Flowing lights on pins 2 to 9, on an Arduino Uno

// Flowing lights: eight LEDs on pins 2 to 9, lit one after another.
//
//   pins 2 to 9 -> one 220 ohm resistor and one LED each -> GND
//
// Because the LEDs sit on pins with consecutive numbers, one for loop can walk
// along them: the loop variable is the pin. Up the row, then back down.

const int FIRST = 2;
const int LAST = 9;
const int STEP_MS = 100;

void setup() {
  for (int pin = FIRST; pin <= LAST; pin++) pinMode(pin, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: flowing lights on pins 2 to 9");
}

void loop() {
  for (int pin = FIRST; pin <= LAST; pin++) {
    digitalWrite(pin, HIGH);
    delay(STEP_MS);
    digitalWrite(pin, LOW);
  }
  for (int pin = LAST; pin >= FIRST; pin--) {
    digitalWrite(pin, HIGH);
    delay(STEP_MS);
    digitalWrite(pin, LOW);
  }
}