MATLAB: A complicated matrix manipulation

Question

So I have a matrix, let's say

A = [1 2;
    1 0;
    3 2;
    0 1;
    2 1;
    1 2];
[r,c] = size(A);

In general, what I would like to do is:

for i = 1:r-1
   x = A(6,1)*A(i,1) + A(6,2)*A(i,2); 
end

However, here is the issue: if A(i,1) and/or A(i,2) is zero, and let's assume just A(i,1) is 0 and A(i,2) is non-zero, then, instead I would like to do:

x = A(6,1)*A(i-1,1) + A(6,2)*A(i,2);

And in case, even A(i-1,1) is also zero, then, I would like to do:

x = A(6,1)*A(i-2,1) + A(6,2)*A(i,2);

and so on and so forth (same for A(i,2)). Note, A(1,1), A(1,2), A(6,1), A(6,2) are always non-zeros. I am trying to get my head around it, but so far no luck, although it seems it should have a simple and efficient way.

Your help is highly appreciated. Please let me know if anything seems confusing.

Thanks in advance!

Answer 1

Try this reasonably "simple and efficient way":

r = size(A,1);
x = nan(r,1);
for k = 1:r
    r1 = find(A(1:k,1),1,'last');
    r2 = find(A(1:k,2),1,'last');
    x(k) = A(6,1).*A(r1,1) + A(6,2).*A(r2,2);
end