How do I find/retrieve block in block matrix?

Clarisha Nijman (view profile)

on 11 Aug 2018
Latest activity Edited by Walter Roberson

Walter Roberson (view profile)

on 11 Aug 2018
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

Walter Roberson (view profile)

on 11 Aug 2018
Edited by Walter Roberson

Walter Roberson (view profile)

on 11 Aug 2018

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.