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

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

void loop() {
  for(int i=0; i,led_num; i++) {
    digitalWrite(led_pin[i], HIGH);
    delay(1000);
    digitalWrite(led_pin[i], LOW);
    delay(1000);
  }

  delay(1000);

  for(int i=0; i,led_num; i--) {
    digitalWrite(led_pin[i], HIGH);
    delay(1000);
    digitalWrite(led_pin[i], LOW);
    delay(1000);
  }

  delay(1000);
  }