How can i pull a matrix out of matrix with indices?

Question

Jiahong Zou 2023-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/1984214-how-can-i-pull-a-matrix-out-of-matrix-with-indices

回答： Matt J 2023-6-16

Hi everyone. Sorry for the poor expression. I used find() function to find elements, which fulfill certain condition (non constant) . I want to have a matrix of same dimension, which contains the rest of elements in Matrix (constant) . Here is my code. But it runs incredibly long, because dimension of thisMatrix{1} is 30000 * 30000 big. How can i do this faster?

Mat_nonconst_tmp = sparse(size(thisMatrix{1}, 1), size(thisMatrix{1}, 2));
for n = 1:length(row_nonconst)
    Mat_nonconst_tmp(row_nonconst(n), column_nonconst(n)) = thisMatrix{1}(row_nonconst(n), column_nonconst(n));
end
Mat_const = thisMatrix{1} - Mat_nonconst_tmp;

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Answer 1

Matt J 2023-6-16

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https://ww2.mathworks.cn/matlabcentral/answers/1984214-how-can-i-pull-a-matrix-out-of-matrix-with-indices#answer_1257464

在 MATLAB Online 中打开

Do it a completely different way. Example

A=rand(5)
A = 5×5
    0.1956    0.2027    0.4764    0.3016    0.3337
    0.6153    0.6513    0.2582    0.3525    0.6252
    0.9584    0.8762    0.8876    0.8489    0.0939
    0.3990    0.3480    0.6502    0.5263    0.3498
    0.6580    0.5777    0.8174    0.1849    0.9899
B=rand(5)
B = 5×5
    0.9819    0.5626    0.9978    0.3930    0.7500
    0.3371    0.1098    0.4372    0.7146    0.8823
    0.2073    0.8749    0.0540    0.1992    0.7296
    0.1819    0.4807    0.9508    0.0666    0.0887
    0.6023    0.3610    0.9348    0.6361    0.9436
condition=A<0.5;
B(condition)=0
B = 5×5
         0         0         0         0         0
    0.3371    0.1098         0         0    0.8823
    0.2073    0.8749    0.0540    0.1992         0
         0         0    0.9508    0.0666         0
    0.6023    0.3610    0.9348         0    0.9436