using Perl productively. You really need very little knowledge to save time. Imagine driving a car for years, then realising it has five gears, not four. That's the sort of improvement learning Perl means to your daily sysadminery. When you are proficient, you find the difference like realising the same car has a reverse gear and you don't have to push it backwards. Perl means you can be lazier. Lazy sysadmins are good sysadmins, as I keep telling my boss.
A few examples of how I use Perl to ease NT sysadmin life:
The question is, "what shouldn't I do with Perl". Write office suites is one answer. Perl, like most scripting languages, is a glue language designed for short and relatively simple tasks. Just don't equate this philosophy with a lack of power or "serious" features.
See the FAQs at www.perl.com. Of course there are Usenet groups, but also many mailing lists. Microsoft Windows users will be interested in those hosted by http://www.activestate.com/ which discuss all things Perl and Windows.
Please, before you ask any question, anywhere:
Think to yourself -- honestly -- if I was a busy Perl Professional, would I want to answer my own question?
Does it clearly state what I want an answer to? Preferably just one question at a time. Am I being unreasonable, for example asking for someone to code it for me? Have I shown evidence that I have tried to help myself? Have I made any mistakes in grammar? Is it polite? Is there enough information in there for the answer to be given?
Why should you care? Well, if you ask poorly-formed questions or those already answered in the FAQ...let's just say you won't get the answers you want. If you care about your online reputation and wasting other people's time -- two more reasons.
There are four stages:
An old version of Perl for Win32 is included with the Windows NT Resource Kit. It is sadly out of date. Follow the steps below to get a newer version. Having said that, you can complete the tutorial with the Resource Kit version but you should upgrade as soon as you can.
Go to http://www.activestate.com and follow the links to download ActivePerl.
It will be a single file, and the name will be something like api508e.exe. The i stands for Intel. If you have an Alpha, download apaXXXe.exe. If you're not sure, download the Intel version.
The 508e is the version number, so expect this to change quite rapidly. The file size will be just over 5Mb, so it will take a while to download via modem. If you know how to use FTP, try ftp.activestate.com/activeperl/.
When you find ActivePerl, save the file into any directory you please. I like to organise my downloads into c:\downloads but that is just personal preference. As long as ActivePerl ends up on your hard disk somewhere it doesn't matter.
So you now have apixxxx.exe. If you forget where you saved it, don't panic, just run Windows Explorer and search for api*e.exe
apixxxx.exe. You'll see the fantastic ActivePerl graphic and be advised to close all open applications before proceeding. The lizard thing is a gecko, which adorns the famous O'Reilly book "Learning Perl on Win32 Systems". This tutorial is aimed at a more basic level than that book, in terms of the author's knowledge, intended audience and quality of humour.
c:\progs\perl rather than c:\program files\perl because many Win32 programs don't properly handle long filenames, let alone those with spaces in. Or you could accept the default. Your choice.perl myscript.plmyscript.pl. Personally, I prefer double-clicking to allow me to edit the file so I do not select this option. Also, perl has a plethora of command line arguments which are difficult to pass to a script if you run it by association. For the purposes of this tutorial I'm assuming that you haven't associated .pl with perl.So you know what this tutorial is designed to do. You know what Perl is designed to do, and you have even installed it. It is now time to start the tutorial proper, and actually hack some code.
Assuming all has gone to plan, you can now create your first Perl script. Follow these instructions, but before you start read them through once, then begin. That's a good idea with any form of computer-related procedure. So, to begin:
c:\scripts\, which is
what I'll assume you are using in this tutorial.
print "My first Perl script\n";
c:\scripts\myfirst.pl. Be careful! Notepad will may save files with a .txt extension, so you will end up with myfirst.txt.pl by default. Perl won't mind, it'll still execute the file. If your version of Notepad does this, select "All files" before saving or rename the file then load it again. Better yet, use a decent text editor!
cd \scripts .
perl myfirst.pl
and you'll see the output. Welcome to the world of Perl ! See what I mean about it being easy to start ? However, it is difficult to finish with Perl once you begin :-)
perl myfirst.pl and you didn't see My first Perl script on the screen. If you saw "bad command or filename" then either you haven't installed Perl or perl.exe is not in your path. Probably the latter. Reboot, then try again.
If you saw Can't open perl script "xxxx.pl": No such file or directory then perl is defintely installed, but you have either got the name of the script wrong or the script is not in the same directory as where you are trying to run it from. For example, maybe you saved in script in c:\windows and you are in c:\scripts so of course Perl complains it can't find the script. Could you? Well, don't expect Perl to then. You don't have to run the script from the directory in which it resides, but it is easier.
; . Almost all lines of code in Perl have to end with semicolons, and those that don't have to will accept semicolons anyway. The moral is -- use semicolons. Sorry; the moral is; use semicolons.
Oh, one more thing -- if you haven't already done so, continue breathing.
Also note the \n . This is the code to tell Perl
to output a newline. What's a newline? Delete the \n from the program and run it again:
print "My first Perl script";
and all should become clear. You have now written your first Perl script.
Almost every Perl book is written for UN*X, which is a problem for
Win32. This leads to scripts like:
#!c:/perl/perl.exe
print "I'm a cool Perl hacker\n";
The function of the 'shebang' line is to tell the shell how to execute the file. Under UNIX, this makes sense. Under Win32, the system must already know how to execute the file before it is loaded so the line is not needed.
However, the line is not completely ignored, as it is searched for
any switches you may have given Perl (for example -w to turn on warnings).
You may also choose to add the line so your scripts run directly on UNIX without modification,
as UNIX boxes probably do need it. Win32 systems do not. We shall continue with the lesson.
$var=10 which
sets the variables $var to the value of 10. Later, we'll look at lists like arrays and hashes, where @var refers to more than
one value. For the moment, remember that Scalar is Singular. If weird metaphors help, think of lots of scaly snakes at a singles bar. If that didn't help, I apologise for putting the thought into your mind.
If you have any experience with other programming languages you might be surprised by the code
$var=10.
With most languages, if you want to assign the value
10
to a variable called
var you'd write var=10.
Not so in Perl. This is a Feature. All variables are prefixed with a
symbol such as $ @ % . This has certain advantages, like making programs easier to read. Honestly, I'm serious! It just takes some getting used to.
The prefixes mean that you can see where the variables are quite easily.
And not only that, what sort of variable it is. The human language German
has a similar principle (except nouns are capitalised, not prefixed with
$ and Perl is easier to pronounce). You'll agree later, I think.
So, ever onwards. Time to try some more variables:
$string="perl";
$num1=20;
$num2=10.75;
print "The string is $string, number 1 is $num1 and number 2 is $num2\n";
A closer look...notice you don't have to say what type of variable you
are declaring. In other languages you need to say if the variable is a string,
array, what sort of number it is and so on. You might even have to declare what type of number
it is. As an example, in Java you'd been saying things like int var=10
which defines the variable var as an integer, with the value 10.
So, why do these other programming languages force you to declare exactly what your variables are? Wouldn't it be easier if we could just not bother?
For short programs, yes. For really big projects with many programmers working on the same application, no. That's because forcing variable type declaration also forces a certain discipline and rigour which is what you need on big projects.
As you know, Perl is not designed for gigantic software engineering efforts. It is all about small, quick programs. For these purposes you don't need the rigour of variable controls as much, so Perl doesn't bother.
This idea of forcing a programmer to declare what sort of variable is being created is called typing. As Perl doesn't by default enforce any rules on typing, it is said to be a loosely typed language, as opposed to something like C++ which is strongly typed.
We still haven't finished learning from that humble bit of code. To refresh your memory, here it is again:
$string="perl";
$num1=20;
$num2=10.75;
print "The string is $string, number 1 is $num1 and number 2 is $num2\n";
Notice the way the variables are used in the string. Sticking variables
inside of strings has a technical term - "variable interpolation".
Now, if we didn't have the handy $
prefix for we'd have to do
something like the example below, which is pseudocode. Pseudocode is code
to demonstrate a concept, not designed to be run. Like certain Microsoft
software.
print "The string is ".string." and the number is ".num."\n";
which is much more work. Convinced about those prefixes yet ?
Try running the following code:
$string="perl";
$num=20;
print "Doubles: The string is $string and the number is $num\n";
print 'Singles: The string is $string and the number is $num\n';
Double quotes allow the aforementioned variable interpolation. Single quotes do not. Both have their uses as you will see later, depending on whether you wish to interpolate anything.
If you want to add 1 to a variable you can, logically, do this;
$num=$num+1 .
There is a shorter way to do this, which is $num++. This is an autoincrement. Guess what this is; $num-- . Yes, an autodecrement.
This example illustrates the above:
$num=10;
print "\$num is $num\n";
$num++;
print "\$num is $num\n";
$num--;
print "\$num is $num\n";
$num+=3;
print "\$num is $num\n";
The last example demonstrates that it doesn't have to be just 1 you can add or decrease by.
\ . You can
see what this does -- it 'escapes' the special meaning of $ .
Escaping means that just the $ symbol is printed instead of it referring
to a variable.
Actually \ has a deeper meaning -- it escapes all of Perl's special characters,
not just $ . Also, it turns some
non-special characters into something special. Like what ? Like n . Add the magic \ and the humble 'n' becomes the mighty NewLine !
The \ character can also escape itself. So if you want to print a single \ try:
print "the MS-DOS path is c:\\scripts\\";
Oh, '\' is also used for other things like references. But that's not even covered here.
There is a technical term for these 'special characters' such as @ $ %. They are called metacharacters. Perl uses plenty of metacharacters. In fact, you'll wear your keyboard pretty evenly during a night's perl hacking. I think it is safe to say that Perl uses every possible keystroke and shifted keystroke on a standard US PC keyboard.
You'll be working with all sorts of obscure characters in your Perl hacking career, and I also mean those on your keyboard. This has earned perl a reputation for being difficult to understand. That's entirely true. Perl does have such a reputation, no doubt about it.
Is the reputation justified? In my opinion, Perl does have a short but steep learning curve to begin with simply because it is so different. However, once you learn the character meanings reading perl code becomes much easier precisely because of all these strange characters.
. can join two variables together, act as a wildcard or become a range operator if there are two of them together. The caret
^ has different effects in
[^abc] as opposed to
[a^bc] .
If this sounds crazy, think about the English language. What do the following mean to you ?
Mean is, in one context, is a word to used describe the purpose of something. It is also another word for average. Furthermore, it describes a nasty person, or a person who doesn't like spending money, and is used in slang to refer to something impressive and good.
That's five different uses for 'mean', and you don't have any trouble understanding which one I
Polish, when capitalised, can either mean pertaining to the country Poland, or the act of making something shiny. And 'like' can mean similar to, or affection for.
So, when you speak or write English (think of two, to and too) you know what these words mean by their context. It is exactly the same way with Perl. Just don't assume a given metacharacter always means what you first thought it did.
To finish off this section, try the following:
$string="perl"; $num=20; $mx=3; print "The string is $string and the number is $num\n"; $num*=$mx; $string++; print "The string is $string and the number is $num\n";Note the easy shortcut
*= meaning 'multiply $num by $mx' or,
$num=$num*$mx .
Of course Perl supports the usual + - * / ** % operators. The
last two are exponentiation (to the power of) and modulus (remainder of
x divided by y). Also note the way you can increment a string ! Is this
language flexible or what ?
print function is a list operator. That means it accepts a list of things to print, separated by commas. As an example:
print "a doublequoted string ", $var, 'that was a variable called var', $num," and a newline \n";Of course, you just put all the above inside a single doublequoted string:
print "a doublequoted string $var that was a variable called var $num and a newline \n";to achieve the same effect. The advantage of using the
print function in
list context is that expressions are evaluated before being printed. For example, try this:
$var="Perl"; $num=10; print "Two \$nums are $num * 2 and adding one to \$var makes $var++\n"; print "Two \$nums are ", $num * 2," and adding one to \$var makes ", $var++,"\n";
You might have been slightly surprised by the result of that last experiment. In particular, what happened to our variable
$var ? It should have been incremented by one, resulting in Perm. The reason being that 'm' is the next letter after 'l' :-)
Actually, it was incremented by 1. We are postincrementing $var++ the variable, rather than preincrementing it.
The difference is that with postincrements, the value of the variable is returned, then the operation is performed on it. So in the example above, the current value of $var was returned to the print function, then 1 was added. You can prove this to yourself by adding the line
print "\$var is now $var\n"; to the end of the example above.
If we want the operation to be performed on $var before the value is returned to the print function, then preincrement is the way to go. ++$var will do the trick.
Let's take a another look at the example we used to show how the autoincrement system works. Messy, isn't it ? This is Batch File Writing Mentality. Notice how we use exactly the same code four times. Why not just put it in a subroutine?
$num=10; # sets $num to 10
&print_results; # prints variable $num
$num++;
&print_results;
$num*=3;
&print_results;
$num/=3;
&print_results;
sub print_results {
print "\$num is $num\n";
}
Easier and neater. The subroutine can go anywhere in your script, at the beginning, end, middle...makes no difference. Personally I put all mine at the bottom and reserve the top part for setting variables and main program flow.
A subroutine is just some code you want to use more than once in the same script. In Perl, a subroutine is a user-defined function. There is no difference. For the purposes of clarity I'll refer to them as subroutines.
A subroutine is defined by starting with sub then the name.
After that you need a curly left bracket { , then
all the code for your subroutine. Finish it off with a closing brace } . The area between the two braces is called a block. Remember this. There are such things as anonymous subroutines but not here. Everything here has a name.
Subroutines are usually called by prefixing their name with an ampersand, that is one of these -- & , like
so &print_results; . It used to be cool to omit the
& prefix but all perl hackers are now encouraged to use it to avoid ambiguity. Ambiguity can hurt you if you don't avoid it.
If you are worrying about variable visibility, don't. All the variables we are using so far are visible everywhere. You can restrict visibility quite easily, but that's not important right now. If you weren't worrying about variable visibility, please don't start. I'd tell you it's not important but that'll only make you worried. (paranoid ?) We'll cover it later.
# crept in there. That's a comment. Everything after
a # is ignored. You can't continue it onto a newline however,
so if your comment won't fit on one line start a new one with # .
There are ways to create Plain Old Documentation (POD) and more ways to
comment but they are not detailed here.
if statement is simple.
if the day is Sunday, then lie in bed. A simple
test, with two outcomes. Perl conversion (don't run this):
if ($day eq "sunday") {
&lie_in_bed;
}
You already know that &lie_in_bed is a call to a subroutine.
We assume $day is set earlier in the program. If
$day is
not equal to 'Sunday' &lie_in_bed
is not executed (pity). You don't need to say anything else. Try this:
$day="sunday";
if ($day eq "sunday") {
print "Zzzzz....\n";
}
Note the syntax. The if statement requires something to test for
Truth. This expression must be in (parens), then you have the braces to
form a block.
There are many Perl functions which test for Truth. Some are
if, while, unless . So it is important you know what truth is, as defined
by Perl, not your tax forms. There are three main rules:
"" and "0".0. This includes negative numbers.
&isit; # $test1 is at this moment undefined
$test1="hello"; # a string, not equal to "" or "0"
&isit;
$test1=0.0; # $test1 is now a number, effectively 0
&isit;
$test1="0.0"; # $test1 is a string, but NOT effectively 0 !
&isit;
sub isit {
if ($test1) { # tests $test1 for truth or not
print "$test1 is true\n";
} else { # else statement if it is not true
print "$test1 is false\n";
}
}
The first test fails because $test1 is undefined.
This means it has not been created by assigning a value to it. So according to Rule 3
it is false. The last two tests are interesting. Of course, 0.0 is the
same as 0 in a numeric context. But it is not the same as
0 in a string context, so in that case it is true.
So here we are testing single variables. What's more useful is testing
the result of an expression. For example, this is an expression;
$x * 2 and so is this;
$var1 + $var2 . It is the end result
of these expressions that is evaluated for truth.
An example demonstrates the point:
$x=5;
$y=5;
if ($x - $y) {
print '$x - $y is ',$x-$y," which is true\n";
} else {
print '$x - $y is ',$x-$y," which is false\n";
}
The test fails because 5-5 of course is 0, which is false. The
print statement might look a little strange. Remember that print is a list operator? So we hand it a list. First item, a single-quoted string. It is single quoted because it we do not want to perform variable interpolation on it. Next item is an expression which is evaluated, and the result printed. Finally, a double-quoted string is used because we want to print a newline, and without the doublequotes the \n won't be interpolated.
What is probably more useful than testing a specific variable for truth is equality testing. For example, has your lucky number been drawn?
$lucky=15;
$drawnum=15;
if ($lucky == $drawnum) {
print "Congratulations!\n";
} else {
print "Guess who hasn't won!\n";
}
The important point about the above code is the equality operator,
== .
The symbol = is an
assignment operator, not a comparison operator. Therefore:
if ($x = 10) is always true, because
$x has been assigned the value 10 successfully.if ($x == 10) compares the two values, which might not be equal.So far we have been testing numbers, but there is more to life than numbers. There are strings too, and these need testing too.
$name = 'Mark';
$goodguy = 'Tony';
if ($name == $goodguy) {
print "Hello, Sir.\n";
} else {
print "Begone, evil peon!\n";
}
Something seems to have gone wrong here. Obviously Mark is different to Tony, so why does perl consider them equal?
Mark and Tony are equal -- numerically. We should be testing them as strings, not as numbers. To do this, simply substitute == for
eq and everything will work as expected.
== and
eq. Run this:
$foo=291;
$bar=30;
if ($foo < $bar) {
print "$foo is less than $bar (numeric)\n";
}
if ($foo lt $bar) {
print "$foo is less than $bar (string)\n";
}
The lt operator compares in a string context, and of course
< compares in a numeric context.
Alphabetically, that is in a string context, 291 comes before 30. It is actually decided by the ASCII value, but alphabetically is close enough. Change the numbers around a little. Notice how Perl doesn't care whether it uses a string comparison operator on a numeric value, or vice versa. This is typical of Perl's flexibility.
Bondage and discipline are pretty much alien concepts to Perl (and the author). This flexibility does have a drawback. If you're on a programming precipice, threatening suicide by jumping off, Perl won't talk you out of your decision but will provide several ways of jumping, stepping or falling to your doom while silently watching your early conclusion. So be careful.
| Comparison | Numeric | String |
| Equal | == | eq |
| Not equal | != | ne |
| Greater than | > | gt |
| Less than | < | lt |
| Greater than or equal to | >= | ge |
| Less than or equal to | <= | le |
More about if statements. Run this:
$age=25;
$max=30;
if ($age > $max) {
print "Too old !\n";
} else {
print "Young person !\n";
}
It is easy to see what else does. If the expression is false then
whatever is in the else block is evaluated
(or carried out, executed, whatever term you choose to use).
Simple. But what if you want another test ? Perl can do that too.
$age=25;
$max=30;
$min=18;
if ($age > $max) {
print "Too old !\n";
} elsif ($age < $min) {
print "Too young !\n";
} else {
print "Just right !\n";
}
If the first test fails, the second is evaluated. This carries on until
there are no more elsif statements, or an
else statement is reached. An
else statement is optional, and no
elsif statements should come after it. Logical, really.
There is a big difference between the above example the one below:
if ($age > $max) {
print "Too old !\n";
}
if ($age < $min) {
print "Too young !\n";
}
If you run it, it will return the same result - in this case. However, it is Bad Programming
Practice. In this case we are testing a number, but suppose we were testing
a string to see if it contained R or S. It is possible that a string could
contain both R and S. So it would pass both 'if' tests. Using an
elsif avoids this. As soon as the first statement is true, no
more elsif statements (and no else statement) are executed.
You don't need to take up a whole three lines:
print "Too old\n" if $age > $max;
print "Too old\n" unless $age < $max;
I added some whitespace there for aesthetic beauty. There are other
operators that you can use instead of if and
unless , but that's for later on.
Incidentally, the two lines of code above do not do exactly the same thing. Consider a maximum age of 50 and input age of 50. Therefore, you should be very careful about your logic when writing code (nice obvious statement there).
For those that were wondering, Perl has no case statement. This is all explained in the FAQ, which is located at http://www.perl.com/.
print "Please tell me your name: "; $name=<STDIN>; print "Thanks for making me happy, $name !\n";New things to learn here. Firstly,
<STDIN> . STDIN is a filehandle. Filehandles are what you use to interact with things such as files, console input, socket connections and more.
You could say STDIN is the standard source for input. Guess what STDIN stands for. In this case the STDIN filehandle is reading from the console.
The angle brackets
<> read data from a filehandle. Exactly how much is dependent on what you do, but in this case it is whatever was input at the prompt.
So we are reading from the STDIN filehandle. The value is assigned to
$name and printed. Any idea why the ! ends up on a new line ?
on a new line on a newline ????
As you pressed Enter, you of course included a newline with your name.
The easy way to get rid of it is to chop it off:
print "Please tell me your name: "; $name=<STDIN>; chop $name print "Thanks for making me happy, $name !\n"and that fails with a syntax error. Can you spot why? Look at the error code, look at the line number and see where the syntax is wrong. The answer is a missing semicolon (
; ) on the end of the last two lines.
If you add a ; to the end of line 3, but not to the last line, then the program works as it should. This is because Perl doesn't need a semicolon to end the last statement of a block. However, I'd advise ending all your statements with semicolons because you may well be adding more code to them and it is only one little keystroke.
When you add the semicolon(s), the program runs correctly. The chop function removes the last character of whatever it is given to chop, in this case removing the newline for us. In fact, that can be shortened:
print "Please tell me your name: ";
chop ($name=<STDIN>);
print "Thanks for making me happy, $name !";
The parentheses ( )
force chop to act on the result of what is
inside them. So $name=<STDIN> is evaluated first, then the
result from that, which is $name , is chopped. Try it without.
You can read from STDIN as much as you like. For your entertainment I have created a sophisticated multinational greeting machine:
print "Please tell me your name: ";
chop ($name=<STDIN>);
print "Please tell me your nationality: ";
chop ($nation=<STDIN>);
if ($nation eq "British" or $nation eq "New Zealand") {
print "Hallo $name, pleased to meet you!\n";
} elsif ($nation eq "Dutch" or $nation eq "Flemish") {
print "Hoi $name, hoe gaat het met u vandaag?!\n";
} else {
print "HELLO!!! SPEAKEEE ENGLIEESH???\n";
}
Aside from demonstrating the native English speaker's linguistic talents, this script also introduces the or logical operator. We'll cover
or and its associates in more detail later on. First, a word of warning.
Chopping is dangerous, as my friend One Hand Harold will tell you. Everyone is concerned about various forms of safety these days, and your perl code should be no exception.
chomp :
chomp ($name=<STDIN>);
At this point the perl gurus are screaming "I found an error !".
Well, chomp doesn't always remove the last character if it is a newline
but if it doesn't, you have set a special variable, namely $/ , to something
different. I presume that if you do set $/ you know what it does.
It is explained later in this very document. Of course, being a good pupil, you wouldn't experiment
with the unknown, blindly changing things just for the hell of it to see what happens.
If you don't, you'll never learn anything useful.
Perl has two types of array, associative arrays (hashes) and arrays. Both types are lists. A list is just a collection of variables referred to as the collection, not as individual elements.
You can think of Perl's lists as a herd of animals. List context refers to the entire herd, scalar context refers to a single element. A list is a herd of variables. The variables don't have to be all of the same type -- you might have a herd of ten sheep, three lions and two wolves. It would probably be just three lions and one wolf before long, but bear with me. In the same way, you might have a Perl list of three scalar variables, two array elements and ten hash elements.
Certain types of lists are known by certain names. Just as a herd of sheep is called a flock, a herd of lions is called a pride, a herd of wolves is called a pack and a herd of managers a confusion, some types of Perl list have a special names.
@names . It puts five values into the array.
@names=("Muriel","Gavin","Susanne","Sarah","Anna");
print "The elements of \@names are @names\n";
print "The first element is $names[0] \n";
print "The third element is $names[2] \n";
print 'There are ',scalar(@names)," elements in the array\n";
Firstly, notice how we define @names . As it is in a list context,
we are using parens. Each value is comma separated, which is Perl's
default list delimiter. The double quotes are not necessary, but as
these are string values it makes it easier to read and change later on.
Next, notice how we print it. Simply refer to it as a whole, that is
in list context.. List context means referring to more than one
element of a list at a time. The code
print @names; will work
perfectly well too. But....
I usually learn something about Perl every time I work with it. When running a course, a student taught me this trick which he had discovered:
@names=("Muriel","Gavin","Susanne","Sarah","Anna","Paul","Trish","Simon");
print @names;
print "\n";
print "@names";
When a list is placed inside doublequotes, it is space delimited when interpolated. Useful.
If we want to do anything with the array as a list,
that is doing something with more than one value, then refer to the array
as @array . That's important. The
@ prefix is used when you want to refer to more than one element of a list.
When you refer to more than one, but not all elements of an array that is known as a slice . Cake analogies are appropriate. Pie analogies are probably healthier but equally accurate.
@ which refers to multiple elements of the array. It is a single,
scalar variable, so $ is used. Secondly, we must specify which
element we want. That's easy - $array[0] for the first,
$array[1]
for the second and so forth. Array indexes start at 0, unless you do something
which is so highly deprecated ('deprecated' means allowed, usually for backwards
compatibility, but disapproved of because there are better ways) I'm not
even going to mention it.
Finally, we force what is normally list context (more than one element)
into scalar context (single element) to give us the amount of elements
in the array. Without the scalar , it would be the same as the
second line of the program.
Please understand this:
$myvar="scalar variable";
@myvar=("one","element","of","an","array","called","myvar");
print $myvar; # refers to the contents of a scalar variable called myvar
print $myvar[1]; # refers to the second element of the array myvar
print @myvar; # refers to all the elements of array myvar
The two variables $myvar and
@myvar
are not, in any way, related. Not even distantly. Technically, they are in different namespaces.
Going back to the animal analogy, it is like having a dog named 'Myvar' and a goldfish called 'Myvar'. You'll never get the two mixed up because when you call 'Myvar !!!!' or open a can of dog food the 'Myvar' dog will come running and goldfish won't. Now, you couldn't have two dogs called 'Myvar' and in the same way you can't have two Perl variables in the same namespace called 'Myvar'.
print "Enter a number :";
chomp ($x=<STDIN>);
@names=("Muriel","Gavin","Susanne","Sarah","Anna");
print "You requested element $x who is $names[$x]\n";
print "The index number of the last element is $#names \n";
This is useful. Notice the last line of the example. It returns the index
number of the last element. Of course you could always just do this
$last=scalar(@names)-1;
but this is more efficient. It is an easy way to get the last element,
as follows:
print "Enter the number of the element you wish to view :";
chomp ($x=<STDIN>);
@names=("Muriel","Gavin","Susanne","Sarah","Anna","Paul","Trish","Simon");
print "The first two elements are @names[0,1]\n";
print "The first three elements are @names[0..2]\n";
print "You requested element $x who is $names[$x-1]\n"; # starts at 0
print "The elements before and after are : @names[$x-2,$x]\n";
print "The first, second, third and fifth elements are @names[0..2,4]\n";
print "a) The last element is $names[$#names]\n"; # one way
print "b) The last element is @names[-1]\n"; # different way
It looks complex, but it is not. Really. Notice you can have multiple values separated
by a comma. As many as you like, in whatever order. The range operator
.. gives you everything between and including the values. And
finally look at how we print the last element - remember $#names gives
us a number ? Simply enclose it inside square brackets and you have the
last element.
Do also note that because element accesses such as [0,1] are more than
one variable, we cannot use the scalar prefix, namely the $ symbol. We
are accessing the array in list context, so we use the @ symbol. Doesn't
matter that it is not the entire array. Remember, accessing more than one
element of an array but not the entire array is called a slice. I won't go over the food analogies again.
@names=("Muriel","Gavin","Susanne","Sarah","Anna","Paul","Trish","Simon");
for ($x=0; $x <= $#names; $x++) {
print "$names[$x]\n";
}
which sets $x to 0, runs the loop once, then adds one to
$x , checks it
is less than $#names , if so carries on. By the way, that was your introduction
to for loops. Just to go into a little detail there, the
for loop has three parts to it:
$x is initialised to 0. It is immediately tested to see
if it is smaller than, or equal to $#names . If that is true, then the block is executed once. Critically, if it is not true the block is not executed at all.
Once the block has been executed, the modification expression is evaluated. That's
$x++ . Then, the test condition is checked to see if the block should be executed or not.
There is a another version:
for $x (0 .. $#names) {
print "$names[$x]\n";
}
which takes advantage of the range operator .. (two dots together).
This simply gives $x the value of 0, then increments
$x by 1 until it is equal to $#names .
For true beauty we must use foreach .
foreach $person (@names) {
print "$person";
}
This goes through each element ('iterates', another good technical word to use) of
@names , and assigns each element in turn to the variable
$person .
Then you can do what you like with the variable. Much easier. You can use
for $person (@names) {
print "$person";
}
if you want. Makes no difference at all, aside from a little clarity.
In fact, that gets shorter. And now I need to introduce you to $_ ,
which is the Default Input and Pattern Searching Variable.
foreach (@names) {
print "$_";
}
If you don't specify a variable to put each element into, $_ is
used instead as it is the default for this operation, and many, many others
in Perl. Including the print function :
foreach (@names) {
print ;
}
As we haven't supplied any arguments to print ,
$_ is printed as default. You'll
be seeing a lot of $_ in Perl. Actually, that statement is not exactly true.
You will be seeing lot of places where $_ is used, but quite often when
it is used, it is not actually written. In the above example, you
don't actually see $_ but you know it is there.
The old jokes are the best, aren't they?
The joke above is a loop. You continue re-reading the sentence until you realise I'm trying to be funny. Then you exit the loop. Or maybe somebody doesn't exit it. Whatever, loops always run until the expression they are testing returns false. In the case of the examples above, a false value is returned when all the elements of the array have been cycled through, and the loop ends.
If you want an everlasting loop, just test an condition you know will always be true:
while (1) {
$x++;
print "$x: Did you know you can press CTRL-C to interrupt a perl program?\n";
}
Another way to exit a loop is a simple foreach over the elements, as we have seen. But if we don't know when we want to exit a loop? For example, suppose we want to print out a list of names but stop when we find one with a particular title? You are throwing a huge party, someone is allergic to vodka, and this person has drunk from the punch bowl despite being assured by someone holding two empty bottles of Absolut that he was just using the bottles to convey yet more orange juice into said punch bowl. So you need a doctor, and so you write a Perl script to find one from the list of attendees, wanting the doctor's name to be the last item printed:
@names=('Mrs Smith','Mr Jones','Ms Samuel','Dr Jansen','Sir Philip');
foreach $person (@names) {
print "$person\n";
last if $person=~/Dr /;
}
The last operator is our friend. Don't worry about the /Dr / business -- that is a regular expression which we cover next. All you need to know is that it returns true if the name begins with 'Dr '. When it does return true,
last is operated and the loop ends early.
@names =('Mrs Smith','Mr Jones','Ms Samuel','Dr Jansen','Sir Philip');
@medics =('Dr Black','Dr Waymour','Dr Jansen','Dr Pettle');
foreach $person (@names) {
print "$person\n";
if ($person=~/Dr /) {
foreach $doc (@medics) {
print "\t$doc\n";
last if $doc eq $person;
}
}
}
Aside from showing one way to indent your code, this also demonstrates a nested loop. A nested loop is a loop within a loop. What happens is that the @names array is searched for a 'Dr ', and if it is found then the @medics array is searched to make sure the doctor is a human-fixing doctor not a professor of physics or something. The regular expression has been shifted into an if statement, where it works nicely as it only returns true or false.
The problem with the code is that after we find our medical doctor we want it to stop. But it doesn't. It only stops the loop it is in, so Dr Pettle never gets printed. However, the code just carries on with Sir Philip who is terribly sorry old chap, but can't be of any bally use at all, what ho! What we need is a way to break out of the entire loop from within a nest. Like so:
@names =('Mrs Smith','Mr Jones','Ms Samuel','Dr Jansen','Sir Philip');
@medics =('Dr Black','Dr Waymour','Dr Jansen','Dr Pettle');
LBL: foreach $person (@names) {
print "$person\n";
if ($person=~/Dr /) {
foreach $doc (@medics) {
print "\t$doc\n";
last LBL if $doc eq $person;
}
}
}
Only two changes here. We have defined a label, namely LBL. Instead of breaking out from the current loop, which is the default, we specify a label to break out to, which is in the outer loop. This works with as many nested loops as your brain can handle. You don't have to use uppercase names but for namespace reasons it is recommended, and you can call your labels whatever you please. I was just being unimaginative with the name of LBL, feel free to invent labels called DORIS or MATILDA if that's what floats your personal boat.
@names . We want to change it. Run this:
print "Enter a name :";
chomp ($x=<STDIN>);
@names=("Muriel","Gavin","Susanne","Sarah");
print "@names\n";
push (@names, $x);
print "@names\n";
Fairly self explanatory. The push function just adds a value on to the end of the
array. Of course, Perl being Perl, it doesn't have to be just the one value:
print "Enter a name :";
chop ($x=<STDIN>);
@names=("Muriel","Gavin","Susanne","Sarah");
@cities=("Brussels","Hamburg","London","Breda");
print "@names\n";
push (@names, $x, 10, @cities[2..4]);
print "@names\n";
This is worth looking at in more detail. It appears there is no fifth element of
@cities , as referred to by
@cities[2..4] .
Actually, there is a fifth element. Add this to the end of the example :
print "There are ",scalar(@names)," elements in \@names\n";
There appear to be 8 elements in @names . However, we have just proved there are in fact 9. The reason there are 9 is that we referred to non-existent elements of @cities , and Perl has quite happily extended @names to suit. The array
@cities remains unchanged. Try poping the array if you don't believe me.
So that's push . Now for some...
@names=("Muriel","Gavin","Susanne","Sarah");
@cities=("Brussels","Hamburg","London","Breda");
&look;
$last=pop(@names);
unshift (@cities, $last);
&look;
sub look {
print "Names : @names\n";
print "Cities: @cities\n";
}
Now we have two arrays. The pop
function removes the last element of an array
and returns it, which means you can do something like assign the returned
value to a variable. The unshift function adds a value
to the beginning of the array. Hope you didn't forget that
&subroutinename calls a subroutine. Presented below are the functions you can use to work with arrays:
| push | Adds value to the end of the array |
| pop | Removes and returns value from end of array |
| shift | Removes and returns value from beginning of array |
| unshift | Adds value to the beginning of array |
Now, accessing other elements of arrays. May I present the splice function ?
@names=("Muriel","Sarah","Susanne","Gavin");
&look;
@middle=splice (@names, 1, 2);
&look;
sub look {
print "Names : @names\n";
print "The Splice Girls are: @middle\n";
}
The first argument for splice is an array. Then second is the offset.
The offset is the index number of the list element to begin splicing at. In this
case it is 1. Then comes the number of elements to remove, which is sensibly
1 or more in this case. You can set it to 0 and perl, in true perl style,
won't complain. Setting to 0 is handy because splice can add elements
to the middle of an array, and if you don't want any deleted 0 is the number
to use. Like so:
@names=("Muriel","Gavin","Susanne","Sarah");
@cities=("Brussels","Hamburg","London","Breda");
&look;
splice (@names, 1, 0, @cities[1..3]);
&look;
sub look {
print "Names : @names\n";
print "Cities: @cities\n";
}
Notice how the assignment to @middle has gone -- it is no longer relevant.
If you assign the result of a splice to a scalar then:
@names=("Muriel","Sarah","Susanne","Gavin");
&look;
$middle=splice (@names, 1, 2);
&look;
sub look {
print "Names : @names\n";
print "The Splice Girls are: $middle\n";
}
then the scalar is assigned the last element removed, or undef if it doesn't work at all.
The
splice function
is also a way to delete elements from an array. In fact, a
discussion of :
@cities=("Brussels","Hamburg","London","Breda");
&look;
$cities[1]="";
&look;
sub look {
print "Cities: ",scalar(@cities), ": @cities\n";
}
would be appropriate. Certainly Hamburg is removed. Shame, such a great
lake. But note, the array element still exists. There are still four elements
in @cities. So what we need is the appropriate
splice function, which removes
the element entirely.
splice (@cities, 1, 1);
Now that's all well and good for arrays. What about ordinary variables,
such as these:
$car ="Porsche 911";
$aircraft="G-BBNX";
&look;
$car="";
&look;
sub look {
print "Car :$car: Aircraft:$aircraft:\n";
print "Aircraft exists !\n" if $aircraft;
print "Car exists !\n" if $car;
}
It looks like we have deleted the $car variable. Pity. But think about
it. It is not deleted, it is just set to the null string "". As you recall
(hopefully) from previous ramblings, the null string evaluates to false
so the if test fails.
Just because something is false doesn't mean to say it doesn't exist.
A wig is false hair, but a wig exists. Your variable is still there. Perl
does have a function to test if something exists. Existence, in Perl terms,
means defined. So:
print "Car is defined !\n" if defined $car;
will evaluate to true, as the $car variable does in fact exist.
This begs the question of how to really wipe variables from the face
of the earth, or at least your Perl script. Simple.
$car ="Porsche 911";
$aircraft="G-BBNX";
&look;
undef $car; # this undefines $car
&look;
sub look {
print "Car :$car: Aircraft:$aircraft:\n";
print "Aircraft exists !\n" if $aircraft;
print "Car exists !\n" if defined $car;
}
This variable $car is eradicated, deleted, killed,
destroyed.
And now for something completely different....
Perl's regex often look like this:
$name=~/piper/
That is saying "If 'piper' is inside $name, then True."
The regular expression itself is between / /
slashes, and the =~ operator assigns the target for the search.
An example is called for. Run this, and answer it with 'the faq'. Then
try 'my tealeaves' and see what happens.
print "What do you read before joining any Perl discussion ? ";
chomp ($_=<STDIN>);
print "Your answer was : $_\n";
if ($_=~/the faq/) {
print "Right ! Join up !\n";
} else {
print "Begone, vile creature !\n";
}
So here $_ is searched for 'the faq'. Guess what we don't need ! The =~ . This works just as well:
if (/the faq/) {
because if you don't specify a variable, then perl searches $_ by default.
In this particular case, it would be better to use if ($_ eq "the faq") { as we are testing for exact matches.
But what if someone enters 'The FAQ' ? It fails, because the regex is case sensitive. We can easily fix that:
if (/the faq/i) {
with the /i switch, which specifies case-insensitivity. Now it works for
all variations, such as "the Faq" and "the FAQ".
Now you can appreciate why a regular expression is better in this situation than a simple test using eq . As the regex searches one string for another string, a response of "I would read the FAQ first !" will also work, because "the FAQ" will match the regex.
Study this example just to clarify the above. Tabs and spaces have been added for aesthetic beauty:
$_="perl for Win32"; # sets the string to be searched
if ($_=~/perl/) { print "Found perl\n" }; # is 'perl' inside $_ ? $_ is "perl for Win32".
if (/perl/) { print "Found perl\n" }; # same as the regex above. Don't need the =~ as we are testing $_
if (/PeRl/) { print "Found PeRl\n" }; # this will fail because of case sensitivity
if (/er/) { print "Found er\n" }; # this will work, because there is an 'er' in 'perl'
if (/n3/) { print "Found n3\n" }; # this will work, because there is an 'n3' in 'Win32'
if (/win32/) { print "Found win32\n" }; # this will fail because of case sensitivity
if (/win32/i) { print "Found win32 (i)\n" }; # this will *work* because of case insensitivity (note the /i)
print "Found!\n" if / /; # another way of doing it, this time looking for a space
print "Found!!\n" unless $_!~/ /; # both these are the same, but reversing the logic with unless and !
print "Found!!\n" unless !/ /; # don't do this, it will always never not confuse nobody :-)
# the ~ stays the same, but = is changed to ! (negation)
$find=32; # Create some variables to search for
$find2=" for "; # some spaces in the variable too
if (/$find/) { print "Found '$find'\n" }; # you can search for variables like numbers
if (/$find2/) { print "Found '$find2'\n" }; # and of course strings !
print "Found $find2\n" if /$find2/; # different way to do the above
As you can see from the last example, you can embed a variable in the regex
too. Regular expressions could fill entire books (and they have done, see
the book critiques at http://www.perl.com/) but here are some useful tricks:
@names=qw(Karlson Carleon Karla Carla Karin Carina Needanotherword);
foreach (@names) { # sets each element of @names to $_ in turn
if (/[KC]arl/) { # this line will be changed a few times in the examples below
print "Match ! $_\n";
} else {
print "Sorry. $_\n";
}
}
This time @names is initialised using whitespace as a delimiter
instead of a comma. qw refers to 'quote words', which means split
the list by words. A word ends with whitespace (like tabs, spaces, newlines etc).
The square brackets enclose single characters to be
matched. Here either Karl or
Carl must be in each element. It doesn't
have to be two characters, and you can use more than one set. Change Line
4 in the above program to:
if (/[KCZ]arl[sa]/) {
matches if something begins with K, C, or Z, then arl, then either s
or a. It does not match KCZarl. Negation is possible too, so try
this :
if (/[KCZ]arl[^sa]/) {
which returns things beginning with K, C or Z, then arl, and then anything
EXCEPT s or a. The caret ^ has to be the first
character, otherwise it doesn't work as the negation. Having said [ ]
defines single characters only, I should mention than these two are the same :
/[abcdeZ]arl/;
/[a-eZ]arl/;
if you use a hyphen then you get the list of characters including the start
and finish characters. And if you want to match a special character (metacharacter),
you must escape it:
/[\-K]arl/;
matches Karl or -arl. Although the - character is
represented by two characters, it is just the one character to match.