var speed = 1;

​

​

​

​

function setup() {

  createCanvas(300, 300);

}

​

​

​

​

function draw() {

  background(255);

​

​

  strokeWeight(3);

​

  stroke(0);

  fill(255);

  rect(0, 0, 300, 300);

​

  ellipseMode(CENTER);

​

​

  xpos = xpos + speed;

​

  if ((xpos > width-100) || (xpos < 100)) {

    speed = speed * -1;

  }

​

  fill(0);

  ellipse(height/2, xpos, 100, 100);//vertical

​

​

}