Perl

1. Perl ESA 2011/2012 Adam Belloum a.s.z.belloum@uva.nl Material Prepared by EelcoSchatborn

2. ESA: Perl • Today: 1. Perl introduction 2. Basic Perl: types, variables, statements, . . . 3. Object Oriented Perl 4. Documentation

3. Regular Expressions • “Practical Extraction and Report Language"? • Developed by Larry Wall, as system administrator at NASA, in the late 1980s • Created as a way to make report processing easier. • It's an interpreted language, but can be compiled as well • Is now being used for: • System administration automation • Glue between systems, conversion • CGI backend for websites • Much more. . .

4. Simple Perl A simple program to start with: #!/usr/bin/perl print "Hi there!\n"; print "This is a very"; print "\tsimple program.\n";

5. Things to know about perl • Perl vocabulary is extremely rich and its grammar is very flexible. • Perl programmers usually say "There's more than one way to do it". • In fact, you can literally write a Perl script your way, with your own style. Some people even do "poetry" in Perl • Because of this, some Perl scripts can be verydifficult to read. Writing them though, is always a pleasure. Clement Lefebvre: http://www.linuxforums.org/articles/learn-perl-in-10-easy-lessons-lesson-1_120.html

6. Another Perl script http://www.perlmonks.org/?node_id=1590

7. http://www.perlmonks.org/?node_id=1590

8. The Perl interpreter • Perl is an interpreted language. • This means that the code you will write using the Perl language will need a program called the Perl interpreter in order to be run. • For instance, if you write a Perl script in a file called myScript.pl (.pl is commonly used as an extension for Perl scripts), there are two ways to run a perl script. call the interpreter to run it: $ perlmyScript.pl Clement Lefebvre: http://www.linuxforums.org/articles/learn-perl-in-10-easy-lessons-lesson-1_120.html

9. Numbers • simple numbers as per usual: • decimal: 12, -17, 255 . . . • octal, start with 0: 015, -023, 0777 . . . • hexadecimal, start with 0x: 0xc, -0x11, 0XfF, . . . either case allowed for the x or hex digits abcdef. Floating point numbers: • “one and a quarter": 1.25 • “7.25 times 10 to the 45th power": 7.25e45. • “negative 12 times 10 to the -24th": -12e-24.

10. String manipulation • Escape sequences represent special characters: \n, \t, . . . • Only in double quoted (‘ “ ’) strings • For verbatim strings use single quotes (‘ ‘ ’) Perl: print "Hi there! \n \t It's me.\n"; print 'And me \n \tas well!'; • output: Hi there! It's me. And me \n \t as well!

11. Variables • Three different types of variables: scalars, arrays and hashes • Start with the dollar (‘$’), at (‘@’) or (‘%’) depending on the type of variable $a = 2; $message = "Hello World!"; @colors = ("red", "green", "blue"); @primaryNumbers = (1, 2, 3, 5, 7); %phoneNumbers = (Alicia => "090-64-773315",Tom => "085-153-3214",Jimmy => "085-285-4545"); • Variable names are a punctuation character, a letter or underscore, and one or more alphanumeric characters or underscores .

12. Your second Perl script, the Calculator! #!/usr/bin/perl $nbArguments = $#ARGV + 1; print "number of arguments: $nbArgumentsn"; exit(1) unless $nbArguments == 3; $a = $ARGV[0]; $b = $ARGV[2]; $operation = $ARGV[1]; if ($operation eq "+") { $result = $a + $b;} elsif ($operation eq "-") { $result = $a - $b;} elsif ($operation eq "/") { $result = $a / $b;} elsif ($operation eq "x") { $result = $a * $b;} print "$a $operation $b = $resultn"; Make the script executable: $ chmoda+rxcalculator.pl Run the script : $ ./calculator.pl 5 + 6./calculator.pl 11 - 2./calculator.pl 4 x 3./calculator.pl 33 / 3 The command line arguments are stored in array @ARGV - first element $ARGV[0] - second element $ARGV[1]... etc. - the index of the last element in the array can be obtained by $#ARGV. Two operators for comparing scalar variables == and eq Unless is used as a control statement

13. Scalar variables • Start with the dollar (‘$’) sign. • Can be a number or a string (or an object reference) • The usual operators apply: +, -, *, /, =, +=, -=, *=, /=, ++, -- • Perl automatically converts between numbers and strings. $n = 8 + "2" ! $n holds "10” • concatenation of strings using a period `.’ $n = 8 . "2" ! $nholds "82”

14. Array variables (1) • Start with the at (`@') sign. • Arrays represent ordered lists of scalars. • Definition using parenthesized, comma delimited lists: @numbers = ( 1, 2, 4, 8, 16 ); @strings = ( "one", "two", "3" ); • The index of an array starts at 0, not 1 • Processing single items as a scalar using `[' and `]': print $strings[1]; # prints "two" $strings[1] = "2"; # we changed @strings

15. Array variables (2) • Arrays can be created using scalar assignment: $names[0] = "Eelco";  creates @names to hold ("Eelco") • The length of an array: $#strings, returns the number of items in the array minus one. If equal to -1, the array does not exist or is empty. print $#strings; ! prints 2 • Assignment to $#strings changes the size of the array. $#strings = 0; @strings now holds ( "one" )

16. Hash variables (1) • Start with the percentage (`%') sign. • Made up of keys and values. • Each key has exactly one corresponding value. • Definition using parenthesized, comma delimited lists of keys and values: %numbers = ("one" => "one", "two" => 2); • Processing a single hash as a scalar using `{' and `}': print $numbers{"two"}; ! prints "2" $numbers{"one"} = "1"; ! Changed %numbers

17. Hash variables (2) • Like arrays, hashes can be created using scalar assignment: $strings{"1"} = "one";  creates %strings to hold ( "1" => "one" ) • The keys <hash> function will return an array filled with all the keys in the hash. print keys%numbers; ! prints ( "one", "two" )

18. Flow control • A block of code contain a list of statements. • Code blocks are delimited by `{' and `}’ • A perl program is also a block of code. • Flow control can be excreted using looping or conditional Statements.

19. Looping Statements • For loops are for repetition • Using a list and an index variable a block of code will be executed for each value in the list. for $i ( 1, 2, 3, 4 ) { . . . } • Using `..' you can also give a range: for $i ( 1 .. 3, 4 ) { . . . } • The list can contain any scalars or other lists. @range = ( 1 .. 3 ); for $i ( @range, 4 ) { . . . }

20. Conditional Statements (1) • Conditional statements: if ( test ) { . . . } elsif{ . . . } else { . . . } unless ( test ) { . . . } else { . . . } while ( test ) { . . . } until ( test ) { . . . } • The usual comparison operators for testing: <, >, ==, !=, <=, >= • But comparison of strings using `eq'. 0xa == " 10 "  true 0xa eq " 10 "  false

21. Conditional Statements (2) if ($a == 5) { print "It's five!\n"; } elsif ($a == 6) { print "It's six!\n"; } else { print "It's something else.\n"; } unless ($pie eq 'apple') { print "Ew, I don't like $pie flavored pie.\n"; } else { print "Apple! My favorite!\n"; }

22. Conditional Statements (3) $a = 0; while ($a != 3) { $a++; print "Counting up to $a...\n"; } until ($a == 0) { $a--; print "Counting down to $a...\n"; }

23. Play Around 1 • At this point you have some basic knowledge of Perl syntax and a few simple toys to play with. Try writing some simple programs with them. • Here are two suggestions, one simple and the other a little more complex: • A word frequency counter. How often does each word show up in an array of words? Print out a report. (Hint: Use a hash to count of the number of appearances of each word.) • Given a month and the day of the week that's the first of that month, print a calendar for the month.

24. Idea of a solution • Use split to divide you string into words, separator “ “ . • For $words(split(‘ ‘, $strings) ) • $count{$words} +=1;

25. Perl Regular expressions (1) • Very mature, can be highly complex • Can be used for matching (testing) or transliteration. • Simple syntax is / . . . / • The matching operator `=~' is used for testing Perl: if ("This is perl!" =~ /is/) { print "Match!"; } output: Match!

26. Perl Regular expressions (2) • `^' and `$' match the beginning and end of a string respectively. • You can use character classes (`[' . . . `]'). • You can quantify matches using `*', `+' or `?' after a character or character class. • `.' matches any character. • Use a backslash (`\') to escape characters like: `.', `^', `$', `/', `\', `{', `}', `[', `]', . . . .

27. Perl Regular expressions (3) Metacharacters in regular expressions: \d Matches a single digit character \w Matches a single word character (a letter, digit or underscore) \s Matches a whitespace character . . . . . . Flags that can follow a regular expression: i Match case insensitively g Remember the current position for the next matching

28. Perl Regular expressions (4) • Matching subexpressions: • You can match subexpressions by encapsulating them with `(' and `)’ • Each subexpression is stored in a variable $1 . . . $n, which can be used later on. Perl: "This is it!" =~ /\s(..)/; print $1; • Question: What is printed by the above lines?

29. Perl Regular expressions (4) Search and replace: • Using s/ regexp/ replacement / • Perl: $a = "This is it, or is it?" $a =~ s/\s.(s)/ wa$1/; print $a; • output: This was it, or is it? • Using the g flag the replacement will be made for all matches, instead of just the first one. • Perl: $a = "This is it, or is it?” $a =~ s/\s.(s)/ wa$1/g; print $a; • output: This was it, or was it?

30. File Handling • The Perl language was designed to make file handling easy and efficient, so you'll probably won't have any problem opening files and reading them. • Opening and closing files • “open” instruction takes two arguments: the name of a filehandleand the name of the file itself. • filehandleis like a variable which represents the handling of the file within the script. open (CLIENTS, "clients.txt"); • By default, the file is open in read-mode. To open a file in write-mode or append-mode simply add a “>” or “>>” in front of the file name: open (CLIENTS, ">clients.txt"); open (CLIENTS, ">>clients.txt");

31. File Handling: open • The “open” instruction returns • true if it managed to open the file, false otherwise. • You can use the return value to test the success of the operation open (CLIENTS, ">clients.txt") or print "Insufficient privileges\n"; • Remember to always close the files once you're finished with them. If you don't your changes might be lost. • to close a file, simply use the “close” instruction on the filehandle: close (CLIENTS);

32. File Handling: write • Writing into files • Once the file is open in write mode you can write into it simply by writing into its filehandle. • The “print” instruction writes things on the screen by default, but you can specify a filehandle to write into. open (CLIENTS, ">>clients.txt") or die "Insufficientprivilegesn";print CLIENTS "Mr John Doen";close (CLIENTS);

33. File Handling: read • Copying the content of the file into an array • Each line will then correspond to an element of the array. Here is an example: open(CLIENTS, "clients.txt");@lines = <CLIENTS> ;close(CLIENTS);print @lines; • Looping through the filehandle • you can loop through the filehandle in a while loop by writing while($line = ) • think of it as “while” which assign lines in the clients file to $line open (CLIENTS, "clients.txt");while ($line = <CLIENTS>) {print $line;}close (CLIENTS);

34. Your third Perl script, list of employees ! #open the employeesfile open (EMPLOYEES,"employees.txt"); #for each line while (<EMPLOYEES>) { #remove the carriage return $line=$_; chomp $line; #split the line between tabs #and get the different elements ($name, $department, $salary) =split(“\t”, $line); #go to the next line unless the name starts with “Mr “ Next unless $name =~ /^Mr /; #go to the next line unless the salary is more than 25000. Next unless $salary > 25000; #go to the next line unless the department is R&D. Next unless $department eq “R&D”; #since all employees here are male, #remove the particle in front of their name $name =~ s/Mr//; Print ”$name”; } Close (EMPLOYEES); What this script does to the list of employees ? What can you learn from the example (new things we did not discuss)

35. Play Around 2 • A rule of good writing is "avoid the passive voice." Instead of The report was read by Carl, say Carl read the report. • Write a program that reads a file of sentences (one per line), detects and eliminates the passive voice, and prints the result. (Don't worry about irregular verbs or capitalization, though.)

36. Subroutines • Subs are functions, they can have any arguments • The arguments are handed down using the special array @_ • A value can be returned using return Perl: sub multiply { my (@ops) = @_; return $ops[0] * $ops[1]; } … multiply(2, 3); output: 6

37. Play around 3

38. Packages (1) • Packages are a way to split up perl code into several pieces • A package can be included into a piece of code by using “use” • Each package represents a new namespace • Create a package by creating a new file, generally ending in .pm • The code for the class you are creating goes in there. • The classname often is the same as the filename, this is easier to use, but not obligatory.

39. Packages (2) • Create a new file, for instance MyPackage.pm • The first line should provide the name of the class in the package: package MyPackage; • Next comes the code for the package . . . • The file should end with 1; to indicate to the loading module that the entire package was read successfully. • Example: packageMyPackage; print "WOW, it worked!\n"; 1;

40. Packages (3) • To use our package run the following perl code: #!/usr/bin/perl useMyPackage; • The output of the program: WOW, it worked!;

41. Packages (4) • You can refer to variables and filehandlesin other packages by prefixing the identifier with the package name and a double colon: $Package::Variable • You can create subs in a package in the normal way • They can be called from the program using the package: $Package::the_sub("well?") • Special code blocks can be used for initialization and destruction: INIT {}, CHECK {}, BEGIN {}, END {} • Use perldocperlmod to get more information

42. Objects (1) “An Object is simply areferencethat happens to know which class it belongs to.” • No special syntax for constructors, you have to make your own • A constructor is merely a subroutine that returns a reference to something "blessed" into a class, generally the class that the subroutine is defined in. “ • An object is created using the bless() • The bless() function creates a reference to an object http://perldoc.perl.org/perlobj.html#An-Object-is-Simply-a-Reference

43. Objects (2) • Using an anonymous reference (Here is a typical constructor) package Critter; sub new { bless {} } Note: • {}allocates an anonymous, empty hash and returns a reference to it • bless takes that reference and tells the object it references that it's now a Critter, and returns the reference • Using anonymous objects the class will not know itself • using a known reference: package Critter; sub new { my $self = {}; bless $self; return $self; } http://perldoc.perl.org/perlobj.html#An-Object-is-Simply-a-Reference

44. Classes (1) “A class is simply a packagethat happens to provide methods to deal with object references.” • A class provides the methods for dealing with its reference • Construction by defining a constructor function, generally new() package Critter; sub new { my $self = {}; bless $self; return $self; } http://perldoc.perl.org/perlobj.html

45. Classes (2) • A method is simply a subroutine that expects as the first argument an: • object reference • a package name (for a class method) • A constructor can call methods using the reference sub new { my $self = {}; bless $self; $self->initialize(); return $self; } http://perldoc.perl.org/perlobj.html

46. Classes (3) • Classes can be inherited • For that use the two-argument version of bless • Bless an object into a class Sub new { my $class = shift; my $self = {}; bless $self, $class; $self->initialize(); return $self; } http://perldoc.perl.org/perlobj.html

47. documentation • BOOKS!, they are in the back of the classroom. . . • Use the web, there are a lot of websites on perl • Check www.perl.com for help. • Find out about perldoc Material for these slides was taken from http://www.perl.com/pub/a/2000/10/begperl1.html

48. Assignment Create a simple program that can keep track of your mp3's.

1. first program the following using only packages, no OO

   1.1. store the names and locations of your mp3's in a large array or
   hash, and allow for additional parameters to be stored as well, like
   preference (1-5 stars), number of times played, etc.

   1.2. create a simple interface for searching/adding/deleting/editing
   your database. Make sure to use subs for these functions. Make use of RE...

2. recreate, using the previous code, the assignment using OO [OPTIONAL].

   2.1 create a main program that can run multiple databases at the same.
   Each database must be a separate object instance. Provide an interface
   that allows the user to choose and switch between the databases at will.

3. (BONUS) add functions for (re)storing your databases


49. Perl Script Exercise: Netswitch • In this exercise, we want to be able to switch between networks. We defined network configuration files in a directory called "networks". • For instance, here is the content of ./networks/home: interface=eth2 type=dhcp proxy=none And here is the content of ./networks/work: interface=eth1 type=dhcp proxy=www-proxy.work.com proxy_port=8080

50. Perl Script Exercise: Netswitch • The Perl script should: • takes a network name as its command line argument, • opens the file with the same name from ./networks and sets the network interface with the data taken from the content of that file