Store matrices under different variable names within a loop?

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Ashton Linney 2020-4-5

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https://ww2.mathworks.cn/matlabcentral/answers/515495-store-matrices-under-different-variable-names-within-a-loop

评论： Les Beckham 2020-4-5

I have a 4 by 16 matrix, say mat, and it can be random for this example. I would like to convert it into four seperate 4 by 4 matricies, where the first row of mat is reshaped into the first 4 by 4 matrix.

At the moment I have this code...

mat = randi([0, 9], [4,16])
for k = 1:size(mat,1)
    vec = mat(k,:);
    A = reshape(vec,[4,4]);
end

This achieves what I would like it to, however it stores all four of the matricies under A, so after it has run I can only access the fourth matrix.

Is there an efficient way to title the four matricies seperately so I can access them all?

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Stephen23 2020-4-5

"Is there an efficient way to title the four matricies seperately so I can access them all?"

Yes, by allocating them explicitly to four variables:

A = ...
B = ...
C = ...
D = ...

Although some beginners use assignin, evalin, eval etc., none of these are efficient. Your basic concept of dynamically defining variable names is fundementally inefficient, slow, obfuscated, liable to bugs, and difficult to debug.

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Usually the best solution is to NOT split up data, but to use MATOLAB efficient indexing, grouping, etc. methods.

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Answer 1

Les Beckham 2020-4-5

2
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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/515495-store-matrices-under-different-variable-names-within-a-loop#answer_424132

编辑：Les Beckham 2020-4-5

Make A a 4x4x4 three-dimensional matrix instead of creating multiple 4x4 matrices:

mat = randi([0, 9], [4,16])
A = zeros(4,4,4); % allocate space for A and set the size
for k = 1:size(mat,1)
    vec = mat(k,:);
    A(:,:,k) = reshape(vec,[4,4]);
end
A % check results

Note that if you want the elements in the rows of A to be filled from mat in order, instead of filling first into the columns of A, transpose the output of the reshape command like this:

A(:,:,k) = reshape(vec,[4,4])';

Experiment to make sure you get what you want/expect.