Programming in Perl Lecture 5 Examples

Some pragmas:

         use strict;
         
         -- strict flags variables not declared with my, 
         as well as bareword strings at compile time

         use integer;

         -- forces match operations to be done in integer

         10/3 now returns 3, not 3.333..

         use diagnostics;

         -- compiler messages now more verbose along with turning on -w

The default package (main):

        use strict;

        $sum = 0;
# Global symbol "sum" requires explicit package name at ./largest2.plx line 9.
# expects sum to come from another package, can eliminate using 
#  my($sum) = 0;
# or
#  $main::sum = 0;

Examples of modules (see Chapter 7, Programming Perl):

       use English;

       -- allows you to use English Names for many special variables

       e.g., instead of $& use $MATCH
                        $`     $PREMATCH
                        $'     $POSTMATCH
       (a full list is on Prog. Perl, p. 403)

       use Env;

       -- gives access to environment variables like and allows them
          to be treated as simple variables (e.g., $HOSTNAME, $PATH,
          $USER etc.) rather than by using the %ENV hash

Example of use of a class from a module:

       use Math::Complex;
       # under UNIX, will look for module Math/Complex.pm 
       # in some standard paths

       o a module generally defines a particular type called a CLASS

       # in this case, the class is Math::Complex

       o the module provides a way to create instantiations of the
     class, called OBJECTS,

       $cnum1 = Math::Complex->new(1,1);
                # called a CLASS METHOD; called as
                #   <classname>-><method>(<params>)
                # new() is the name often given to
                # an object constructor
                # new IS NOT a built-in function in Perl
       $x = 2;
       $y = 3;
       $cnum2 = new Math::Complex($x,$y);
                # another way to call new

       o module has other functions called OBJECT METHODS which
     operate on objects of the class

       # examples of object methods;
       # called as 
       #   <objectname>-><method>(<params>)
       $x = $cnum1->Re(); # returns real portion
       $y = $cnum1->Im(); # returns imaginary portion

       o some modules overload operators

       # create an empty complex number object
       $cnum3 = Math::Complex->new();

       $cnum3 = $cnum1 + $cnum2;

Creation of Hard References

    -- backslash operator (functions much like & in C/C++)

    $ref_scalar = \$scalar;
    $ref_array = \@array;
    $ref_hash = \%hash;
    $ref_sub = \&subname;

    -- ANONYMOUS ARRAY COMPOSER

    $ref_array = [$a, $b, $c];
    # reference to an unnamed array consisting of
    # of the values of $a, $b, and $c at the time of creation

    # contrast with
    @array_of_refs = \($a, $b, $c);
    # a three element array containing references 
    # to $a, $b, and $c
    # same as
    @array_of_refs = (\$a, \$b, $\c);

    -- ANONYMOUS HASH COMPOSER

    $ref_hash = { # note the brackets!
        lunch => 'noon',
        dinner => 'six pm'
    };

Using Hard References

    $c = 3;
    $ref_scalar = \$c;
    # rightmost $ dereferences, leftmost $ indicates scalar value
    $value = $$ref_scalar; # $value is 3
    # could also write
    $value = ${$ref_scalar}; # same as above
    $ref_array = [1, 2, 3];
    $$ref_array[2] = 4;
    # the following produces the same result
    # it says $ref_array is a reference;
    # dereference it and return the second element
    # of the result (it always defaults to a scalar
    # context -- i.e., always returns a scalar value)
    $ref_array->[2] = 4;
    print("@$ref_array\n"); # prints 1 2 4<newline>
    $$ref_hash{'breakfast'} = 'seven am';
    @meals = keys(%$ref_hash); 
    $c = &$ref_sub(2);  # calls a sub

To illustrate the difference between [ and \(:

$a = 1; $b = 2; $c = 3;

$ref_array = [$a, $b, $c];
# assign 100 to the first array location
$$ref_array[0] = 100;
print("@$ref_array\n");
print("$a\n");
# 100 2 3
# 1

@array_of_refs = \($a, $b, $c);
# note $$array_of_refs[0] == ${$array_of_refs}[0],
#   which would be incorrect because @array_of_refs
#   is NOT a reference; therefore, must use {}
${$array_of_refs[0]} = 100;
# assign 100 to the location referred to
# by the first element of the array
foreach $ref (@array_of_refs) {
    print ($$ref, " ");
}
print("\n");
print("$a\n");
# 100 2 3 
# 100

Passing References as Parameters

      ($ref_first, $ref_second) = order_by_means(\@a, \@b);
      print("First: @$ref_first\n");
      print("Second: @$ref_second\n");

      sub order_by_means {
         my($ref_a) = $_[0];
         my($ref_b) = $_[1];

         my($mean_a) = calc_mean($ref_a);
         my($mean_b) = calc_mean($ref_b);

         if ($mean_a > $mean_b) {
           return ($ref_b, $ref_a);
         }
         else {
           return ($ref_a, $ref_b);
         }
     }

     sub calc_mean {
         my($ref_array) = $_[0];

         my($sum) = 0;

         if ( @$ref_array == 0 ) {
           return 0;
         }
         foreach $elem (@$ref_array) {
           $sum += $elem;
         }
         return ( $sum / @$ref_array ) ;
     }

Passing Filehandles as Parameters:

       o  special syntax used (typeglob)

       $rec = get_rec(\*STDIN);
       sub get_rec {
         my($handle) = $_[0];
         return (scalar <$handle>);
       }