Problems with time travel. A (t)=Variable(t-2)

I want to make one value of a variable in a for loop dependent on the value of another variable 2 timesteps ago.

When I am on t=1, the code will lookup Variable(-1) which is an invalid index number because array index numbers must start at 1.

Is there a way to say?

if variable(x) has an invalid reference number do y?

Can you please give me an example?

[Azzi Abdelmalek]

what is the value of y(-1). give an example

Answer 1

[Azzi Abdelmalek]

eg

 y=[0 rand(1,20)]

use

 y(max(1,n-n0))

Answer 2

[Michael]

This is a simplified version of my problem.

for(t=1:10),

Array1=[1 4 5 6 7 8 9]; Variable=(Array1(t-1))

end

The error is: Attempted to access Array1(0); index must be a positive integer or logical.

I want Variable to simply skip t's or return some specific value where the index is bad