Hi!
I'm an almost complete beginner with Processing and programming at all, but I'm interested in learning. However i've got really stuck at a tiny program i've created. It's just a grid board which you can click in to fill in the squares. The problem I'm having is that if I first click gridX[0] gridY[0] then click gridX[0] gridY[2] then gridX[0] gridY[1] gets filled in too! I really can't get my head around this one, so if anyone wants to help i would be really grateful. Thanks!

Code:

boolean gridX[] = new boolean[200];
boolean gridY[] = new boolean[200];

void setup() {
  frameRate(10);
  size(200,200);
  background(255);
  for (int x = 0; x < width; x++) {
    for (int y = 0; y < height; y++) {
      line(x * 10, 0, x * 10, height);    
      line(0, y* 10, width, y * 10);
      }
    }
}
void draw() {

  for (int x = 0; x < width; x++) {
    for (int y = 0; y < height; y++) {
      if( ( gridX[x] && gridY[y] ) == true) {
        fill(0);
        rect(x * 10, y * 10, 10, 10);
      }
    }
  }
}

void mousePressed() {
      if (!(gridX[mouseX/10] && gridY[mouseY/10])) {
      gridX[mouseX/10] = true;
      gridY[mouseY/10] = true;
    }
}
 

Your data (gridX and gridY) don't represent a two-dimensional grid but two one-dimensional vectors.

You need something like
boolean grid[][] = new boolean[20][20];
to represent a grid of 20x20 cells.

If you click on a square, you currently do not activate a single grid cell,
grid[x][y] = true;
but one column of your grid,
gridX[x] = true;
and one row of your grid,
gridY[y] = true;

In your draw() function, you don't ask whether a certain grid cell is activated, but whether the row and the column which define the position of this cell are activated.

This modification of your draw() visualizes this:
Code:

void draw() {

  for (int x = 0; x < width; x++) {
    for (int y = 0; y < height; y++) {
      if( ( gridX[x] || gridY[y] ) == true) {
        fill(200);
        rect(x * 10, y * 10, 10, 10);
      }
      if( ( gridX[x] && gridY[y] ) == true) {
        fill(0);
        rect(x * 10, y * 10, 10, 10);
      }
    }
  }
}