October 7 lecture notes

 * Touching base - test , assignments to date

 * Questions ?

 * A few more thoughts on the technicalities
   of perl subroutine arguments : 

   # What does this do?
   sub swap {
     $_[0] = $_[1];
     $_[1] = $_[0];
   }

   # What does this do?
   my $one = 1;
   my @list = ('red', 'blue', 'green');
   my $two = 2;
   my @answer = something( $one, @list, $two );
   sub something {
     my ($first, @second, $third) = @_;
     print " \$first is '$first'.\n";
     print " \@second is '$first'.\n";
     print " \$third is '$third'.\n";
   }

 * When in doubt - try it out!
    * Caveat: "my" in interactive debugger doesn't work - each line is a sub.

 * ** PROJECT DUE IN 2 WEEKS ** see website for details

 * References
    * big topic; all languages have 'em
    * leads to various data structures: trees, linked lists, ...
    * not a simple topic; don't be worried if this takes some time.
    * all references are ** scalars ** 

    * reference to a scalar:  
         my $one = 1; 
         my $one_ptr = \$one; 
         print " one is '$one' or '$$one_ptr' \n";

    * reference to an array:
         my @some = ('this', 'that'); 
         my $some_ptr = \@some;
         print " print '@some' or '@$some_ptr' \n";

    * reference to a hash:
         my %ages = ( george => 42, alice => 21 );
         my $age_ptr = \%ages;
         print " %ages is '", %ages, "' or '", %$age_ptr, "' \n";


    * alternate syntax for arrays and hashes

         my $array_ptr = [10,11,12];
         my $hash_ptr  = { jim => 44, cindy => 45 };

         print $array_ptr->[0];
	 print $$array_ptr[0];

         print $hash_ptr->{jim};
         print $$hash_ptr{jim}


    * The -> thing means "de-reference".

    * This is how you put arrays or hashes inside other arrays or hashes,
      to make things like matrices.

      # what do these do?
      my $grid = [  [1,2,3], [4,5,6], [7,8,9] ];    
      my $people = [

                       { name=>'george', phone=>'111-222'},
		       { name=>'mary',   phone=>'123-456'},
                   ];


      print $grid->[0]->[1];
      print $grid->[0][1];          # shorthand for the same thing.

      print $people->[1]->{name};
      print $people->[1]{name};     # shorthand for the same thing


    * You can put references inside lists and hashes to make
      arbitrarily complicated network-like structures

      Try this in the debugger

        $dad      = { name => 'John' };

        $son      = { name => 'Junior' };
        $daughter = { name => 'Alice' };

        $dad->{children} = [ $son, $daughter ];

        x $dad                        # examine $dad reference in debugger