Draw a 2D grid of objects, filling the viewport, using Processing 3.x

grid.pde

import processing.svg.*; // for SVG output

// SET THESE
int numCols = 50;
int numRows = 50;

void setup() {
  size(500, 500); // for screen display
  //size(864, 864, SVG, "new.svg"); // 9in x 9in SVG
  background(255);
  smooth();
}

void draw() {
  // Draw a 2D grid of something, filling the viewport
  float colWidth = (width / numCols);
  float rowHeight = (height / numRows);
  float xpos = (colWidth / 2); // start at middle of first column
  float ypos = (rowHeight / 2); // and start in middle of first row
  float ystart = ypos;
  
  for (int i=0; i<numCols; i++) {
    ypos = ystart;
    for (int j=0; j<numRows; j++) {
      ellipse(xpos,ypos,10,10); // or whatever you want to draw
      ypos += rowHeight;
    }
    xpos += colWidth;
  }
  
  noLoop();
}

This is a quick-and-dirty method of drawing a regular grid of items in Processing 3.
Probably you would want to replace the ellipse() on Line 25 with translate(), followed by a separate function to draw whatever you want.