Please Speed UP This Algoritm!!!

Question

Paolo Rossi 2012-2-13

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https://ww2.mathworks.cn/matlabcentral/answers/28955-please-speed-up-this-algoritm

编辑： Cedric 2013-10-10

hi i'd like to know if there's is a way to speed up this algoritm where:, lphi is a vector of 120 elements, IN4 is a matrix of 4x8192, aaa is a 3D matrix of 4x8192x120

for k = 1:lphi;

B4(k,:) = sum(IN4.*aaa(:,:,k));

end

B = sum(abs(B4).^2,2);

thanks, i don't know if in c++ it work faster any ideas?

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the cyclist 2012-2-13

It's not clear to me what your loop does. You say that lphi is a vector, but then you loop over k=1:lphi. That seems odd to me, and not really what you want.

Paolo Rossi 2012-2-13

Sorry, lphi is not a vector is the length of a vector lphi = 120

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Answer 1

Jonathan Sullivan 2012-2-13

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在 MATLAB Online 中打开

B4 = reshape(permute(bsxfun(@tims,IN4,aaa),[3 1 2]),size(aaa,3),[]);
B = sum(abs(B4).^2,2);

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Sean de Wolski 2012-2-13

And it still doesn't give the correct results.

Sean de Wolski 2012-2-13

Like, I said, this does not produce correct results, nor does it work as written. PERMUTEing a big array will likely never be faster than a well written for-loop.

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Answer 2

Sean de Wolski 2012-2-13

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在 MATLAB Online 中打开

bsxfun is lurking (update: and staying hidden)!

The fastest I could get it was to dynamically preallocate your for-loop.

    for k = 120:-1:1
        B5(k,:) = sum(IN4.*aaa(:,:,k));
    end
    B2 = sum(abs(B5).^2,2);

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Paolo Rossi 2012-2-13

i will study this function

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Answer 3

James Tursa 2012-2-13

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在 MATLAB Online 中打开

Another option you can try using some FEX submissions (slightly better at memory management than your original code):

uninit; % One time only, to built the mex routine
mtimesx; % One time only, to build the mex routine
B4 = uninit(8192,1,lphi); % Fast create variable
for k = 1:lphi;
    B4(:,:,k) = sum(IN4.*aaa(:,:,k)).'; % Contiguous result
end
B = mtimesx(B4,'t',B4,'speedomp'); % Fast nD dot product
B = B(:); % Reshape result to column

You can find uninit and mtimesx here:

http://www.mathworks.com/matlabcentral/fileexchange/31362-uninit-create-an-uninitialized-variable-like-zeros-but-faster

http://www.mathworks.com/matlabcentral/fileexchange/25977-mtimesx-fast-matrix-multiply-with-multi-dimensional-support

Note: This won't give exactly the same answer as the MATLAB code because the last dot product operation is done internally in mtimesx differently from MATLAB.

Q: Is everything real? What is the point of the abs( )?