Iteration over digital outputs.

Iteration with a "for" structure over output pins. LEDs are wired from pins 0 to 7

 
void setup() {
  Serial.begin(9600);
  for(int i = 0; i<8; i++) {  // sets pins 0-7 as outputs
    pinMode(i, OUTPUT);
  }
}

void loop() {
  for(int i = 0; i<8; i++) {  // sets pins 0-7 to HIGH
    digitalWrite(i, HIGH);
    delay(100);  // delay to see the sequence
  }
  for(int i = 0; i<8; i++) {  // sets pins 0-7 to LOW
    digitalWrite(i, LOW);
    delay(100);  // delay to see the sequence
  }
}