int i;
int direction;

void setup(){

 size(300,300);
 direction = 1;
}

void loop(){

  background(255,255,255);
  
  line(0,i,300,i);
  i += direction;
  
 if(i > 300){
    direction= -1;
  }
  if(i < 0){
    direction= 1;
  } 
}