Thread: #ifndef explanation?

I was wondering how #ifndef worked. to me, the following should compile. (i get syntax errors) any hints are greatly appreciated!

Code:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#ifndef a
#define a int
#endif

#ifndef b
#define b int
#endif

#ifndef LT
#define LT (a,b)(a<b)
#endif

main() {
	int i;

	while(i = 0; LT( i, 9); i++; )
		printf("Compiled/n");
//or
                LT( 5, 9){
		printf("Compiled/n");
	}
}

my reference manual claims "if true, the code that is between one of the preceding and an #endif will be compiled"

Also, while doesn't take three things in a loop guard, but for does.