You seem to be treating this in a sorta symbolic way trying to tell MATLAB that the c(i-1) element is really element c(16,0) when i==1. However, what you have done in practice is an assignment statement,
which is not valid because you can't assign to element c(0,:).
I think the easiest way to accomplish what you want is to use the CIRCSHIFT command:
>> index = (1:16)'
>> down_one_index = circshift(index,1)
The guts of CIRCSHIFT, in your case, is the following use of MOD:
This creates the periodic indexing that you want. Hope that helps.