int pin_nums[8] = {2, 3, 4, 5, 6, 7, 8, 9};

void setup() {
  for(int i = 0; i <= 7; i++) {
    pinMode(pin_nums[i], OUTPUT);
  }
}

void loop() {
  for(int i = 0; i <= 7; i++) {
    digitalWrite(pin_nums[i], HIGH);
    delay(200);
    digitalWrite(pin_nums[i], LOW);
  }

  for(int i = 7; i >= 0; i--) {
    digitalWrite(pin_nums[i], HIGH);
    delay(200);
    digitalWrite(pin_nums[i], LOW);
  }
  

}