.. _enumerator:

Enumeration Types and Typedef
===========================================

.. index:: enum, typedef

Here we present another mechanisms for user-defined data types.
Like :keyword:`typedef`, it is not needed to write any program, but can help
with the clarity of a program.

.. _enum:

Enumerator
----------

The :keyword:`enum` keyword allows a programmer to create a named finite set
of elements. The names of the elements of the set are also defined
by the programmer.


- 
   The sole purpose of the enumerator is to increase the clarity of
   the program.

- 
   There is no input or output mechanism for enumerators, so the
   program must have hard coded statements which set the value of the
   enumerator.

- 
   Enumerator values are stored as integers, so integers can be used
   either in setting the value of an enumerator or testing the value
   of an enumerator.

- 
   The declaration of an :keyword:`enum` defines the elements of the
   enumerator. It is necessary define a variable to use the :keyword:`enum`.

- 
   By default the values stored for an :keyword:`enum` variable is 
   0 through (N - 1), where the enumerator has N elements. This can be changed,
   however.


    ::

        enum month {Jan, Feb, March, April, May, June, July, Aug, Sept, Oct,
        Nov, Dec};
        
        /* Jan == 0, Feb == 1,... Dec == 11 */
        
        enum month month_due, month_late;
        int input;
        int next_month;
        
        ...
        
        if ( input == 1 ) month_due = Jan;
        
        ...
        
        next_month = (input + 1) % 12;
        month_late = (month)next_month - 1;
        
        ...
        
        if ( month_due == Jan )
           printf( "Your bill is due to be paid in January\n" );


    ::

        enum cars { ford = 3, dodge, honda = 8, toyota };
           /* dodge == 4, toyota == 9 */



.. _typedef:

Typedef
-------

The :keyword:`typedef` keyword allows us to rename a data type to a name
that has more meaning to our program.


-  The only purpose of :keyword:`typedef` is increased program clarity.

- 
   :keyword:`typedef` can be used to rename any data type including
   :keyword:`enum` and
   :keyword:`struct` definitions, which we will study shortly.

- 
   The :keyword:`typedef` statement may be specified by itself or as part of a
   :keyword:`enum` or :keyword:`struct` definition.

- 
   The :keyword:`typedef` defined new name is often the given the same name as
   the :keyword:`enum` or :keyword:`struct` definition.

- 
   Sometimes when :keyword:`typedef` is used with an individual data type
   (:keyword:`char`, :keyword:`int`, :keyword:`float` ...), it is done so to
   make a program more easily ported to another platform. For example, we
   could typedef a name like ``fullword`` to be what ever variable type is
   32 bits. Then if needed when porting the program to another platform, the
   typedef definition can be changed and the data declarations do not
   need to be modified.


    ::

        typedef char   Letter;
        typedef int    Inches;
        typedef double math;
        
        Letter ch;
        Inches height; 
        math x;

    ::

        
        typedef enum month billing_cycle;
        billing_cycle month_due;
        
        typedef enum day {sun, mon, tue, wed, thu, fri, sat} day;
        day today;
        
        typedef struct date {
           day day_of_week;
           enum month month_of_year;
           int day_of_month;
        } date;
        
        date due_date;