Associative Arrays

                    CHAPTER 8: ASSOCIATIVE ARRAYS


   What Is An Associative Array?

     - Collection of scalar data

     - Elements have no particular order

     - Index values are arbitrary scalars and not necessarily integers, 
       as in an array

     - Index values are called keys


   Associative Array Variables

     - Variable that holds a single associative array

     - Name begins with a %

     - Usually an associative array is created and accessed by referring 
       to its elements

     - Associative array variables have their own namespace

   
   Associative Array Assignment Operator (=)

     - Used to give a value to an element of an associative array variable

     - Associative array variables have the empty list value before they 
       are first assigned

     - Ex.

         $names{"bob"} = "tarr";   # Key "bob", value "tarr"
         $ssn{"bob"} = 123456789;  # Key "bob", value 123456789

     - Note use of {} around the key

     - Accessing an element that does not exist returns the undef value


   Associative Array Literals

     - An associative array literal is represented as a list of key-value 
       pairs

     - Ex.

         %names = ("bob", 1, "joe", 2);
         $x{"bob"} = 1;
         $x{"joe"} = 2;            # %x is same as %names
         @y = %x;                  # @y is ("bob", 1, "joe", 2)
         %z = @y;                  # %z is same as %x
         $w = %x;                  # $w is a junk string!  Can NOT
                                   #   assign an associative array to
                                   #   a scalar in any meaningful way
        

   Associative Array Variable Interpolation

     - Elements of an associative array are interpolated in 
       a double-quoted string

     - BUT an entire associative array can NOT be interpolated in a 
       double-quoted string

     - Ex.

         $x{"bob"} = 1;
         $y = "bob";
         print ("$x{$y}\n");       # Prints: 1
         print ("%x\n");           # Prints: %x


   Keys Function

     - Gives a list of the keys of an associative array when used in an 
       array context.  (Gives the empty list, if the associative array 
       is empty.)

     - Gives the number of elements (key-value pairs) when used in a 
       scalar context.  (Gives the value 0, if the associative array 
       is empty.)

     - The list of keys is not returned in sorted order, but rather in 
       an (seemingly) random order determined by the internal way Perl 
       stores an associative array

     - Ex.

         $x{"bob"} = 10;
         $x{"joe"} = 20;
         @y = keys(%x);            # @y is ("bob", "joe")
         $z = keys(%x);            # $z is 2

     - Ex.

         foreach $key (keys %x)
         {
           print ("Key $key has value $x{$key}\n");
         }


   Values Function

     - Gives a list of the values of an associative array when used in 
       an array context.  (Gives the empty list, if the associative 
       array is empty.)

     - Gives the number of elements (key-value pairs) when used in a 
       scalar context.  (Gives the value 0, if the associative array 
       is empty.)

     - Ex.

         $x{"bob"} = 10;
         $x{"joe"} = 20;
         @y = values(%x);          # @y is (10, 20)
         $z = values(%x);          # $z is 2


   Each Function

     - Gives an element (key-value pair) of an associative array as a 
       two-element list when used in an array context.  (Gives the empty 
       list, if the associative array is empty.)

     - Each associative array has a corresponding iterator.  On every use 
       of the each operator, the iterator advances to the next key-value 
       pair.  When the end of the associative array is reached, the empty 
       list is returned.  The next use of each after that starts the 
       process over.

     - The iterator can be reset ONLY by reading all the elements from the 
       array or when a new value is assigned to the ENTIRE array

     - Adding or deleting elements of the array should not be done while 
       using the iterator

     - Gives just the next value when used in a scalar context.
 
     - Ex.

         $x{"bob"} = 1;
         $x{"joe"} = 2;

         while (($name, $numb) = each (%x))
         {
           print ("The number for $name is $numb\n");
         }


   Delete Function

     - Used to delete an element (key-value pair) of an associative array 

     - Returns the element as a two-element list

     - Ex.

         $x{"bob"} = 1;
         $x{"joe"} = 2;

         delete ($x{"bob"});