How can i speed up the following?
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Bill Hannon
on 25 Apr 2016
Commented: Bill Hannon
on 29 Apr 2016
Can anyone help me speed up what is written below? I have a large code and this routine, while seemingly fast, gets called a great deal.
a = rand(31,31);
TmTn = rand(301,301,31,31);
f = zeros(size(TmTn,1),size(TmTn,2));
[qe pe] = size(a);
for p = 1:pe
for q = 1:qe;
f = f + a(q,p).*TmTn(:,:,p,q);
end
end
Note: I have tried various sequences of permute, repmat, direct multiplication, moving the sum, parfor and reshape.
Thanks in advance, Bill
Accepted Answer
Roger Stafford
on 29 Apr 2016
You can turn this into ordinary matrix multiplication. Since matlab is specially optimized for matrix multiplication, it might be faster.
a = rand(31,31);
TmTn = rand(301,301,31,31);
a2 = reshape(a.',[],1); % Transpose 'a' and make it a column vector
TmTn2 = reshape(TmTn,[],31^2); % Change TmTn to a 2D matrix
f = reshape(TmTn2*a2,301,301); % Use ordinary matrix multiplication and then reshape back
More Answers (2)
Arnab Sen
on 28 Apr 2016
Hi Bill, You can try to replace the loop by vectorization. Something like below:
a = rand(31,31); TmTn = rand(301,301,31,31);
f = zeros(size(TmTn,1),size(TmTn,2));
[qe pe] = size(a);
TmTn2=reshape(TmTn,301*31,301*31);
TmTn1=mat2cell(TmTn2,size(TmTn,1)*ones(1,p),size(TmTn,2)*ones(1,q));
b=a';
C= cellfun(@(x,y) x.*y, mat2cell(b,ones(1,pe),ones(1,qe)),TmTn1, 'UniformOutput',false);
f=sum(cat(3,C{:,:}),3);
Using 'perfor' is another option where we can multiple for loops in parallel. For more detail refer to the following link:
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