Can someone explain to me the codes?

Question

#include <stdio.h>
#define IN 1 /* inside a word */
#define OUT 0 /* outside a word */
/* count lines, words, and characters in input */
main()
{
int c, nl, nw, nc, state;
state = OUT;
nl = nw = nc = 0;
while ((c = getchar()) != EOF) {
++nc;
if (c == '\n')
++nl;
if (c == ' ' || c == '\n' || c == '\t')
state = OUT;
else if (state == OUT) {
state = IN;
++nw;
}
}
printf("%d %d %d\n", nl, nw, nc);
}

Why using IN and OUT? What is the purpose of doing that? Thank you.

Answer 1

#define IN 1
#define OUT 0
are called preprocessors in C which are nothing but constants that you use in your programm. Every occurrence of IN and OUT is replaced with their respective values 1 and 0 during the preprocessing phase which occurs before the compilation process.

Answer 2

I'd say it's to make it a bit more readable. state = OUT is understandable, but state = 0 is not really.