Scroll backward circular array

Question

I have an array and I want to scroll the positions in it backward.

I have a boolean array and I need to count how many true are there from a given cell going forward and backward.

I know that if N is the number of elements into the array, I can go ahead doing i=(i+1)%N. But what if I need to scroll it in the opposite way?

Thank you in advance.

Answer 1

This should do it:

i--;
if (i == -1)
  i = N-1;

Or in 1 line:

i = ((i-1 == -1) ? N-1 : i-1);

But noise's variation of the above is probably better. A related if-statement version:

if (i == 0)
  i = N-1;
else
  i--;

A related 1-line version:

i = (i == 0 ? N-1 : i-1)