let rows = 10;

let cellWidth;

let cellHeight;

let grid = [];

​

function setup() {

  createCanvas(400, 400);

  cellWidth = width / cols;

  cellHeight = height / rows;

  for (let i = 0; i < rows; i++) {

    grid[i] = [];

    for (let j = 0; j < cols; j++) {

      grid[i][j] = false;  

    }

  }

}

​

function draw() {

  background(255);

  for (let i = 0; i < rows; i++) {

    for (let j = 0; j < cols; j++) {

      if (grid[i][j]) {

        fill(255, 0, 0); 

      } else {

        fill(255); 

      }

      stroke(0);

      rect(j * cellWidth, i * cellHeight, cellWidth, cellHeight);

    }

  }

}

​

function mousePressed() {

  let j = floor(mouseX / cellWidth);

  let i = floor(mouseY / cellHeight);

  if (i >= 0 && i < rows && j >= 0 && j < cols) {

    grid[i][j] = !grid[i][j];  

  }

}

​