How to assign boundary condition

Suppose I have a vector like the following:

I=[8, 5, 12, 7, 9]

suppose also that for the purpose of some program I need to use the quantity

I(i)

where i is an integer, but not necessarily 0<i<6. I need a function (or a method, possibly a smart one) such that:

I(6)

returns 8 (=I(1))

I(0)

returns 9 (=I(5))

and so on: I(-1) returns I(4), I(-2) returns I(3), ... I(-5) returns I(5)... I(7) returns I(2), ... I(11) returns I(1)...

More exactly: I need periodic condition for the vector I (imagine that I(i) describes some physical quantity of a crystal primitive cell, that's require periodicity).

Note that the function

mod(i,5)

does not the correct job for i<1. How can I "fix" it for this range of i (i<1)?

Sorry for my bad english, and thank you for your help!

Answer 1

[Guillaume]

The mod function is exactly what you need. But since matlab use 1-based indexing, you need to shift your indices by 1 before and after modding:

boundedindex = mod(index-1, numel(I)) + 1