Question

Hello I am writing a program for a programming class and I am getting a:

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 18
    at Life.countNeighbors(Life.java:200)
    at Life.genNextGrid(Life.java:160)

I’ve gotten ArrayIndexOutOfBoundsException errors before, usually caused when I try to add or substract a index of an array. However this time it is quite different and I was hoping someone here can point out why the error is occuring.

info about my program: the program is like John Conway game of life. using a 2d array and then setting certain elements to (true = alive) or (false = dead) the program then determine if a cell lives or dies in the next generation base on the number of neighbors it has. (3 neighbors = birth of a new cell) (2,3 neighbor = they stay alive) anything else they died in the next generation.

The IndexOutOfBound error is caused by this line according to my editor:

if(!(grid[1][1]) && !(grid[1][18]) && !(grid[18][1]) && !(grid[18][18]))

I created the above line as a constraint, and it should not tell java to search a index beyond the bounds of the original array since they are merely booleans (true/false)statement in the end. If anyone can help me debug this error that would be splendid.

HERE IS MY CODE: (does not include main method)

 public static void clearGrid ( boolean[][] grid )
{
    int col;
    int row = 1;

    while(row < 18){
       for(col = 1; col < 18; col++){
          grid[row][col]= false;//set each row to false
       }
         row++;
    }//set all elements in array to false
}

public static void genNextGrid ( boolean[][] grid )
{
    //new tempprary grid
     boolean[][] TempGrid = new boolean[GRIDSIZE][GRIDSIZE];

     TempGrid= grid; // copy the current grid to a temporary grid

     int row = 1;
     int col = 1;

    countNeighbors(TempGrid, row, col); // passes the TempGrid to countNieghbor method

 for(row = 1; row < 18; row++){

        countNeighbors(TempGrid, row, col);

        for(col = 1; col < 18; col++){

            countNeighbors(TempGrid, row, col);

            if(countNeighbors(grid, row, col) == 3)
            {
                TempGrid[row][col] = true;
            }
            else if(countNeighbors(grid, row, col) == 2 || countNeighbors(grid, row, col) == 3)
    {
                TempGrid[row][col] = true;
            }
            else
            {
                TempGrid[row][col] = false;
            }

        }
    }
}

public static int countNeighbors ( final boolean[][] grid, final int row, final int col )
{
    int n = 0; //int used to store the # of neighbors
    int temprow = row;
    int tempcol = col;
//count # of neighbors for the cell on the edge but not the corner
            for(temprow = row; temprow <= 18; temprow++)
            {
                for(tempcol = row; tempcol <= 18; tempcol++)
                {
                     if(temprow == 1 || temprow == 18 || tempcol == 1 || tempcol ==18)
                     {
                         if(!(grid[1][1]) && !(grid[1][18]) && !(grid[18][1]) && !(grid[18][18]))
                         {
                             if(grid[temprow][tempcol] == true)
                             {
                                 n++;
                             }
                         }
                     }
                }
            }


//count # of neighbors for the corner cells
     for(temprow = row; temprow <= 18; temprow++)
     {
         for(tempcol = row; tempcol <= 18; tempcol++)
         {
            if(grid[1][1] || grid[1][18] || grid[18][1] || grid[18][18])
                    {
                        if(grid[temprow][tempcol] == true)
            {
                n++;
            }
                    }
         }
     }

// count the cells that are not on the edge or corner
    while(temprow >= 2 && tempcol >= 2 && temprow <= 17 && tempcol <= 17)
    {
    for(temprow = row; temprow-1 <= temprow+1; temprow++)
    {
            for(tempcol = col; tempcol-1 <= tempcol+1; tempcol++)
            {
                if(grid[temprow][tempcol] == true)
                {
                    n++;
                }
            }
    }
    }
    return n; // return the number of neighbors
}

Answer 1

Without a full stack trace and an indication as to where the problem lies, this is my best guess:

grid[18][1]

The value 18 is beyond the size of the array you can access, in Java arrays are zero based (0). Since I have seen 17 all throughout your post, this seems like the most logical reason why.