Improving the performance of a for loop in Matlab

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Yodish 2017-7-19

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编辑： Yodish 2017-7-20

Hello,

I am trying to improve the speed of the following for loop in Matlab. As it is now is incredibly slow. Maybe vectorizing? But one vector slides on the other and constantly changes.

for m = 1:size(phi,1) - (constant)/2
    phi(m) = phi(m).*(mean(conj(phi(1+m:(constant)/2+m))));
end

Thanks!

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dpb 2017-7-19

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What are phi and constant? Is phi actually complex; and even if so,

mean(conj(x)) --> conj(mean(x))

so can be outside the loop.

Yodish 2017-7-20

Hi and thanks for the answer. phi would be a complex vector whereas constant is a constant number (of elements) < than the length of the aforementioned vector phi

Michele

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Answer 1

David Goodmanson 2017-7-20

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编辑：David Goodmanson 2017-7-20

在 MATLAB Online 中打开

Hello Yodish,

'one vector slides on the other'. This is basically convolution, and since you are finding a mean it is convolution with a vector of ones, of length constant/2. Since the code is not iterative, all the means can be found first.

N = 10000;
constant = 5000;
phi_orig = rand(1,N)+i*rand(1,N);
tic                                      % first way
phi = phi_orig;
for m = 1:length(phi) - (constant)/2
    phi(m) = phi(m).*(mean(conj(phi(1+m:(constant)/2+m))));
end
toc
tic                                      % second way
phi2 = phi_orig;
c2 = constant/2;
A = conv(phi2,ones(1,c2),'valid')/c2;   % means
A(1) = [];       
sA = length(A);
phi2(1:sA) = phi2(1:sA).*conj(A);
toc
d = max(abs(phi2-phi))    % should be zero