How do I find/retrieve block in block matrix?

Question

Clarisha Nijman 2018-8-11

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编辑： Walter Roberson 2018-8-11

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

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Answer 1

Walter Roberson 2018-8-11

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https://ww2.mathworks.cn/matlabcentral/answers/414516-how-do-i-find-retrieve-block-in-block-matrix#answer_332436

编辑：Walter Roberson 2018-8-11

在 MATLAB Online 中打开

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.