int led_0=0, led_1=1, led_2=2, led_3=3;
int led_4=4, led_5=5, led_6=6, led_7=7;

void setup() {
  pinMode(led_0, OUTPUT);pinMode(led_1, OUTPUT);
  pinMode(led_2, OUTPUT);pinMode(led_3, OUTPUT);
  pinMode(led_4, OUTPUT);pinMode(led_5, OUTPUT);
  pinMode(led_6, OUTPUT);pinMode(led_7, OUTPUT);
}

void loop() {
  digitalWrite(led_0, LOW);delay (50);
  digitalWrite(led_1, LOW);delay (50);
  digitalWrite(led_2, LOW);delay (50);
  digitalWrite(led_3, LOW);delay (50);
  digitalWrite(led_4, LOW);delay (50);
  digitalWrite(led_5, LOW);delay (50);
  digitalWrite(led_6, LOW);delay (50);
  digitalWrite(led_7, LOW);delay (50);

  digitalWrite(led_7, HIGH);delay (50);
  digitalWrite(led_6, HIGH);delay (50);
  digitalWrite(led_5, HIGH);delay (50);
  digitalWrite(led_4, HIGH);delay (50);
  digitalWrite(led_3, HIGH);delay (50);
  digitalWrite(led_2, HIGH);delay (50);
  digitalWrite(led_1, HIGH);delay (50);
  digitalWrite(led_0, HIGH);delay (50);
}