notes_part_5.txt

---------- Notes on Chapter 8 of Beginning Perl (Simon Cozens, 2000). ----------

-------------------------------- SUBROUTINES -----------------------------------

Subroutines don't return anything unless they are explicitly told to. Here's how
to define one:
   sub name_of_sub {
       # code goes here
   }

Prototypes:
   sub name_of_sub($$$) {
       # takes 3 scalars as input
   }
   sub another_sub(\@\@) {
       # takes 2 array references as input
   }

To pre-declare a subroutine:
   sub name_of_sub;
   sub another_sub($$); # Prototyped to take two scalars
Or, as popular in Perl 4:
   &name_of_sub;

To pass agruments into a subroutine:
   name_of_sub(1,2,3);
   name_of_sub(1...100);
We can pass any list into a subroutine as we would into a print statement. Each
element that gets passed in ends up in the special array @_

Example subroutine to sum all numbers passed in:
   sub total {
      my $total = 0;
      $total += $_ for @_;
      print "The total is $total\n";
   }

---------------------------------- CACHING -------------------------------------

Store each answer generated from a set of parameters into a cache, usually hash.
If we need to calculate using those parameters again, we get the answer from
the cache rather than needing to recompute.

   my %cache;
   sub first_line {
      my $filename = shift;
      return $cache{$filename} if exists $cache{$filename}
      open FILE, $filename or return "";
      my $line = <FILE>;
      $cache{filename} = $line;
      return $line;
   }

----------------------------------- SCOPE --------------------------------------

Global is the default if "my" is not specified.

By default, we are in the package called "main".

Thus, saying $x = 10 is really saying $main::x = 10.

We can create other packages to use the same variable name like so:
   $Joe::name = "Joe Johnson";
   $Naim::name = "Naim Ayat";

We can change the package we are currently working in by saying:
   package Joe;
   print $name;         # this now returns "Joe Johnson"

------------------------- REFERENCES TO SUBROUTINES ----------------------------

Use ampersand to declare:
    sub my_sub { print "Hello\n"; }
    my $subReference = \&my_sub

Call like any other reference:
    &{$subReference};
    &{$subReference}(@parameters);
    $subReference->();
    $subReference->(@parameters);
________________________________________________________________________________