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

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Benson Gou on 7 Jul 2021
Commented: James Tursa on 9 Jul 2021
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.
Benson
James Tursa on 9 Jul 2021
What are the actual dimensions of A and selectedCol?

Matt J on 7 Jul 2021
Edited: Matt J on 7 Jul 2021
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.
##### 2 CommentsShowHide 1 older comment
Matt J on 7 Jul 2021
Longer on my example?

Matt J on 7 Jul 2021
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.
Stephen23 on 9 Jul 2021
Edited: Stephen23 on 9 Jul 2021
"I do not know if you have a faster way to replace find."
Have you considered using logical indexing?