Using Arrays and LED's to represent Piano Scales
Your current code will not show all notes of a scale. Only the notes at 1 and 2 will light up, since you don't sum up the numbers, that you have stored in MajorScale. But I think it is not a great idea to sum them up programatically, since this is a constant and you don't win much by representing the data this way. Instead you should directly initiate the MajorScale variable with the summed up values:

int MajorScale[] = {0, 2, 4, 5, 7, 9, 11};


Then you can enclose your for loop with another for loop, that adds an offset corresponding to the octave:

for(int j=0; j&lt;NUM_LEDS; j+=12){
    for(int i=0; i&lt;=6 &amp;&amp; MajorScale[i]+j&lt;NUM_LEDS;i++){
        leds[8+MajorScale[i]+j] = CRGB::BlueViolet;
        FastLED.show();
        delay(delayvalue);
    }
}


So we set the LED number 8+MajorScale[i]+j. 8 for the offset to C, MajorScale[i] as the position of the note in the current octave and j for the offset of the current octave relative to the C.

Note, that this is not tested in any way.