SelfAvoidingWalk.java

Below is the syntax highlighted version of SelfAvoidingWalk.java from §1.4 Arrays.

/******************************************************************************
 *  Compilation:  javac SelfAvoidingWalk.java
 *  Execution:    java SelfAvoidingWalk n trials
 *
 *  Generate trials self-avoiding walks in an n-by-n grid. For each walk,
 *  continue until either it reaches the boundary (coordinate 0 or n-1)
 *  or reaches a dead end (all neighboring sites have been visited).
 *  Prints the fraction of random walks that lead to dead ends.
 *
 ******************************************************************************/

public class SelfAvoidingWalk {
    public static void main(String[] args) {
        int n = Integer.parseInt(args[0]);        // lattice size
        int trials = Integer.parseInt(args[1]);   // number of trials
        int deadEnds = 0;                         // trials resulting in a dead end

        // simulate trials self-avoiding walks
        for (int t = 0; t < trials; t++) {
            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)  {

                // 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
                double r = Math.random();
                if (r < 0.25) {
                    if (!a[x+1][y])
                        x++;
                }
                else if (r < 0.50) {
                    if (!a[x-1][y])
                        x--;
                }
                else if (r < 0.75) {
                    if (!a[x][y+1])
                        y++;
                }
                else if (r < 1.00) {
                    if (!a[x][y-1])
                        y--;
                }
            }
        }

        System.out.println(100*deadEnds/trials + "% dead ends");
    }
}