define undef

This is the simplest form of macro subsitution. We have seen:

#define FALSE 0

#define TRUE 1

Similarly, you might have

#define PI 3.142

although

#define PI ( (double) (3.142) )

is better. Just to emphasise that this is a preprocessor operation, note that everything to the end of the line is substituted, so in

#define TRUE 1  /* Any non-zero value will do */

the comment statement will be included: wherever you have TRUE in your program you will henceforth get:

1  /* Any non-zero value will do */

You can write longer macros, extending over several lines. Use the backslash symbol at the end of each line except the last:

#define BIG_MACRO   first line \\ 
	second_line \\ 
	last_line

You can also write macros with arguments:

#define MAX(A,B) ( (A) > (B) ? (A) : (B) )

With this definition, a line

	x= MAX(p+q, r+s);

will be replaced by:

	x= ( (p+q) > (r+s) ? (p+q) : (r+s) );

Note the use of `extra' brackets around A and B to ensure that, whatever is written there is correctly evaluated. Consider what you would get with the unsound form:

#define square(x) x*x

and square(a + 1).

Finally, #undef causes the definition to be forgotten.