int timer = 100;           // The higher the number, the slower the timing.
 
void setup() {
  // use a for loop to initialize each pin as an output:
  for (int thisPin = 3; thisPin < 8; thisPin++) {
    pinMode(thisPin, OUTPUT);
  }
}
 
void loop() {
  // loop from the lowest pin to the highest:
  for (int thisPin = 3; thisPin < 8; thisPin++) {
    // turn the pin on:
    digitalWrite(thisPin, HIGH);
    delay(timer);
    // turn the pin off:
    digitalWrite(thisPin, LOW);
  }
 
  // loop from the highest pin to the lowest:
  for (int thisPin = 7; thisPin >= 3; thisPin--) {
    // turn the pin on:
    digitalWrite(thisPin, HIGH);
    delay(timer);
    // turn the pin off:
    digitalWrite(thisPin, LOW);
  }
}