Mike Gossland's Perl Tutorial Course for Windows
What's Perl | Task Examples | Running Perl | Variables | Flow Control | Simple Scripts
Chapter 1: Introduction
Some Simple Perl Scripts
To get an idea of how Perl works, we'll finish off the first lesson with some simple Perl scripts. We'll build on the items you've learned earlier: scalars and arrays, the if-else, while and for constructs, and the print statement.
Since we're writing real perl programs here, there are a few more things you should know about.
Statements
Statements are complete thoughts in perl, just like sentences. In English, sentences end with a period. In Perl, statements must end with a semi-colon. This is for good reason — in perl the period is used for something else.
Comments
If you've ever had the nerve to scribble notes in the margin of a novel, you'll know what comments are for.
In perl, comments are words within the program that perl itself ignores. They are written by you for your own benefit. They help explain the program so when you look at it later you know what the heck is going on. In perl, comments are set off with the "#" character. Anything following the # to the end of the line is a comment. For example:
#This illustrates a comment. #Note there is a semi-colon on the next line print "The quick brown fox jumped over the lazy dog\n";
There is another form of comment which is very useful when you want to chop out large sections of code. Borrowing a technique from Perl's embedded documentation, I like to use it in this way:
=comment until cut $variable = 1; print "The variable has the value of $variable\n"; #more lines ... =cut
Commenting out large sections like this can be extremely helpful.
The Newline Character
In the examples leading up to this section, I've used the print statement a lot. Each time, I've added the funny ending "\n" onto the print statement. This odd combo of \n is the newline character. Without it, the print statement would not go on to the next line.
Many people are surprised to learn you have to tell the program to go to a new line each time. But if it did this all by itself every time, then you could never print out complete lines a bit at a time, like this:
$num_words = "eight"; print "There are "; print $num_words; print " words altogether in this sentence.\n";
Instead the output would be:
There are eight words altogether in this sentence.
Short Forms
In perl, some operations are so common they have a shortened form that saves time. These may be strange to the novice, so I'll be careful here. Where appropriate I'll make a comment near the statement to describe the short form.
Programming Errors
Programming errors? Already? Yes. Programming errors, also affectionately known as bugs, are a fact of programming life. You'll be encountering them soon enough.
If you have a mistake in your perl script that makes your meaning unclear, you'll get a message from the perl compiler when you try to run it. To check for these errors before running, you can run perl with the -c flag. And turn on the warnings flag, -w, while you're at it. This picks up hard to find errors too. As an example you'd type in:
perl -wc hello.pl
to check on the health of the perl script, hello.pl. If there are any syntax errors, you'll hear about them alright!
Running the example scripts
You can copy any of the following script examples into a file in Notepad and save it as, say, perltest.pl. Then check it for errors by typing
perl -wc perltest.pl
If it comes back saying "syntax ok", then go ahead and run it by typing
perl perltest.pl
If it doesn't say "syntax ok", then go and fix the reported error, and try again.
Script 1: Adding the numbers 1 to 100, Version 1
$top_number = 100;
$x = 1;
$total = 0;
while ( $x <= $top_number ) {
$total = $total + $x; # short form: $total += $x;
$x += 1; # do you follow this short form?
}
print "The total from 1 to $top_number is $total\n";
Script 2: Adding the numbers 1 to 100. Version 2
This script uses a form of the for loop to go through the integers 1 through 100:
$total = 0;
#the for loop gives $x the value of all the
#numbers from 1 to 100;
for $x ( 1 .. 100 ) {
$total += $x; # again, the short form
}
print "The total from 1 to 100 is $total\n";
Script 3: Printing a menu
This script uses an array to store flavors. It also uses a terrific form of the for loop to go through them.
@flavors = ( "vanilla", "chocolate", "strawberry" );
for $flavor ( @flavors ) {
print "We have $flavor milkshakes\n";
}
print "They are 2.95 each\n";
print "Please email your order for home delivery\n";
Script 4: Going one way or the other:
This allows you to program in a word to make a decision. The "ne" in the if statement stands for "not equal" and is used to compare text. The "die" statement shows you a way to get out of a program when you're in trouble.
#You can program answer to be heads or tails
$answer = "heads";
if ( $answer ne "heads" and $answer ne "tails" ) {
die "Answer has a bad value: $answer!";
}
print "Answer is programmed to be $answer.\n";
if ( $answer eq "heads" ) {
print "HEADS! you WON!\n";
} else {
print "TAILS?! you lost. Try your coding again!\n";
}
Script 5: Going one way or the other, interactively:
This allows you to type in a word to make a decision. A shameless sneak peek at the next lesson on input and output. <STDIN> allows us to read a word from the keyboard, and "chomp" is a function to remove the newline that's attached to our answer after hitting the carriage return.
print "Please type in either heads or tails: ";
#The <STDIN> is the way to read keyboard input
$answer = <STDIN>;
chomp $answer;
while ( $answer ne "heads" and $answer ne "tails" ) {
print "I asked you to type heads or tails. Please do so: ";
$answer = <STDIN>;
chomp $answer;
}
print "Thanks. You chose $answer.\n";
print "Hit enter key to continue: ";
#This line is here to pause the script
#until you hit the carriage return
#but the input is never used for anything.
$_ = <STDIN>;
if ( $answer eq "heads" ) {
print "HEADS! you WON!\n";
} else {
print "TAILS?! you lost. Try again!\n";
}
Next section: matching in perl!
