PROBLEM
I am working on a code where I am simulating a dog walking in a city - trying to escape the city. The dog makes random choices of which way to go to at each intersection with equal probability.If stuck at a dead end the dog will come directly back to the middle of a big city and start all over again. The dog will do this again and again until it gets out of the city or until it gets tired after T number of trials. But by the time the the dog starts again from the middle(N/2,N/2) on each try, it will have forgotten all the intersections it had visited in the previous attempt.
IDEA
The idea is to mimic a code given in our textbook and come up with the solution. We were given input N, T - where N is the number of north-south and east-west streets in the city and T is the number of times the dog will try to get out of the city before it gives up. We have to draw it out, using StdDraw. We have been given how to make random movements - generate a number between 0 and 4 - up: 0 right: 1 down: 2 left: 3
My Approach
import java.util.Random;
public class RandomWalk {
private static final Random RNG = new Random (Long.getLong ("seed",
System.nanoTime()));
public static void main(String[] args) {
int N = Integer.parseInt(args[0]); // lattice size
int T = Integer.parseInt(args[1]); // number of trials
int deadEnds = 0; // trials resulting in a dead end
StdDraw.setCanvasSize();
StdDraw.setXscale(0,N);
StdDraw.setYscale(0,N);
// simulate T self-avoiding walks
for (int t = 0; t < T; t++) {
StdDraw.clear();
StdDraw.setPenRadius(0.002);
StdDraw.setPenColor(StdDraw.LIGHT_GRAY);
for(int i=0;i<N;i++){
StdDraw.line(i, 0, i, N);
StdDraw.line(0, i, N, i);
}
StdDraw.setPenColor(StdDraw.RED);
StdDraw.setPenRadius(0.01);
boolean[][] a = new boolean[N][N]; // intersections visited
int x = N/2, y = N/2; // current position
// repeatedly take a random step, unless you've already escaped
while (x > 0 && x < N-1 && y > 0 && y < N-1) {
int t_x = x;
int t_y=y;
// dead-end, so break out of loop
if (a[x-1][y] && a[x+1][y] && a[x][y-1] && a[x][y+1]) {
deadEnds++;
break;
}
// mark (x, y) as visited
a[x][y] = true;
// take a random step to unvisited neighbor
int r = RNG.nextInt(4);
if (r ==3) {
//move left
if (!a[x-1][y])
t_x--;
}
else if (r == 1 ) {
//move right
if (!a[x+1][y])
t_x++;
}
else if (r == 2) {
//move down
if (!a[x][y-1])
t_y--;
}
else if (r == 0) {
//move up
if (!a[x][y+1])
t_y++;
}
StdDraw.line(t_x, t_y, x, y);
x = t_x;
y = t_y;
}
System.out.println("T: "+t);
}
System.out.println(100*deadEnds/T + "% dead ends");
}
}
ISSUE
Given N - 15, T - 10, -Dseed=5463786 we should get an output like - http://postimg.org/image/s5iekbkpf/
I am getting - see http://postimg.org/image/nxipit0pp/
I don't know where I am going wrong. I know this is very specific in nature, but I am really confused so as to what I am doing wrong. I tried all 24 permutations of 0,1,2,3 but none of them gave the output desired. So, I conclude that the issue in in my code.
