Basic animation, variables & conditional

gistfile1.pde

/* variables for pos, width & height of rectangle */
int leftPos;
int speed;
int rectW, rectH;

void setup() {
  size(300, 300);

  leftPos = 0;
  speed = 1;
  rectW = 30;
  rectH = 30;
}


void draw() {
  background(200, 200, 200);

  fill(random(0, 255), random(0, 255), random(0, 255));
  rect(leftPos, 30, rectW, rectH);
  
  /* move our rect postion by speed every frame */
  leftPos += speed; 
  
  /* ask whether it its left edge has gone past the right
     edge of our screen
  */
  if (leftPos > width) {
    // if beyond screen, set postion to negative width of our rect
    leftPos = -rectW;
  }
}