Game of Life ArrayIndexOutofBounds

Question

I'm doing the Conway's game of life. I'm pretty sure I'm close to finished, but when I run it, I get Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1 at game.of.life.GameOfLife.generation(GameOfLife.java:77) at game.of.life.GameOfLife.main(GameOfLife.java:32) Java Result: 1

I'm assuming when the method that checks neighbors at the edges of the array, there's nothing there so it dies or something. I just don't know how to make it so that doesn't happen. Does anyone have any thoughts? Code below.

package game.of.life;
import java.util.Scanner;
public class GameOfLife {
static boolean[][] current = new boolean[10][10];
static boolean[][] old = new boolean[10][10];
static int population = 10;
public static void main(String[] args) {
    String a = " @ ";
    String b = " ' ";
    int choice = 9;
    int gencount = 0;
    Scanner input = new Scanner(System.in);
    System.out.print("Choose population density. i.e. 10 = 10%: ");
    population = input.nextInt();
    populate();
    copy();
    for(int r = 0; r < current.length; r++){
        for(int c = 0; c < current[r].length; c++){
            if(current[r][c] == true){
                System.out.print(a);
            }
            else
                System.out.print(b);
        }
        System.out.println();
    }
    System.out.print("Generation " + gencount + ".");
    while(choice != 0){
        System.out.print("Make a selection: 1 - Advance Generation 0 - Exit");
        choice = input.nextInt();
        if(choice == 1){
            generation();
            for(int r = 0; r < current.length; r++){
                for(int c = 0; c < current[r].length; c++){
                    if(current[r][c] == true){
                        System.out.print(a);
                    }
                    else
                        System.out.print(b);
                }
                System.out.println();
            }
            copy();
            gencount += 1;
            System.out.println("Generation" + gencount + ".");
        }
    }
}
private static void generation(){
    for(int r = 0; r < old.length; r++){
        for(int c = 0; c < old[r].length; c++){
            if (old[r][c] == true){
                int neighbors = 0;
                if(old[r + 1][c] == true)
                    neighbors += 1;
                if(old[r - 1][c] == true)
                    neighbors += 1;
                if(old[r][c + 1] == true)
                    neighbors += 1;
                if(old[r][c - 1] == true)
                    neighbors += 1;
                if(old[r + 1][c + 1] == true)
                    neighbors += 1;
                if(old[r + 1][c - 1] == true)
                    neighbors += 1;
                if(old[r - 1][c - 1] == true)
                    neighbors += 1;
                if(old[r - 1][c + 1] == true)
                    neighbors += 1;
                if(neighbors != 3 || neighbors != 2)
                    current[r][c] = false;
            }
            else if(old[r][c] == false){
                int neighbors = 0;
                if(old[r + 1][c] == true)
                    neighbors += 1;
                if(old[r - 1][c] == true)
                    neighbors += 1;
                if(old[r][c + 1] == true)
                    neighbors += 1;
                if(old[r][c - 1] == true)
                    neighbors += 1;
                if(old[r + 1][c + 1] == true)
                    neighbors += 1;
                if(old[r + 1][c - 1] == true)
                    neighbors += 1;
                if(old[r - 1][c - 1] == true)
                    neighbors += 1;
                if(old[r - 1][c + 1] == true)
                    neighbors += 1;
                if(neighbors == 3)
                    current[r][c] = true;
            }
        }
    }
}
private static void populate(){
    for(int r = 0; r < current.length; r++){
        for(int c = 0; c < current[r].length; c++){
            int q = (int)(Math.random() * 100);
            if(q < population){
                current[r][c] = true;
            }
            else{
                current[r][c] = false;
            }
        }
    }
}
private static void copy(){
    for(int r = 0; r < old.length; r++){
        for(int c = 0; c < old[r].length; c++)
            old[r][c] = current[r][c];
    }
}
}

If anyone can help me out it would be much appreciated.

Answer 1

When r is 0, this one is not valid: old[r - 1][c].
Thus you get the exception you posted.

I suggest you simplify it like this.

    boolean isValidPosition(int r, int c){
        return 
            0 <= r && r < N &&
            0 <= c && c < M;

    }

    int getNeighboursCount(boolean[][] old, int r, int c){
        int neighbors = 0;
        for (int i=-1; i<=1; i++){
            for (int j=-1; j<=1; j++){
                if (i!=0 || j!=0){
                    if (isValidPosition(r + i, c + j)){
                        if(old[r + i][c + j])
                        {
                            neighbors++;
                        }
                    }
                }
            }
        }
        return neighbors;
    }

Answer 2

As I can see, you basically have two choices:

1. apply finite bounds, that is, for the cells in the first and last columns and rows, you implement an additional check when counting the number of 'living' neighbours.

2. apply periodic bounds, that is, the cells on the leftmost column and the cells on the rightmost column are considered as neighbours. With the help of modular arithmetic, these cells don't need to be handled separately from others.