const int LED_PINS[] = {5,18,19,21} ;
const int NUM_LEDS = sizeof(LED_PINS)/sizeof(int) ; 

#define OFF (LOW) 
#define ON (HIGH)


void setup() {
  // put your setup code here, to run once:
    for (int i=0 ; i < NUM_LEDS ; i++) {
    pinMode(LED_PINS[i], OUTPUT);
    }

}


void loop() {
  // put your main code here, to run repeatedly:
  int i = 0;
  for ( i ; i < NUM_LEDS ; i++){
    digitalWrite(LED_PINS[i],ON);
    delay(1000);

  }
  if (i == NUM_LEDS) {
    for ( i-- ; i >= 0; i--){
    digitalWrite(LED_PINS[i],OFF);
    delay(1000);
  }

  }
}