How to vectorize the loops

2016 9 22
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更新时间：2016 9 22
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0 个投票

Dear all,
Could anyone tell me how to vectorize the follow loops:
Best regards.
Yeping Sun

4 个评论
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Andrei Bobrov
Andrei Bobrov 2016-9-22
What is it - dv? Scalar or vector?
Jan
Jan 2016-9-22
The EXP function is very expensive. It seems in your example like you apply it for a constant repeatedly. The inner loop overwrites the value of Hr(i,j) 5596 times, so you can easily get a speedup of factor 5596 if you omit the loop over k.
Is Hr pre-allocated before the loops?
Summary: It is hard to optimize code, which is shown partially only. Better post the real code together with some test data.
Yeping Sun
Yeping Sun 2016-9-22
编辑：Yeping Sun 2016-9-22
The code I mean should be:
for i=1:1:40
for j=1:1:40
for k=1:1:5596
Hr(i,j)=sum(delta_2(k,i,j)*exp(dv(k));
end
end
end
delta is a (5596x40x40) matrix,dv is a 5596-element vector. The attached test.mat file contains these two entities.
What I want to do is to multiple an exponent factor (exp(dv)) to each element of the first dimension of the 3D matrix delta_2 and then sum them up, and the sums are assigned as the element of the result Hr matrix (40x40).
Now I have two questions: (1) it takes over 8000 s for the code to finish the calculation in my computer, which is too long; (2)the elements in dv are too large and exp(dv) cannot be properly calculated in matlab. Would you minding helping me solve these questions?
Warm regards.
Yeping Sun
Andrei Bobrov
Andrei Bobrov 2016-9-22
编辑：Andrei Bobrov 2016-9-22
Your code should be:
ev = exp(dv(:))';
Hr = zeros(40,40);
for ii = 1:40
for jj = 1:40
Hr(ii,jj) = ev*delta_2(:,ii,jj);
end
end
Vectorize form see my answer in part 3.

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回答（1 个）

Andrei Bobrov
Andrei Bobrov 2016-9-22
编辑：Andrei Bobrov 2016-9-22

2 个投票

if dv -scalar:
out = permute(sum(delta*exp(dv)),[2,3,1]);
if dv - vector:
out = permute(sum(...
reshape(sum(bsxfun(@plus,reshape(delta,[],1),exp(dv(:)')),2),size(delta))...
),[2,3,1]);
part 3
ev = exp(dv)';
[~,n,k] = size(delta_2);
out = reshape(ev*reshape(delta_2,k,[]),n,[]);

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