How do I avoid infinite loopings in an array?

Question

"Mount an F matrix of size n x n representing a forest. Each element of this matrix can have 3 values:

0 - if empty 1 - contains a tree 2 - contains fire

With probability p, given by the user, put a tree in each position of F (use Math.random()). Choose a tree to set on fire (change its value to 2).

Apply the following rule until there are no more items with a value of 2:

• If there is a tree in position (i,j) and there is a fire in some neighboring position, change tree to fire.
• If there is fire in a position (i,j) and there is no tree in the vicinity, change fire to void.

Determine the percentage of trees that survived."

I dare say I got it working. Here’s my code:

package aula10;
import java.util.*;
public class aula10_1 {
    public static void main(String[] args) {
        int n, i, j;
        double p;
        Scanner ent = new Scanner(System.in);
        ent.useLocale(Locale.US);
        System.out.println("Digite a dimensão da matriz (que é quadrada):");
        n = ent.nextInt();
        System.out.println("Digite a probabilidade de haver uma árvore em qualquer ponto da floresta:");
        p = ent.nextDouble();
        int [][] F = new int[n][n];
        Random rnd = new Random();        
        for (i=0; i<n; i++) {
            for (j=0; j<n; j++) {
                F[i][j] = rnd.nextInt();
                if (F[i][j] < p) {
                    F[i][j] = 1;
                } else {
                    F[i][j] = 0;
                }
                System.out.print(F[i][j]+" ");
            }
        }
        int k = rnd.nextInt(n-1);
        int l = rnd.nextInt(n-1);
        while (true) {
        if (F[k][l] == 1) {
            F[k][l] = 2;
            break;
        }
    }
        System.out.println(Arrays.deepToString(F));

    }

}

I tested it several times and went into infinite looping some. With Debug I was able to identify and correct some errors, but even so sometimes gives problem with small probabilities. I don’t know exactly what happens... My knowledge in Java is very limited, so I was satisfied with this code. But I still accept suggestions for improvement.

Answer 1

Given a position i,j in the forest, the neighbors of that position are:

F[i-1][j-1]
F[i-1][j]
F[i-1][j+1]
F[i][j-1]
F[i][j+1]
F[i+1][j-1]
F[i+1][j]
F[i+1][j+1]

Every position you travel through the forest, that is, for every (i,j) within the for more internal (the for j), you have to check each of the neighbors by the conditions proposed in the problem (taking care not to check outside the forest area, ie limiting yourself to i-1 >=0, j-1 >= 0, i+1 < n and j+1 < n).

You must repeat these for i and for j as long as there are any trees on fire.