large 3D Matrix calculation

Question

rahman 2015-1-20

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评论： rahman 2015-1-25

Hi All I have two large matrix and I want to calculate an expression without for loop. this expression is something like matrix d as below

 f=[f1 f2 f3]
 r=[r1 r2 r3]
 d=[f1-r1 f2-r1 f3-r1
    f1-r2 f2-r2 f3-r2
    f1-r3 f2-r2 f3-r3]

can any one help me?! consider that size(f)=1*1*400 and size(r)=50*50*900 and fii=f(1,1,ii) and rjj=r(:,:,jj)

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Answer 1

Stephen23 2015-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/170652-large-3d-matrix-calculation#answer_165351

编辑：Stephen23 2015-1-21

在 MATLAB Online 中打开

You gave this code in a comment to my other answer:

size(f)==[1,350]
size(r)==[50,50,900] 
for ii=1:350
    for jj=1:900
        s(ii,jj) = sum(sum(sum( repmat(f(ii),[50,50,1]) - r(:,:,jj) ))) ;
    end
end

You can try this instead:

A = 50*50*reshape(f,1,[]);
B = reshape(sum(sum(r,1),2),[],1);
C = bsxfun(@minus,A,B);

It produces the same result as your nested loops.

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rahman 2015-1-25

tnx Stephen Cobeldick. your code was completely helpfull ;)

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Answer 2

Stephen23 2015-1-20

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https://ww2.mathworks.cn/matlabcentral/answers/170652-large-3d-matrix-calculation#answer_165332

编辑：Stephen23 2015-1-20

在 MATLAB Online 中打开

This is exactly what bsxfun is for:

f = [f1,f2,f3];
r = [r1,r2,r3];
d = bsxfun(@minus,f,r.');

bsxfun calculates the output without requiring large intermediate arrays (eg using repmat). Although, depending on the size of d, you might still run out of memory...

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Stephen23 2015-1-20

编辑：Stephen23 2015-1-20

What you have now described is a different problem to the one that you posed in your original question. My code exactly solves your original question.

John D'Errico 2015-1-20

The heartache of those who write code - shifting specs. The correct answer to the question asked but not the question intended. :)

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Answer 3

dpb 2015-1-20

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在 MATLAB Online 中打开

d=repmat(f,size(r,2),1).-repmat(r.',1,size(f,2));

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rahman 2015-1-20

tnx dpb but this operation needs much RAMs according to what I said ( consider that size(f)=1*1*400 and size(r)=50*50*900 and fii=f(1,1,ii) and rjj=r(:,:,jj) )

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