let circleY = 0;

​

function setup() {

  createCanvas(400, 400);

  background(0);

}

​

function draw() {

  if(random() > 0.5){

    circle(circleX,circleY,20);

  }

  if(circleX > width){

     circleY = circleY + 20;

     circleX = 0;

  }

  circleX = circleX + 20;

}

​

​