Object Oriented Programming
Tuesday's Homework
	Two style suggestions
Store mixed data in a hash
	my $record = [$address,$state,$zip];
	$record[1] # Less clear
	
	my $record = {
	  'address' => $address,
	  'state' => $state,
	  'zip' => $zip,
	$record{'zip'} # More clear
Memory is cheap
	Make multiple copies of data if it's easier to work with
	my %name_index;
	my %state_city_index;
	Counterpoint: multiple copies makes it easier to diverge
Store parsed data, not raw strings
	$name_index{$name} = $line;
		More work to use later
	$name_index{$name} = {
	  'state' => $state,
	  'city' => $city,
	}
		Easier to use later
Wednesday's Homework
A simple module:
	use FindBin;
	print "I was run from $FindBin::Bin\n";
	There is a module called "FindBin", and it contains a variable $Bin.
A quick trick
	My module is next to my script
	(ex path/main.pl)
	% ./main.pl
	Hello, world!
	% cd ..
	% path/main.pl
	Can't locate Alan/Hello.pm in @INC (@INC contains: /afs/cs.wisc.edu/u/a/d/adesmet/bin/opt/perlmodules/lib BUNCH OF STUFF DELETED /usr/lib/perl5/5.8.8 .) at 12-oo/hello_world.pl line 8.
Why?
	(ex path/main2.pl)
	My directory isn't in the search path
	Can set the environment variable PERL5LIB
	Lazy answer:
		use FindBin;
		use lib $FindBin::Bin;
	"use lib 'path'" lets you add to Perl's search path
	FindBin returns the directory your script is in
Another module:
	use Digest::MD5;
	my $encoded = Digest::MD5::md5_hex($password);
	Digest::MD5 provides the function md5_hex
Packages
	Obviously Perl provides some sort of namespaces (C++) or packages (Java).
package
	package Alan::MyPackageName;
	Everything from that point forward is in the Alan::MyPackagename package.
	Stop at end of file, or the next package line.
	You start in a package cleverly called main.
package
	A single file can hold multiple packages
	(ex. two-in-one.pl)
	A::hi();
	B::hi();
	package A;
	sub hi { print "Hello from A\n"; }
	package B;
	sub hi { print "Hello from B\n"; }
package per file/module
	But if you put a package in a file with the same name as the package, this works:
	(ex hello_world.pl, Alan/Hello.pm)
	In the file "Alan/Hello.pm":
		package Alan::Hello;
		sub hi { print "Hello, world!"; }
		1;
	In "hello_world.pl":
		use Alan::Hello;
		Alan::Hello::hi();
That's all nice, but where at the objects?
A module implementing an object
	use FileHandle;
	my $fh = new FileHandle;
	$fh->open("</etc/services");
	my $line = $fh->getline;
	$fh->close;
A Do-It-Yourself Object System
	package
	module
	blessing
bless my reference
	An object is a reference that is "blessed" into a package
	Usually the package is in a file/module of the same name
An example
Common (simplified) usage
	sub new {
	  my($class, $name) = @_;
	  my($self) = {
	    'name' => $name,
	    'count' => 0,
	  }, $class;
	  bless $self, $class;
	  return $self;
	}
Common (simplified) usage
	sub new {
	  my($class, $name) = @_;
	  return bless {
	    'name' => $name,
	    'count' => 0,
	  }, $class;
	}
Create your own idiom?
	"new" isn't special
	You could call it "create" or "fred".
	new is a good name. Use that.
Other details
Alternate calls
	alan->new();
	new alan;
	$existing_instance->new();
$existing_instance->new();
	Requires a tweak to support...
	sub new {
	  my($class, $name) = @_;
	  return bless {
	    'name' => $name,
	    'count' => 0,
	  }, (ref($class) || $class);
	}
Inheritance
	@ISA = ("OtherPackageName");
Python
	class hello(object):
	  def __init__(self, name):
	    self._count = 0
	    self._name = name
	  def hi(self):
	    self._count += 1
	    print "Hello, %s (I've said hello to %s %d times)" % \
	      (self._name, self._name, self._count)
	#
	hello_alan = hello("Alan")
	hello_alan.hi()
Ruby
	class Hello
	  def initialize(name)
	    @name = name
	    @count = 0
	  end
	  def hi
	    @count = @count + 1
	    puts "Hello, #{@name} (I've said hello to #{@name} #{@count} times)"
	  end
	end
	#
	hello_alan = Hello.new("Alan")
	hello_alan.hi
Javascript
	Function as class
	function hello(name) {
	  this.name = name;
	  this.count = 0;
	  this.hi = function() {
	    this.count++;
	    document.write("Hello, " + this.name + " (I've said hello to " +
	      this.name + " " + this.count + " times)\n");
	  }
	}
	//
	hello_alan = new hello("Alan");
	hello_tim = new hello("Tim");
	//
	hello_alan.hi();