for(i = 0; i<MAXROCKS; i++) if(rock[i].active) {
rock[i].x += rock[i].xvel*movementrate;
rock[i].y += rock[i].yvel*movementrate + yscroll;
for(i = 0; i<MAXROCKS; i++) if(rock[i].active) {
rock[i].x += rock[i].xvel*movementrate;
rock[i].y += rock[i].yvel*movementrate + yscroll;