int pinsCount=8;                        // declaring the integer variable pinsCount
int pins[] = {2,4,5,18,19,21,13,12};          // declaring the array pins[]
 
void setup() {                
  for (int i=0; i<pinsCount; i=i+1){    // counting the variable i from 0 to 9
    pinMode(pins[i], OUTPUT);            // initialising the pin at index i of the array of pins as OUTPUT
  }
}
void loop() {
  for (int i=0; i<pinsCount; i=i+1){    // chasing right
    digitalWrite(pins[i], HIGH);         // switching the LED at index i on
    delay(500);                          // stopping the program for 100 milliseconds
    digitalWrite(pins[i], LOW);          // switching the LED at index i off
  }
  for (int i=pinsCount-1; i>0; i=i-1){   // chasing left (except the outer leds)
    digitalWrite(pins[i], HIGH);         // switching the LED at index i on
    delay(500);                          // stopping the program for 100 milliseconds
    digitalWrite(pins[i], LOW);          // switching the LED at index i off
  }
}