Sparse matrix from the columns of an initial matrix

Question

POHL Michel 2021-4-5

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/793087-sparse-matrix-from-the-columns-of-an-initial-matrix

回答： Bjorn Gustavsson 2021-4-5

Hello everyone, given a matrix A of size (m,n),

I would like to construct a matrix B of size (m,m*n) the following way :

for j = 1:n
    col_start = (j-1)*m+1;
    col_end = j*m;
    B(:,col_start:col_end) = diag(A(:,j));
end

This version uses a for loop, is there any faster way of constructing B?

Thank you in advance.

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Answer 1

Bjorn Gustavsson 2021-4-5

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https://ww2.mathworks.cn/matlabcentral/answers/793087-sparse-matrix-from-the-columns-of-an-initial-matrix#answer_667452

在 MATLAB Online 中打开

Something like this might work:

vals = []; 
idx1 = [];                      
idx2 = [];
for j = 1:n
idx1 = [idx1,1:n];
  col_start = (j-1)*m+1;                
  col_end = j*m;                        
  idx2 = [idx2,col_start:col_end];
  vals = [vals,A(:,j)'];
end
B2 = sparse(idx1,idx2,vals);