How to cite the row of the matrix by vectorization?

Question

wei zhang 2019-9-3

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https://ww2.mathworks.cn/matlabcentral/answers/478670-how-to-cite-the-row-of-the-matrix-by-vectorization

评论： wei zhang 2019-9-4

采纳的回答： Andrei Bobrov

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Hi,

I have a problem in speeding my code. In my code, there is a step as below,

 a=repmat(1:6,3,1)';
 index=[1:3;2:4];
 result=zeros(3,3,size(index,1));
 for i=1:size(index,1)
     result(:,:,i)=a(index(i,:),:)
 end

In practice, the matrix a and index are very large n*3 matrix. a is the node coordinates and the index is the triangle face index of the node. The surface is too big. So I really need to speed this loop. I have an idea that vectorization could make the code faster. But I could not get the ideal output result, even with some matrix "resize" or matrix rotation functions, like "resize" or "reshape". I am very thankful for any suggestion.

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Answer 1

Andrei Bobrov 2019-9-3

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https://ww2.mathworks.cn/matlabcentral/answers/478670-how-to-cite-the-row-of-the-matrix-by-vectorization#answer_390169

编辑：Andrei Bobrov 2019-9-3

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For large matrices, the cycle may be faster.

 a = repmat(1:6,3,1)';
 index = [1:3;2:4];
 si = size(index);
 result = permute(reshape(a(index',:),si(2),si(1),[]),[1,3,2]);

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wei zhang 2019-9-4

Thank you for your answer!

I just record the speed comparision here for others. My 'a' matrix is 188802*3, and 'index' is 377644*3. The time with the previous loop is approximately 0.9 s; the time use this answer is 0.08s. The vectorization method is obviously much faster.

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