The example in the learning section wraps around the edges of the board - thus checking for the sides never happens.

http://processing.org/learning/topics/conway.html

Checking to the right:

(x + 1) % width

Checking to the left:

(x + width - 1) % width

There are always 8 neighbours in the example.

Well, I thought I understood, but it looks like I don't. I'm using the terms "row" and "col" to refer to the position of a cell in the overall Cell List. I then use the main "draw" function to update each cell by checking their neighbors and rendering (just drawing the correctly colored square).

I don't know if this is too much for one of you to look at, but I'd appreciate any help!

Code:

   int GridWidth = 60;
   int GridHeight = 60;
   int CellSize = 5;
   Cell[][] CellList = new Cell[GridHeight][GridWidth];
   boolean[][] Ons = new boolean[GridHeight][GridWidth];   
   
   void setup() {
     background(100);
     size(GridWidth*CellSize,GridHeight*CellSize);
     
     Ons[20][20] = true;
     Ons[20][21] = true;
     Ons[20][22] = true;
     
     // Create the grid
     for(int i=0; i < GridWidth; i++) {
	 for(int j=0; j < GridHeight; j++) {
	   CellList[j][i] = new Cell( j,i,CellSize,Ons[j][i] );
	 }
     }
     
     noStroke();
     frameRate(1);
   }
   
   void draw() {
     /*
     for(int i=0; i < GridWidth; i++) {
	 for(int j=0; j < GridHeight; j++) {
	   CellList[j][i].update();
	 }
     } */
   }
 

Code:

class Cell {
  int row, col;   // allocated spot in row/col
  int x, y;	 // actual coordinates on screen
  int Size;	 // size
  boolean State;  // on/off
  
  Cell( int _row, int _col, int _size, boolean _state ) {
    row = _row;
    col = _col;
    Size = _size;
    State = _state;
    
    x = col*Size;
    y = row*Size;
    
    update();
  }
  
  void render() {
    if(State)
	fill(#FFFFFF);
  
    rect(x,y,Size,Size);
    fill( 100 );
  }
  
  void update() {
    // check all neighbors
    // apply rules of game
    int n = countNeighbors();
    if(State==true) {
	if(!(n==3) && !(n==2)) State = false;
    } else {
	if(n==3) State = true;
    }
    
    render();
    println(State);
  }
  
  int countNeighbors() {
    int n = 0;
    
    if(CellList[row][(col+1)%GridWidth].getState()) n++; // right middle
    if(CellList[(row+1)%GridHeight][(col+1)%GridWidth].getState()) n++;	// right bottom
    if(CellList[(row+GridHeight-1)%GridHeight][(col+1)%GridWidth].getState()) n++; // right top
    
    if(CellList[row][(col+GridWidth-1)%GridWidth].getState()) n++;  // left middle
    if(CellList[(row+1)%GridHeight][(col+GridWidth-1)%GridWidth].getState()) n++; // left bottom
    if(CellList[(row+GridHeight-1)%GridHeight][(col+GridWidth-1)%GridWidth].getState()) n++; // left top
    
    return n;
  }
  
  boolean getState() {
    return State;
  }
}
 

The error I'm getting is a NullPointerException at the first line of the logic inside countNeighbors. All signs point to an error on my part inside the logic of counting neighbors.

Some progress has been made, but its still not doing what its supposed to. I've narrowed the problem down to the code that checks for neighbors on the left side of a given cell.

Can anyone see whats wrong with this code?

Code:

if(CellList[row][(col+GridWidth-1)%GridWidth].getState()) n++;  // left middle
if(CellList[(row+1)%GridHeight][(col+GridWidth-1)%GridWidth].getState()) n++; // left bottom
if(CellList[(row+GridHeight-1)%GridHeight][(col+GridWidth-1)%GridWidth].getState()) n++; // left top