How do I find/retrieve block in block matrix?

Hi,

let's say I have 3 matrices A,B,C and store them in a block diagonal matrix, such that I can refer easily to them in a for loop.

Example:

D=[A B C; B A B; C A B];

for j=1:3

      for i=1:9

      E=D(i,j)^1*x

     end

end

If I say D(i,j) I get an entry of matrix D. But I want the block stored in the i-th row and the j-th column in de way D is defined in the example. D(1,2) should return matrix B and D(2,2) matrix A. How can I do this?

kind regards,

Clarisha

Answer 1

[Walter Roberson]

In general you cannot, as you do not know that the matrices are the same size. If you knew the sizes of the matrices, you could calculate locations inside D given the block indices.

The better way to handle this is

D = {A B C; B A B; C A B};

then you would use D{1,2} to get back B, even if it is not the same size as the other matrices.