// ball bounces back and forth between left and right!

float posX = 0;
// so it's easy to change between going left and right
float speedX = 1;

void setup() {
  size(400, 100);
  smooth();
  background(255);  
}

void draw() {
  // clear screen to animate
  background(255);
  
  ellipse(posX, 50, 30, 30);
  posX = posX + speedX; 
   
  // if we're off the right edge
  if (posX > width) {
    speedX = -1; // go left
  }  
  
  // if we're off the left edge
  if (posX < 0) {
    speedX = 1; // go right  
  }
}