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
2 Comments
Adam
on 28 Apr 2016
Have you run the profile on your full "large code" to determine how much of the total time is actually spent on this routine?
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:
2 Comments
Adam
on 28 Apr 2016
Edited: Adam
on 28 Apr 2016
Be careful using cellfun if speed is your aim, it is almost always slower than a for loop, but in either case the profiler or a timeit wrapped around the two options should help to check that.
Working with cell arrays in general is a lot less performant than numeric arrays if you are able to use numeric arrays instead.
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