Question

I am supposed to write a program in C only. It’s code for a grid follower.

I have defined a co-ordinate system of the grid (0-5,0-5). Also the bot orientations (+y,-y,+x,-x) and positions are defined. (I would welcome tips on how to write good code for this.)

Now as my bot moves through the grid and goes to some coordinate (x,y) in the grid through some path. Only 90 deg and 180 deg turns are allowed.

How can I reach (0,0), i.e. the starting point, traversing the same path? How can I reverse the orientations and give appropriate turning instructions using C code?

Answer 1

This can be cleaned up quite a lot, but it should be a good framework to understand the concepts. All we’re basically doing is saving every move, and then simply backtracking it.

#include <stdio.h>

#define MAX_STEPS 256

enum CARDINAL_DIRECTIONS { N = 0, W, S, E };

struct _s_robot {
    int x;
    int y;
    int orientation;
    int step;
    int x_history[MAX_STEPS];
    int y_history[MAX_STEPS];
    int turn_history[MAX_STEPS];
} MY_LITTLE_ROBOT = {0, 0, 0, 0, {0}, {0}, {0}};

void robot_go() {
    switch(MY_LITTLE_ROBOT.orientation) {
    case N:
        ++MY_LITTLE_ROBOT.y;
        break;
    case W:
        --MY_LITTLE_ROBOT.x;
        break;
    case S:
        --MY_LITTLE_ROBOT.y;
        break;
    case E:
        ++MY_LITTLE_ROBOT.x;
        break;
    }
    MY_LITTLE_ROBOT.x_history[MY_LITTLE_ROBOT.step] = MY_LITTLE_ROBOT.x;
    MY_LITTLE_ROBOT.y_history[MY_LITTLE_ROBOT.step] = MY_LITTLE_ROBOT.y;
    MY_LITTLE_ROBOT.turn_history[MY_LITTLE_ROBOT.step] = MY_LITTLE_ROBOT.orientation;
    ++MY_LITTLE_ROBOT.step;
}

void robot_change_orientation(int _orientation) {
    MY_LITTLE_ROBOT.orientation = _orientation;
}

void robot_reverse_orientation(int _orientation) {
    if (_orientation == N) MY_LITTLE_ROBOT.orientation = S;
    else if (_orientation == W) MY_LITTLE_ROBOT.orientation = E;
    else if (_orientation == S) MY_LITTLE_ROBOT.orientation = N;
    else if (_orientation == E) MY_LITTLE_ROBOT.orientation = W;
}

void robot_backtrack() {
    int i;
    printf("MY_LITTLE_ROBOT wants to turn around, currently at: %i, %i\n", MY_LITTLE_ROBOT.x, MY_LITTLE_ROBOT.y);
    robot_reverse_orientation(MY_LITTLE_ROBOT.orientation);
    for (i = MY_LITTLE_ROBOT.step-1; i >=  0; --i) {
        printf("Robot is @ %i, %i, with an orientation of: %i \n", MY_LITTLE_ROBOT.x, MY_LITTLE_ROBOT.y,  MY_LITTLE_ROBOT.orientation );
        robot_reverse_orientation(MY_LITTLE_ROBOT.turn_history[i]);
        robot_go();
    }
}

void dump_history() {
    int i;
    for (i = MY_LITTLE_ROBOT.step-1; i >=  0; --i) {
        printf("Robot is @ %i, %i, with an orientation of: %i \n", MY_LITTLE_ROBOT.x_history[i], MY_LITTLE_ROBOT.y_history[i],  MY_LITTLE_ROBOT.turn_history[i] );
    }
}

int main() {
    printf("***START: Robot is @ %i, %i\n", MY_LITTLE_ROBOT.x, MY_LITTLE_ROBOT.y);
    robot_go();
    robot_go();
    robot_go();
    robot_change_orientation(S);
    robot_go();
    robot_go();
    robot_change_orientation(W);
    robot_go();
    robot_go();
    robot_go();
    robot_change_orientation(N);
    robot_go();
    dump_history();
    robot_backtrack();
    printf("***FINISH: Robot is @ %i, %i\n", MY_LITTLE_ROBOT.x, MY_LITTLE_ROBOT.y);
    return 0;
}

To use the robot over and over again, you may have to reset all travel history after a backtrack, which should be somewhat trivial. I purposely avoided mallocs and other confusing aspects of making the program actually useful for the sake of verbosity and simplicity. Hopefully it helps.

I also assumed you didn’t want a true path-finding algorithm (such as A*), because that’s a different ballgame altogether.