Question

How can I create an index-matrix that specifies which elements of a matrix to address?

So for example I have a matrix A which is 80 by 50. I know that A(1:5,:) addresses only the first 5 elements, but what if I want to multiply A with another matrix which also changes the elements to be addressed? So I want to multiply B(1,:) with A(1:5,:), and B(2,:) with A(10:15,:) and so on. Is there a smart way to specify this index-matrix where the information (1:5; 10:15, etc.) is stored?

Answer 1

Yes you can certainly define indices into a matrix using another matrix. Here is a simple example using a cell array to store the index list:

X =[1,2,3,4,5,6]
Idx = { [1, 2, 3], [4, 5, 6] }
Y = X( Idx{1} ) .* X( {Idx{2} )

Y = [ 4, 10, 18]