Getting started in Perl:

Intro to Perl for programmers

Matthew Heusser – xndev.com - matt@xndev.comPresented to the West Michigan Perl User’s Group

Why use Perl?

● Discuss

Getting Perl

● www.activestate.com– The easy way to get started in Windows

● www.cpan.org (Comprehensive Perl Archive Network)– If you have Linux, chances are you have Perl

● Both support modules for almost everything imaginable.

My first Perl script

#!/usr/bin/perl -wuse strict;print "Hello, world\n";

Variables in Perl

● Scalars start with a $ ex: $foo● Arrays start with a @ ex: @foo● Hashs (Associative arrays) start with % ex: %foo● By convention file handles are UPPERCASE (no

funny character).● “my” gives variable lexical (local) scope. Use

this unless you have reason for some other scope.

Scalars

● A Scalar begins with a $● A Scalar holds a single string, number or

reference.● Unitialized scalars are undef.● Examples:

– my $num = 3.1412;– $str = "This is a string";

Type Safe

● Perl is a loosely-typed language● To make a string, evaluate the scalar in string

context– Example: if ($a eq ‘Fifty’) { do_stuff($a); }

● To make a number, evaluate as a number– Example: if ($a == 50) { do_stuff($a); }

● Conversion is like ATOI● To guarantee output, use prinft or sprintf

Some control structures

● Use {} (Curlies) to declare what your control strucuture is working with. Like BEGIN/END in PL/SQL or Pascal.

● Usual “if”-”elsif” (note missing e), “while”, and “for” structures.

● Also supports “unless” (!if), “until” (!while), and foreach strucutures.

● “next;” is like “continue;” in C.● “last;” is like “break;” in C.

Boolean context

● The scalars: “” (null string), 0, “0”, and undef evaluate to false, everything else evalutates to true.

● Lists are put into scalar context, then evaluated for truth value. Zero length lists, and undef arrays evalute to false, all other lists are true.

Some operators

● Perl supports the usual +,-,*,/,%,++,-- (add, substract, multiply, divide, modulo, increment, decrement).

● . (period) concatenates strings.● C style “+=”, “.=” etc. are supported.● Use ==, !=, >, >=, <, <= to compare numbers● Use eq, ne, gt, ge, lt, le to compare strings● All the usual operators are available, but the

syntax may be weird.

Basic I/O● STDIN is a file● To read from a file, do this:

my $str = <STDIN>;Control-Z is the EOF symbol on windows

● To re-direct STDIN from a file, do this:UNIX: perl scriptname.pl < in.txtWin: type in.txt | perl scriptname.pl

● To loop until EOF, do this: while (my $str = <FILEHANDLE>) {

}

Exercise

1) Write a program to add two scalar variables together and print the total.

3) Write a program to:– Read three numbers from the command line– Add them up – Print the total

2) Mod the program to also print an average

Arrays● Array names begin with a @● Arrays are composites of scalars that are indexed

with numbers beginning with 0.● Arrays are named Lists. Subtle differences exist

between Arrays and Lists.● Examples:

– my @stuff = (1,2,3); # use @ for the composite– $stuff[0] = 10; # use $ for a single element (scalar)– $stuff[99] = “Numbers and strings can be mixed”;

Iterating over a list

for (my $i=0; $i< scalar(@a);$i++) {

do_something($a[$i]);

}

for my $val (@a) {

do_something($val);

}

Push and Pop

my @a;push (@a, 2);push (@a, 3);my $var = pop(@a);

● Remember, lists are automatically managed● A stack using arrays is now trivial

● I sure wish I had this for the AP computer science A test!

Exercises1) Write a program to:

– Read three numbers from the command line– … Into a list– Loop and re-print the numbers– Print the total

2) Print the numbers in reverse

– It’s ok to use a c-style for loop

Hashes

● Hash names begin with %● Hashes are composites of scalars that are indexed with

scalars.● Hashes are unordered.● Example:

my %employee;

$employee{"name"} = "Bill Day";

$employee{"SSN"} = "353-27-7625";

print $employee{"name"}, $employee{"SSN"}– - Outputs: Bill Day353-27-7625

Scalar vs List Context

● In an assignment, context is determined by left side of equal sign.

● An array in scalar context evaluates to length of the array: $len = @stuff;

● (parenthesis) will put a scalar into list context: ($thing) = @stuff; # assigns $stuff[0] to $thing.

● Psudeo-function “scalar” can be used to to force a list into a scalar. Example: scalar(@stuff);

Quotes in Perl

● 'Ordinary quotes'● "Interpolated quotes - $vars exapanded\n", This is my

favorite..● `execute a “shell”` command and return the result as a

string.

my $var = " test ";

print " $var\n", '$var\n';

Outputs: test

$var\n

Iterating over a hash#!/usr/bin/perl -w

use strict;

...

my $key, $value, %hash;

...

while (($key, $value) = each %hash) {

print $key $value "\n";

}

Pronouns in Perl●$_ is the default variable●Example:#!/usr/bin/perl -w

use strict;

my @array = ("a", "b", "c");

# the following 2 loops are equivalent

foreach my $element (@array) {

print $element;

}

foreach (@array) {

print;

}

Regular Expressions

● Much like SED

● if (/Bill Day/) # evaluate $_, true if it contains string.

● if ($var =~ /Bill Day/) # evalute $var for string.

● $var =~ s/Bill/William/; # substitue the 1st occurance of “Bill” with “William” in $var.

● Lots of special characters: ., ?, *, +, (, ), [, ], | ^, \, {, },

● One of the most powerful features of Perl.

● Unfortunately beyond the scope of this talk.

I/O in Perl

● open IN, "name"; # open “name” for reading.● open HANDLE, "<name"; # same thing.● open OUT, ">output”; # create or truncate file

for output.● open LOG, ">>logfile "; # append or create file

for output.● close HANDLE; # When done with file.● All the POSIX “C” style stuff works.

I/O in Perl Continued

● <HANDLE> # (diamond operator) to read a line from the file.

● print HANDLE "string"; # prints to file. Note: no comma between HANDLE and string.

● <> with no handle reads each file given on the command line, else if command line blank STDIN. Just like you want your standard Unix utility to do.

Errors and warnings

● die "meltdown in progress"; # message to STDERR for fatal errors (exits program).

● warn "your shoe is untied."; # message to STDERR for non-fatal warnings.

Putting it all together#!/usr/bin/perl -w

use strict;

open ORIGINAL, "<original "

or die " cannot open 'original': $!";

while (<ORIGINAL>) {

s/William/Bill/g;

# substitute Bill for William globally

print;

}

close ORIGINAL;

Where to get help

● In shell: perldoc perl● In shell: perldoc perlre● www.perl.com● www.cpan.org● www.activestate.com● grand-rapids.pm.org

Review

● Perl is the premier open source high performance cross platform enterprise class object oriented language that holds together the world wide web.

● There's more than one way to do it – TMTOWTDI (Pronounced “Tim-Toady”)

● Perl makes Easy things easy, and hard things possible.

● Questions?

Exercises:

1) Create a hash; write a program to loop through a hash and print all hash pairs.

2) Write a program to read some numbers in from STDIN. stopping when the user types in 'quit', and print the total.

3) Write a program to read in some words from the command line, replacing Matthew with Matt, and re-print the words back onto the command line.