Use the index 'n' of any element of the array. Unless that is the first element you can always take the elment n - 1 of the same array. That is the "previous element" of the elment at n. And so on.

I'm not saying that if you have value of any element you could derive the value of the previous or next element from it. I'm saying "it has a previous element" (unless it is the 1st element) meaning there is such a previous element and it is also possible to access that previous element if you know the index of the current element under investigation.