Ali - you could create two vectors of indices that you can then use to perform the subtraction. One vector would be all indices starting from 3, 4, 5,... up to the last index (which would correspond to the length of the input vector), and the other vector would be all indices starting from 1, 2, 3,... up to two less than the end of the input vector. Try the following
A = [1 2 3 4 5 6];
idx2 = 3:length(A);
idx1 = 1:length(A)-2;
myDiff = A(idx2) - A(idx1);
Running the above returns
myDiff =
2 2 2 2
which correspond to the differences of 3-1, 4-2, 5-3, and 6-4.
