[flashgordon]

I been getting this error i run my code for game of life.
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -1
at Life.countNeighbors(Life.java:34)
at Life.update(Life.java:77)
at Life.main(Life.java:148)
I know why its giving me the error but i can't for the life of me figure out how to fix it. Any suggestions?

public static int  countNeighbors(boolean[][] board, int i, int j)
        {
        	
        	
        		
	        int count = 0;
	           
	   
	            
	        if (board[i+1][j] == true){
	           count = count + 1;
	        }
	            
	        if (board[i][j+1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i][j-1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i-1][j] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i+1][j+1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i+1][j-1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i-1][j+1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i-1][j-1] == true){
	            	count = count + 1;
	        }
	        
	        return count;
	        
	            
        }   
            
     
        	
        	
               
        
        
        
        public static void update(boolean[][] board){
                boolean[][] original = Arrays.copyOf(board,board.length);
                
                for (int i = 0; i < original.length ;  i++){
                    for ( int j = 0; j <= original[0].length ; j++){
                    	
                    	
	                    int neighbor = countNeighbors(board,i,j);
	                    	
	                    	
	                    if (original[i][j] == false && neighbor == 3){
	                             original[i][j] = true;
	                    }
	                            
	                    if (original[i][j] == true && neighbor <= 1){
	                             original[i][j] = false;
	                        }
	                            
	                    if (original[i][j] == true && neighbor > 3){
	                              original[i][j] = false;
	                        }
	                        
	                       
                            
                    	
                    	
                    	
                    	
                    	
                    }
                    	

[pbl]

Without seing all your code not easy to match the line number of your stack with your actual code but I'll bet that you are testing

board[i-1][j] or board[i][j-1] with a i or a j == 0

Making a Life game it is a lot easier to make an array of Cell object and have all the neighbors of each of the Cell defined at creation time

or make an horrible test if i == 0 or j == 0 so not check i-1 and j-1

[jon.kiparsky]

So obviously you're checking a cell where i == 0.

One way to fix this would be to guard each of those checks:

  if ((i>0) && (board[i-1][j+1]) == true){

Because of short-circuit evaluation, this will skip the check for [i-1][j+1] if that cell is off the board.

Another way to guard against this would be to leave a border around the play area, whose cells are set to false and are not updated. That is, if your board is an nXn grid, your actual arrays are of length n+2, and you iterate from 1 to n, not zero to n-1. Now i-1, i+1, j-1, and j+1 are all safe operations.

It might be nice to think about how you'd approach this problem in an object-oriented fashion, considering that you're writing this in Java. How would you go about that?

[pbl]

Another solution, if you want to play with a board[N][M] is to make your board[N+2][M+2]
and just play from 1 to N-2 and 1 to M-2
assuming the border will always be filled with 0

000000000000000
0.............0
0.............0
0.............0
000000000000000

play only when the . are

This post has been edited by pbl: 12 October 2012 - 07:14 AM
Reason for edit:: Oups... Jon beat me on that one

[flashgordon]

Here is the complete code

import java.util.Arrays;


public class Life 
{
        /*
        public static boolean[][]life(boolean[][]world)
        {
                int N = world.length;
                boolean [][] a  = new boolean [N][N];
                
                
                return a;
        }
        */
        public static int  countNeighbors(boolean[][] board, int i, int j)
        {
        	
        	
        		
	        int count = 0;
	           
	        
	       

	            
	        if (board[i+1][j] == true){
	           count = count + 1;
	        }
	            
	        if (board[i][j+1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i][j-1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i-1][j] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i+1][j+1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i+1][j-1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i-1][j+1] == true){
	            	count = count + 1;
	        }
	            
	        if (board[i-1][j-1] == true){
	            	count = count + 1;
	        }
	        
	        return count;
	        
	            
        }   
            
     
        	
        	
               
        
        
        
        public static void update(boolean[][] board){
                boolean[][] original = Arrays.copyOf(board,board.length);
                
                for (int i = 1; i < original.length-2 ;  i++){
                    for ( int j = 1; j < original[0].length-2 ; j++){
                    	
                    	
	                    int neighbor = countNeighbors(board,i,j);
	                    
	                    
	                    	
	                    	
	                   
	                        
	                       
	                        if (original[i][j] == true && neighbor == 2){
	                        original[i][j] = true;
	                        }
	                       
	                        if (original[i][j] == true && neighbor <= 1){
	                        original[i][j] = false;
	                        }
	                       
	                        if (original[i][j] == true && neighbor >= 3){
	                        original[i][j] = false;
	                        }
	                        
	                       
                            
                    	
                    	
                    	
                    	
                    	
                    }
                    	
                    
                    
                }
                
            
            
            
            
                
        }
        
        
        public static void drawBoard(boolean[][] board){
                StdDraw.clear();
                StdDraw.show(0);
                for (int i = 0; i < board.length; i++)
                        for (int j = 0; j < board[0].length; j++)
                        { 
                                if (board[i][j] == true)
                                        StdDraw.filledSquare(i, j, .5);
                                else
                                        StdDraw.square(i,j,.5);
                        }
                StdDraw.show(0);
                
        }
        
        public static  boolean[][] initBoard(int numOfRows, int numOfColumns){
                boolean [][] a = new boolean[numOfRows][numOfColumns];
                
                for (int i = 0; i < a.length+2; i++)
                        for ( int j = 0; j < a[0].length+2; j++)
                                a[i][j] = StdRandom.bernoulli();
                
                return a;
        }
        
        
        public static void main(String[] args)
        {
                StdDraw.setScale(0, 10);
                boolean[][] b = initBoard(10,10);
                
                while(true){
                	 	
                        drawBoard(B)/>;
                        StdDraw.show(100);
                        update(B)/>;
                        
                        
                } 

[pbl]

Did you fix something ?
What is StdDraw ?
Why are your method static ? main() should be the only static method which should create a Life object which has the two arrays in its instance variable

[flashgordon]

StdDraw is what is being used to draw it in Eclipse.

[jon.kiparsky]

pbl, on 12 October 2012 - 09:13 AM, said:

Oups... Jon beat me on that one

Just want to savor that moment... it happens so rarely.

[flashgordon]

Thank you both for the help I realize what i need to do now

[baavgai]

Get rid of all the bloody static. It's an object oriented language. Use objects. And use more methods.

e.g.

public class Life {
	private final boolean[][] board;
	public final int ROWS, COLS;
	
	public Life(int rows, int cols) { /* your code here */ }
	public Life(Life other) { /* your code here */ }
	private boolean randCell() { /* your code here */ }
	public void randomFill() { /* your code here */ }
	private boolean isValidPos(int row, int col) {  /* your code here */ }
	private int isAlive(int row, int col) { /* your code here */ }

	private int countNeighbors(int row, int col) {
		int count = 0;
		count += isAlive(row+1, col+1);
		count += isAlive(row+1, col);
		count += isAlive(row+1, col-1);
		count += isAlive(row, col+1);
		count += isAlive(row, col-1);
		count += isAlive(row-1, col+1);
		count += isAlive(row-1, col);
		count += isAlive(row-1, col-1);
		return count;
	}
	
	private boolean isStillAlive(Life other, int row, int col) {  /* your code here */ }

	public void update() {
		Life original = new Life(this);
		for (int row = 0; row < ROWS; row++) {
			for (int col = 0; col < COLS; col++) {
				this.board[row][col] = isStillAlive(original, row, col);
			}
		}
	}

	public void drawBoard() {  /* your code here */ }

	public static void run() {
		StdDraw.setScale(0, 10);
		Life game = new Life(10, 10);
		while (true) {
			game.drawBoard();
			StdDraw.show(100);
			game.update();
		}
	}
}

Don't use the border of doom model; it's confusing. You're already confused with it. Rather, check for boundaries before the check the life of the cell; if it's out of bounds just return false.

Hope this helps.

[jon.kiparsky]

baavgai, on 12 October 2012 - 11:39 AM, said:

Don't use the border of doom model; it's confusing. You're already confused with it. Rather, check for boundaries before the check the life of the cell; if it's out of bounds just return false.

Oh, don't listen to him, he uses K&R braces...

Seriously, this is a good example of reasonable people apparently disagreeing, but not really. Ultimately, I think we'd all agree that your best bet is to move to an object-oriented solution. That being said, you have to make your own decisions about what you want to do, and what ideas work best for you. Obviously, there's a lot of room for discussion about the details, and a number of ways to get this to work.
Play's to you, I think.

[baavgai]

Here we call those Java standard braces.

I agree, do as you like. However, try for OO and not static. The cool thing about making a class is that you can use any type implementation and only the class knows. Indeed, you can even abstract how you use the class enough than only a few methods know.

While learning, your only static method should be main. It might seem like a waste to create an object for something simple, but you really want to be thinking in that direction.

[jon.kiparsky]

Quote

While learning, your only static method should be main. It might seem like a waste to create an object for something simple, but you really want to be thinking in that direction.

Awfully sensible for a K&R man...


(Me, I've never understood the notion that it's somehow wasteful to create an object. What exactly are we supposed to be preserving here? All of the bits can be recycled, right?)

[baavgai]

Newbie programmers believe the word "new" gives them cooties, apparently.

[jon.kiparsky]

What? Everyone knows you get cooties from girls.