int boardWidth  = 8;
int boardHeight = 8;
int ON  = 1;
int OFF = 0;

int[][] board = new int[boardWidth][boardHeight];

float unitX, unitY;

void setup() {
  size(400,400);
  unitX = width  / boardWidth;
  unitY = height / boardHeight;
  
  for( int i = 0 ; i < board.length ; i++ ) {
    for( int j = 0 ; j < board[i].length ; j++ ) {
      board[i][j] = OFF;
    }
  }
  board[2][3] = ON;
  board[2][4] = ON;
}

void draw() {
  background(255);
  stroke(0);
  for( int i = 0 ; i < board.length ; i++ ) {
    for( int j = 0 ; j < board[i].length ; j++ ) {
      if( board[i][j] == ON ) {
        fill(0);
      } else {
        fill(255);
      }
      rect(i * unitX, j * unitY, unitX, unitY);
    }
  }
}

void mouseClicked() {
  int mx = (int)(mouseX / unitX);
  int my = (int)(mouseY / unitY);
  if( board[mx][my] == ON ) {
    board[mx][my] = OFF;
  } else {
    board[mx][my] = ON;
  }
}