Perl and Python ESA 2007/2008. Eelco Schatborn 27 September 2007

Transcription

1 Perl and Python ESA 2007/2008 Eelco Schatborn 27 September 2007

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

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

6 Strings Escape sequences represent special characters: \n, \t,... Only in double quoted ( " ) strings For verbatim strings use single quotes ( ) Perl: output: print "Hi there! \n \t It s me.\n"; print And me \n \t as well! ; Hi there! It s me. And me \n \t as well!

7 Variables Start with the pound ( $ ), at ) or percentage ( % ) sign, depending on the type of variable Variable names are a punctuation character, a letter or underscore, and one or more alphanumeric characters or underscores. Three different types of variables: scalars, arrays and hashes

8 Scalar variables Start with the pound ( $ ) 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" But concatenation of strings using a period. $n = 8. "2" $n holds "82"

9 Array variables (1) Start with the at ) sign. Arrays represent ordered lists of scalars. Definition using parenthesized, comma delimited = ( 1, 2, 4, 8, 16 = ( "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

10 Array variables (2) Arrays can be created using scalar assignment: $names[0] = "Eelco"; 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 = now holds ( "one" )

11 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"; we changed %numbers

12 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" ) Question: How do you find out the number of items in a hash?

13 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 excerted using looping or conditional statements.

14 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 = ( ); for $i 4 ) {... }

15 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

16 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"; }

17 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"; }

18 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!

19 Perl Regular expressions (2) Jaap already treated most of this: ^ 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. You can quantify matches generically using { from, to }.. matches any character. Use a backslash ( \ ) to escape characters like:., ^, $, /, \, {, }, [, ],....

20 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 g Match case insensitively Remember the current position for the next matching

21 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?

22 Search and replace: Perl Regular expressions (4) 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?

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

24 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.

25 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: package MyPackage; print "WOW, it worked!\n"; 1;

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

27 Packages (4) You can refer to variables and filehandles in 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: BEGIN {}, END {}, INIT {}, CHECK {} Use perldoc perlmod to get more information

28 Objects (1) An Object is simply a reference that happens to know which class it belongs to. No special syntax for constructors, you have to make your own An object is created using the bless() The bless() function creates a reference to an object Anything can become blessed bless {} allocates an anonymous, empty hash and returns a reference to it

29 Using an anonymous reference: package Critter; sub new { bless {} } Objects (2) Using anonymous objects the class will not know itself, using a known reference: package Critter; sub new { my $self = {}; bless $self; return $self; }

30 Classes (1) A class is simply a package that 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; }

31 Classes (2) A method is simply a subroutine that expects an object reference (or a package name, for class methods) as the first argument. A constructor can call methods using the reference sub new { my $self = {}; bless $self; $self->initialize(); return $self; }

32 Classes (3) Classes can be inherited 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; }

33 Documentation BOOKS!, they are in the back of the classroom... Use the web, there are a lot of websites on perl Check for help. Find out about perldoc Material for these slides was taken from