Using modular arithmetic to move a robot in 4 directions

modRobot.c

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

#define TOTAL_POS 4

#define addMod(x, n) (x + 1) % n
#define subMod(x, n) (x - 1 + n) % n

int main(int argc, char const *argv[]){
	
	int actualPosition = 0;
	char positions[4][6] = {"North", "East", "South", "West"};
	char pos;

	do {
		pos = getchar();
		if (pos == 'l') actualPosition = subMod(actualPosition, TOTAL_POS);
		if (pos == 'r') actualPosition = addMod(actualPosition, TOTAL_POS);
	} while (pos != '\n');

	printf("%s\n", positions[actualPosition]);

	return 0;
}

// input example: llrlrlrlrlr\n

/* 
	Z(4) = {0, 1, 2, 3}
	Left = -1
	Right = +1

	>>>> RIGHT
	0 + 1 = 1 % 4 = 1
	1 + 1 = 2 % 4 = 2
	2 + 1 = 3 % 4 = 3
	3 + 1 = 4 % 4 = 0
	(actualPosition + 1) % TotalPos

	>>>> LEFT
	3 - 1 = (3 - 1 + 4) % 4 = 6 % 4 = 2
	2 - 1 = (2 - 1 + 4) % 4 = 5 % 4 = 1
	1 - 1 = (1 - 1 + 4) % 4 = 4 % 4 = 0
	0 - 1 = (0 - 1 + 4) % 4 = 3 % 4 = 3
	(actualPosition - 1 + TotalPos) % TotalPos

*/