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Perl Lecture Notes - September 28

• Questions

• Housekeeping
  • putting your work on bob: permissions and html directory
  • submitting assignments

• Perl Documentation
  • man perl; perldoc; pod2*
  • online: see wiki PerlStuff
  • camel book as a reference
  • don't underestimate "ask Google"

• This week's perl syntax
  • Jim's advice: explore; play around; take what you need.
  • The llama is a tour. For the complete picture, read the camel.
  • llama chap 1 - 3 high lights :
    • $_ , Perl's default ('it') variable, as in " print; ", i.e. "print it"
    • warnings : "perl -w" or "#!/usr/bin/perl -w"
    • string interpolation : " His name is $name "
    • size of array: "scalar @a" = number of element; $#array = index of last element. (But in perl people more often loop over arrays with things like 'foreach' or 'map' which don't mention the length explicitly.)
    • array functions: push, pop, shift, unshift, map, grep, order, reverse, join, sort, ...
      
      • @a= (1, 100, 10, 1000 );
      • $left = shift @a;
      • $right = pop @a;
      • push @a, $newRightmost;
      • unshift @a,$newLeftmost;
      • @b = grep { $_ > 10 } @a;
      • @names = qw( George Mary Ringo Charlie );
      • print join ' ', reverse sort map {lc} @names;
  • llama chap 4 : Subroutines
    • another funny character: & (which you can usually omit)
    • all arguments passed in through special array @_
    • modifying elements of @_ (i.e. $_[0], $_[2], ...) changes values in calling routine
    • create private copies with "my ($a,$b) = @_" syntax. (highly recommended.)
    • "local" keyword : ignore this for now; you almost never need it.
    • "my", "our", "local", "use vars qw($a $b $c)"
      issues of namespaces, package global variables, 'lexical' variables
    • 
      # Typical style for subroutine definitions
      sub addTwoNumbers {
         my ($a,$b) = @_;
         return $a + $b;
      }
  • llama chap 5 : Hashes
    • another funny character: %
    • gets used often in perl, like a private little database.
    • auto-creation of elements
    • can be used for counting things
    • the big arrow: =>
    • hash functions: keys, values, exists, delete
      • %car = ( color => 'blue', length => 6 );
      • foreach my $key (keys %car) {
           print $key ." ". $car{$key} . "\n" };
      • print " length is " . $car{length} if exists $car{length};
  • llama chap 6 : Simple I/O and unix pipes
    • unix pipes: this < input | filter | that gt; output
    • STDIN, <>, STDOUT, print : file handles
    • ( $a = <> ) vs ( @a = <> )
    • <> and chomp()
    • Weird command line switches: -p, -n , -i , -e . Example:
      perl -pi.orig -e 's/this/that/' sample.txt
    • printf, sprintf
    • print FILEHANDLE "hello";

• CPAN
  • See list of modules (also called packages or .pm files) at uwinnipeg
  • "perl -MCPAN -e shell"
    a built-in automatic download, compile, install
    • "are you ready for to configure" - just say "NO"
    • getting the first piece - "force install Net::FTP"
  • alternative : download a module like Net-FTP.tar.gz;
    upack with something like "gnutar zxf file.tar.gz";
    then "perl Makefile.PL; make; make test; make install"
  • For more info, see chapter 22 on CPAN in the camel book

• Examples
  • number_formats - source / run it
  • fractions - source / run it

• Assignment:
  • Read chapter 4, 5, 6 in the llama book, on subroutines, hashes, and simple I/O
  • Browse chapters 1-4 in the camel book. This book also covers the basics of the language, but in a more thorough way, with more detail. Get a feel for what's in there, particularly for use as a reference. The camel is the authority for Perl.
  • Pick a CPAN module that looks interesting to you. (See, for example, the list here.) Install it on your linux box or at home ("perl -MCPAN -e shell" is usually the easiest method), and write a short program to try it out.
  • In the llama book, do exercises 4.1, 4.2, 5.2, 6.1