How can I fill a plane without gaps with regular squares?

Question

I'm new to processing, and trying to do this task. Maybe you know how to do it, or know where I could read about it.

I need something like this:

Thanks in advance for your help.

Answer 1

The image is a 4x3 grid with zero spacing between the columns(vg) and rows(hg):

final int _numRows = 3;
final int _numCols = 4;

void rectGrid(int l, int t, int w, int h, int hg, int vg) {
  int left;
  int top;
  for (int k = 0; k < _numRows; k++) {
    for (int j = 0; j < _numCols; j++) {
      left = l + j*(w+vg);
      top = t + k*(h+hg);
      stroke(0);
      strokeWeight(2);
      fill(255);
      rect( left, top, w, h);
    }
  }
}

void setup() {
  size(640, 340);
  background(209);
  rectGrid(20, 20, 150, 100, 0, 0);
}