int timer = 500;           
int LEDPins[] = {13, 12, 8, 4, 2};       // an array of declared pin numbers on the board
int countOFpin = 6;           // the number of arrays
void setup() 
{
  // we have declared an array to intialize the LED pins as OUTPUT
  for (int PIN = 0; PIN < countOFpin; PIN= PIN + 1) 
{
    pinMode(LEDPins[PIN], OUTPUT);
  }
}
void loop()
 {
  // loop starting from the lowest pin in the array to the highest:
  for (int PIN = 0; PIN < countOFpin; PIN++) {
    // turns the pin ON:
    digitalWrite(LEDPins[PIN], HIGH);
    delay(timer);
    // turnS the pin OFF:
    digitalWrite(LEDPins[PIN], LOW);
  }
  // loop from the highest pin in the array to the lowest: 
  // It means the LEDs will light in the reverse direction as used above
  for (int PIN = countOFpin - 1; PIN >= 0; PIN- -)
 { 
    digitalWrite(LEDPins[PIN], HIGH);
    delay(timer);
    digitalWrite(LEDPins[PIN], LOW);
    // We can also specify the time inside the delay( ) instead of the delcaring the timer
  }
}