code for LED wave:

 

void setup()               // run once, when the sketch starts
{                               // even though there's nothing in it
}                               // we still need this process

void loop()                // run over and over again
{
  for (int ledPin = 13; ledPin >= 4; ledPin--)   // Declares ledPin,
                                  // tests if equal to or greater than 4
                                  // then does the following. It repeats
                                  // but decreases the value of ledPin by 1
                     
  {
    pinMode(ledPin, OUTPUT);      // sets the digital pin as output
    digitalWrite(ledPin, HIGH);       // sets the LED-ledPin on
    delay(75);                             // waits for 75 milliseconds
    digitalWrite(ledPin, LOW);        // sets the LED-ledPin off
  }
 for (int ledPin = 4; ledPin <=13; ledPin++)      // Declares ledPin,
                                   // tests if equal to or less than 13
                                   // then does the following. It repeats
                                   // but increases the value of ledPin by 1            
{
    pinMode(ledPin, OUTPUT);        // sets the digital pin as output
    digitalWrite(ledPin, HIGH);        // sets the LED-ledPin on
    delay(75);                              // waits for 75 milliseconds
    digitalWrite(ledPin, LOW);          //set the LED-ledPin off
 }
}

 

P.S.

        This way is much easier than my previous try.