Light only half of the LEDs
I don't know much about neopixel but as a general coding practice if you want to do something for every other object, you use the modulus operator like so

if(i % 2 == 0)
{
  //i is even
  //off
}
else
{
  //i is odd
  //on
}


So just wrap your current for loop in another one and to the above with the iterating variable.