int ledPins[] = {2, 3, 4, 5, 6, 7, 8, 9};
void setup() {
    for(int i = 0; i < 8; i++) {
        pinMode(ledPins[i], OUTPUT);
    }
}

void loop() {
    oneAfterAnotherLoop();
}

void oneAfterAnotherLoop() {
    int delayTime = 100;
    for(int i = 0; i <= 7; i++) {
        digitalWrite(ledPins[i], HIGH);
        delay(delayTime);
    }
    
    for(int i = 7; i >= 0; i--) {
        digitalWrite(ledPins[i], LOW);
        delay(delayTime);
    }
}

void oneOnAtATime() {
    int delayTime = 100;
    for(int i = 0; i <= 7; i++) {
        int offLED = i - 1;
        if(i == 0) {
            offLED = 7;
        }

        digitalWrite(ledPins[i], HIGH);
        digitalWrite(ledPins[offLED], LOW);
        delay(delayTime);
    }
}

void inAndOut() {
    int delayTime = 100;
    for(int i = 0; i <= 3; i++) {
        int offLED = i - 1;
        if(i == 0) {
            offLED = 3;
        }

        int onLED1 = 3 - i;
        int onLED2 = 4 + i;
        int offLED1 = 3 - offLED;
        int offLED2 = 4 + offLED;
        digitalWrite(ledPins[onLED1], HIGH);
        digitalWrite(ledPins[onLED2], HIGH);
        digitalWrite(ledPins[offLED1], LOW);
        digitalWrite(ledPins[offLED2], LOW);
        delay(delayTime);
    }

    for(int i = 3; i >= 0; i--) {
        int offLED = i + 1;
        if(i == 3) {
            offLED = 0;
        }

        int onLED1 = 3 - i;
        int onLED2 = 4 + i;
        int offLED1 = 3 - offLED;
        int offLED2 = 4 + offLED;

        digitalWrite(ledPins[onLED1], HIGH);
        digitalWrite(ledPins[onLED2], HIGH);
        digitalWrite(ledPins[offLED1], LOW);
        digitalWrite(ledPins[offLED2], LOW);
        
        delay(delayTime);
    }
}