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how to vectorize nested for loops

Asked by aliaqa aliaqa on 10 Sep 2019
Latest activity Commented on by Rik
on 17 Sep 2019 at 19:42
Hello everyone
I want to vectorize the following code to reduce operation time. Can you guys help me out?
thanks
% X is a double matrix
s=size(X);
RepSol=repmat(X,2,2);
% Y is a binary matrix
Y=repmat(Y,2,2);
% Preallocation
p=zeros(s);
for m=1:s(1)
for n=1:s(2)
for i=1:s(1)
for j=1:s(2)
if RepSol(i,j)==RepSol(m+i-1,n+j-1)
p(m,n)=p(m,n)+Y(i,j)*Y(m+i-1,n+j-1);
end
end
end
end
end

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2 Answers

Answer by darova
on 10 Sep 2019

IS it correct?
for m=1:s(1)
for n=1:s(2)
cond = RepSol(1:s(1),1:s(2)) == RepSol((1:s(1))+m-1,(1:s(2))+n-1);
% cond = X - RepSol((1:s(1))+m-1,(1:s(2))+n-1);
P = Y(1:s(1),1:s(2)) .* Y((1:s(1))+m-1,(1:s(2))+n-1);
res = cond .* P;
p(m,n) = sum(res(:));
end
end

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darova
on 10 Sep 2019
s(1) and s(2) are size of RepSol before repmat (two times less)
s=size(X);
RepSol=repmat(X,2,2);
Dear darova
Thank you for your prompt response. Your code produce the same answer as mine, yet it does not improve the speeding as much. The reason the speed is incredibly important to me is that this code is a function of a much larger code and it needs to be computed more than 30.000 times; hence I need it to be as simple as possible. My own solution for improving the speed was to vectorize the nested "for" loops and avoid using Repmat cause according to MATLAB Profiler, the "if" part in my code consumes most of the runtime.
I am wondering if it is feasible to vectorize the two remaining "for" loops in your code.
Dear Bob
Thank you for putting your time and energy into my question. As I mentioned in my previous comment, derova’s code produces identical answers but does not improve the speed.
To answer to your question I have to say in my knowledge there is no function in MATLAB that does the same thing. Also, I cannot go through details because the code is somehow simple and hard to explain at the same time.

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Answer by aliaqa aliaqa on 17 Sep 2019 at 17:21

Can anyone suggest anything regarding boosting my code? I have tried everything but nothing seems to workout.

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aliaqa aliaqa on 17 Sep 2019 at 18:05

Thank you for your response. Repsol matrix contains cluster numbers and matrix p do some calculation for identical cluster numbers.

darova
on 17 Sep 2019 at 18:31
Unfortunately is explains nothing
The script does the follwoing if i understood correctly:
img1 - Copy - Copy.png img1 - Copy - Copy (2).png img1 - Copy - Copy (3).png
Rik
on 17 Sep 2019 at 19:42
I agree with darova that your explanation is minimal. Please edit your question, adding what you're trying to achieve. Maybe your problem can be solved with a well-chosen convolution, but with the current level of detail it is impossible to give you a better solution than you already have been given.

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