matrix and steps in column

My matrix A is nx3 matrix and X is entry of A at (t,2) and t is assignmed in previous line.

I expressed it as X(t)=A(t,2). But when I ask for X(t+1), it returns this error:

Index exceeds the number of array elements (3).

Error in sym/subsref (line 907)

R_tilde =

builtin('subsref',L_tilde,Idx);

If I want to calculate X(t+1)-(X), how should I change my code?

Answer 1

[Image Analyst]

Did you also assign X in advance? Why do you think there should be a t+1 element of X if you never assigned one?

n = 9;
A = rand(n, 3);
t = 4; % Whatever.
X(t) = A(t, 2); % Works fine
% The following won't work because X has only 4 elements, not 5
difference = X(t+1) - X(t); % There is no (t+1) element yet!

Not sure what you want to do Maybe you want this:

X = diff(A(:, 2);