User-Specified Data Types
-------------------------

Sometimes it would be nice to be able to define your own data types.
Let's say that you wanted a data type for True and False.  You can
use enumerated data types to make this definition.

Enumerated Data Types
---------------------

The format for an enumerated data type is:

	enum name {item1, item2, ... , itemn};

The example given above could be represented like this:

	enum flag {true, false};

Now when you access flag, you can directly test for "true" or "false" like
this:

	if (flag === true) 

or

	if (flag != false)

In this case, true and false, are not variables.  They are values!

Let us try another example:

	enum days {sun, mon, tue, wed, thu, fri, sat};

A sample C fragment using these looks like this:

	switch (days)
	{
	  case sun:
	  case tue:
	  case thu:
	    printf("It is a ride day!\n");
	    break;

	  case mon:
	  case wed:
	  case fri:
	  case sat:
	    printf("It is a run day!\n");
	    break;

	  default:
	    break;
	}

When you print the value back, the integer value is shown.  The list of
days above corresponds to 0, 1, 2, 3, 4, 5, 6.  If the commands

	day = tue;
	printf("%d\n", day);

were executed.  The value of 2 would come back since tue corresponds to 2.


The typedef Statement
---------------------

The typedef command allows you to rename data types and create new data types.
For example:

	typedef float REAL;

This adds a new data type called REAL that is identical to the float data
type.  So now in a program, you could declare vaariables of type REAL.

	REAL a, b;
	int c,d;

You could also define arrays like this:

	typedef int ARRAY[20];

	ARRAY h;

The variable h is an array of type int size 20.

A practical example in graphics would be to define a 3D point or a 2D
point.  This may be done like this:

	typedef short POINT3D[3];
	typedef short POINT2D[2];

	POINT3D pt1, pt2;
	POINT2D p, q, r;


The #define Statement
---------------------

This was covered previously for simple cases of redefining.  Here I will
present some more complicated #define uses.

If you wanted to define a simple value this is done like the usual:

	#define value 3.1455555

If you wanted to make a simple expression such as square(x) which takes
x and multiplies by itself, do this:

	#define square(x) (x) * (x)

This may be used like:

	square(10)

This value is 100.

One more example:

	#define average(a,b,c) ((a) + (b) + (c)) / 3.0

To use it, try this program:

	#define avg(a,b,c) ((a) + (b) + (c)) / 3.0

	main()
	{
	  int a = 10, b = 7, c = 11;

	  printf("avg = %f\n",avg(a,b,c));
	}

The output is:

	avg = 9.333333