How to quickly calculate the sum of the transpose of a sparse matrix?

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Benson Gou 2021-7-7

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评论： James Tursa 2021-7-9

Dear All,

I have a very big sparse matrix A. I want to obtain the sum of its transpose of the selected columns in A. Here is my code:

B = A(:,selectedCol)';

sumA = sum(B);

I am wondering if there is a faster way to do the above calculation.

Thanks a lot in advance.

Benson

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James Tursa 2021-7-9

What are the actual dimensions of A and selectedCol?

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回答（2 个）

Matt J 2021-7-7

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编辑：Matt J 2021-7-7

在 MATLAB Online 中打开

A=sprand(1e6,1e3,100/1e6);  %Example
selectedCol=1:2:100;
tic;
B=sum( A(:,selectedCol)');
toc
Elapsed time is 0.010279 seconds.
tic
B=sum( A(:,selectedCol) ,2)';
toc
Elapsed time is 0.005409 seconds.

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Benson Gou 2021-7-7

Hi, Matt,

Thanks a lot for your great help. I tested your code, but I found it took longer time than the original code.

Thanks a lot again.

Benson

Matt J 2021-7-7

Longer on my example?

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Matt J 2021-7-7

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https://ww2.mathworks.cn/matlabcentral/answers/874068-how-to-quickly-calculate-the-sum-of-the-transpose-of-a-sparse-matrix#answer_742118

在 MATLAB Online 中打开

Depening on the size of selectedCols, it may also help to cast the operation as a matrix/vector multiplication

A=sprand(1e6,1e5,100/1e6);
selectedCol=1:10:size(A,2);
tic;
B=sum( A(:,selectedCol)');
toc
Elapsed time is 0.140409 seconds.
tic
B=sum( A(:,selectedCol) ,2)';
toc
Elapsed time is 0.182437 seconds.
z=false(size(A,2),1);
tic
x=z;
x(selectedCol)=1;
B=(A*x)';
toc
Elapsed time is 0.086501 seconds.