Perl: Lecture 1

The language

What Perl is

Merger of Unix tools– Very popular under UNIX– shell, sed, awk

Programming language– C syntax

Scripting language– Ability to change everything during runtime– Fast to employ, one-liners possible– Slower than C

What Perl is

Easy to learn– Learning curve similar to human language– More difficult things possible

Tell it to be more strict Object orientation

Esp. suited for the web & text processing– Regular expressions

How to get & use it

http://www.perl.com– ActiveState makes port for Microsoft Windows

Current Version is 5.8.0 Comprehensive documentation included

– C:\Perl\html\index.html– Perldoc

Running perl scripts– perl –w script.pl– #!/usr/bin/perl + chmod (Unix)

Variables

Scalars $ Arrays @ Hash % No need to declare variables Namespace for each variable type Case sensitive

Scalars

Number String Reference Automatic conversion of numbers and strings

$i = 1;$j = "2";print "$i\n";print "$j\n";$k = $i + $j;print "$k\n";print $i . $j . "\n";

Scalar Comparison Operators

Number String== eg

<> ne

< lt

> gt

<= le

=> ge

Truth

Any String is true except for „“ and „0“ Any number is true except for 0 Any reference is true Any undefined variable is false

The Perl if statement

if ($var==1) { commands1; }

elsif ($var==2) { commands2; }

else { commands3; }

unless ($var==3) {commands4; }

Arrays

Multivalued Variable Lookup by number List Assignments

Accessing

@home = ("couch", "chair", "table", "stove")($one, $two, $three, $four) = @home($one, $two) = ($two, $one)

$home[0] - $home[3]

Hashes

Multivalued Variable Lookup by name List Assignments

Accessing

%longday = ( "Sun" => "Sunday","Mon" => "Monday","Tue" => "Tuesday" );

@list = %longday

$longday{"Sun"}, $longday{"Mon"}

Quoting in Perl

Double Quotes ““ interprete variables and backslashes

Single Quotes ‘‘ don‘t

Own Quoting characters

q//, qq//

$one=“two”; $two=“four”;print ‘$one + $one is $two \n’;print “$one + $one is $two \n”;

Operators (1)

Arithmetic

String

Logical

% Modulus

** Exponentiation

. String concatenation

x Repeat operator

&&, and AND

||, or OR

!, not NOT

Operators (2)

File test operators

-e Exists

-r Readable

-w Writable

-d Is a directory

-f Is a regular file

-T Is a Text file

Iterative Structures

For

While

Foreach

while ($x<0.5) { $x = rand; }

for ($x=0; $x<10; $x++) { print “$x\n“; }

foreach $line (@lines) {

if ($line eq ““) last;

print reverse $line; }

Next and Last

next (continue in C) : start the next iteration of the loop.

last (break in C) : exit the loop.

Working with Files

open FILEHANDLE, EXPRESSION– Filehandle : means to access opened file– Expression : path of file to open, file mode

„<“ read (default) „>“ truncate and write „>>“ append

close FILEHANDLE

open LOG, ‘>>/var/log/mylog‘;

close LOG;

Input and Ouput

Print FILEHANDLE OUTPUT

„<FILEHANDLE>“ line reading operator

print LOG “Debug Message“;

$line = <LOG>;

@wholefile = <LOG>;

while ($line = <LOG>) {

print $line;

}

Default Variable

„$_“ default variable for functions

$_ = „222“;print log;

print while(<>);

Subroutines

Equivalent of C functions

Argument list, result list, no need to specify length

sub NAME {} # declaration

&NAME(); # invocation

sub arguments {my @params = @_;return @params;

}

@result = &arguments(@input);

Subroutines 2

„Named Parameters“

sub printtimes {my $string = shift;my $times = shift;while ($times-->0) {

print $string;}

}

$printtimes(„BA Stuggi\n“, 10);

Subroutines bonus

„@_“ for the main program : „@ARGV“

if (defined $ARGV[0]) {$loga = log $ARGV[0];print $loga;

}

print log (shift or exit);

Perl Regular Expressions (1)

(perlrequick)

Simple Pattern Matching

=~ Operator : boolean– TRUE for a match– FALSE for no match

"Hello World" =~ /World/; # matches

regex

delimiters

expression to match

Simple Pattern Matching Usage

1. if ("Hello World" =~ /World/) print "It matches\n";

2. if ("Hello World" !~ /World/) print "No match\n";

3. $teststring= "Hello World"; $greeting = "World";

if ($teststring =~ /$greeting/) print "It matches\n";

4. $_ = "Hello World";

if (/World/) print "It matches\n";

Simple Pattern Matching Bonus

Delimiters changable if „m“ is introduced

Match always at the earliest point possible Special characters („metacharacters“) require

masking

"Hello World" =~ m!World!; # matches

{}[]()^$.|*+?\

"2+2=4" =~ /2\+2/; # matches,'C:\WIN32' =~ /C:\\WIN/; # matches

Scalar Manipulation (1)

chop VAR / chomp VAR– Remove last character / Only if newline and return it

lc EXPR– Returns a lowercased EXPR

length EXPR– Returns number of characters in EXPR

index STR, SUBSTR [, POS]– Returns first position of SUBSTR in STR [after POS]

rindex STR, SUBSTR [,POS]– See above, uses last occurence

Scalar Manipulation (2)

sprintf FORMAT, LIST

%c a character with the given number %s a string %d a signed integer, in decimal %u an unsigned integer, in decimal %x an unsigned integer, in hexadecimal %f a floating-point number, in fixed decimal notation

substr EXPR,OFFSET [,LENGTH]– return the substring of EXPR starting from OFFSET

uc EXPR– Returns an uppercased EXPR