Putting one matrix a varying number of times along the diagonal of another matrix

Question

Zain 2022-6-23

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https://ww2.mathworks.cn/matlabcentral/answers/1746750-putting-one-matrix-a-varying-number-of-times-along-the-diagonal-of-another-matrix

评论： Zain 2022-6-23

I have a matrix A which is 6x6. I need to create another matrix where A is along the diagonal with the other entries being 0. I know I could use blkdiag() for this, but the issue is I need to have as many entries of A as the value of another variable that appears earlier in the code. Let's call it n. Sometimes n could be 2, so the matrix would be a 12x12 with A along the diagonal. Sometimes n could be 100 so the ouput matrix would be 600x600 since A needs to appear 100 times. How could I go about automating this process so I don't have to write blkdiag(A,A,A,A...)? Thanks!

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Answer 1

John D'Errico 2022-6-23

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https://ww2.mathworks.cn/matlabcentral/answers/1746750-putting-one-matrix-a-varying-number-of-times-along-the-diagonal-of-another-matrix#answer_992000

在 MATLAB Online 中打开

A = rand(3);

n = 50;

Acell = repmat({sparse(A)},[1,n]);

B = blkdiag(Acell{:});

spy(B)

Note that I made A sparse in there, so that the final resulting matrix will also be sparse.

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Steven Lord 2022-6-23

在 MATLAB Online 中打开

M = magic(3)
M = 3×3
     8     1     6
     3     5     7
     4     9     2
N = 4;
C = repmat({M}, 1, N)
C = 1×4 cell array
    {3×3 double}    {3×3 double}    {3×3 double}    {3×3 double}
B = blkdiag(C{:}) % Equivalent to blkdiag(M, M, M, M)
B = 12×12
     8     1     6     0     0     0     0     0     0     0     0     0
     3     5     7     0     0     0     0     0     0     0     0     0
     4     9     2     0     0     0     0     0     0     0     0     0
     0     0     0     8     1     6     0     0     0     0     0     0
     0     0     0     3     5     7     0     0     0     0     0     0
     0     0     0     4     9     2     0     0     0     0     0     0
     0     0     0     0     0     0     8     1     6     0     0     0
     0     0     0     0     0     0     3     5     7     0     0     0
     0     0     0     0     0     0     4     9     2     0     0     0
     0     0     0     0     0     0     0     0     0     8     1     6