let posY = 200;

​

function setup() {

  createCanvas(400, 400);

  fill(255,0,0);

}

​

function draw() {

  background(220);

  circle(posX, posY, 100);

  // random function goes between 0 and 1

  posX = posX;

  posY = posY + 1 ;

​

  if(posX > width)  { posX = 0;     }   // left wall

  if(posX < 0)      { posX = width; }   // right wall

  if(posY < 0)      { posY = height;}   // top wall

  if(posY > height) { posY = 0; fill(0,0,255); }   // bottom wall

}