# Efficient submatrix product computation

조회 수: 3(최근 30일)
Aaron Pim 2022년 9월 20일
댓글: Aaron Pim 2022년 9월 20일
I am considering the discrete Smoluchowski equations and I need efficiently compute a matrix product.
For , and a given I want to compute the product
I can easily do this by the following for loop
b = 0
for j = 1:ceil((i-1)/2)
b = b + K(j,i-j)*y(j)*y(i-j);
end
However, I want to know if there is a more efficient way of computing this product in a single line or less.
##### 댓글 수: 1표시숨기기 없음
Chunru 2022년 9월 20일
Have you tested that your above code works?

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### 채택된 답변

Karim 2022년 9월 20일
편집: Karim 2022년 9월 20일
Do note that a for loop can be very efficient. I'm not sure the single line methods will always be faster.
Below you can find one (of many) methods to reduce the number of lines in the code. I used the sub2ind command to get the data directy out of the matrix K.
% initialize data
N = 1000;
y = rand(N,1);
K = rand(N,N);
i = randi(N,1);
% test loop...
tic
b1 = 0;
for j = 1:ceil((i-1)/2)
b1 = b1 + K(j,i-j)*y(j)*y(i-j);
end
toc
Elapsed time is 0.006585 seconds.
b1
b1 = 60.1868
% test single line
tic
% first setup indices
j = (1:ceil((i-1)/2))';
% now compute the sum
b2 = sum( K(sub2ind(size(K),j,i-j)) .* y(j) .* y(i-j) );
toc
Elapsed time is 0.006417 seconds.
b2
b2 = 60.1868
##### 댓글 수: 1표시숨기기 없음
Aaron Pim 2022년 9월 20일
This is perfect, thank you.

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