Removing nested for loops for quicker time

Question

Sanghyun Lee 2019-9-11

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https://ww2.mathworks.cn/matlabcentral/answers/479923-removing-nested-for-loops-for-quicker-time

评论： Bob Thompson 2019-9-12

I need help removing the nested for loop in this code.

The code right now has imagesList, totalDistance which are 4D arrays, and finalPixels is a 3D array and index is a 2D array.

The code goes through each of the rows and columns to replace them with colours in the finalPixels which are each stored in its own 2D array which (:,:,1) is red (:,:,2) is green and (:,:,3) is blue.

[~,index]=max(totalDistance,[],4);
[rows,cols]=size(index);
finalPixels=zeros(rows,cols,3);
for i=1:rows
    for j=1:cols
        finalPixels(i,j,1)=imagesList(i,j,1,index(i,j));
        finalPixels(i,j,2)=imagesList(i,j,2,index(i,j));
        finalPixels(i,j,3)=imagesList(i,j,3,index(i,j));
    end
end

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Answer 1

Bob Thompson 2019-9-11

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I'm shooting in the dark a bit on this one, but I think you can get rid of both loops. The only thing I'm not entirely sure about is whether the indexing for the max 4th dimension value will work correctly.

finalPixels(:,:,1:3)=imagesList(:,:,1:3,index);

Without a sample of data I wouldn't be able to test it, but feel free to post any problems you have.

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Sanghyun Lee 2019-9-12

doens't seem to work

the error i get is this:(its translated so it might be wrong)

The requested 557x495x3x275715 (212.4 GB) array exceeds the maximum array size preference. If you create an array that is larger than this limit, it can take a long time and MATLAB may stop responding. For more information, see Array size limit or Preferences panel.

Bob Thompson 2019-9-12

Ok, so it's trying to use each of the elements in the 2D index array. I thought that might happen, but I had hoped it wouldn't. Unfortunately, I cannot think of another way to avoid using loops. It doesn't mean it doesn't exist, I just don't know it.

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Answer 2

Bruno Luong 2019-9-11

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编辑：Bruno Luong 2019-9-11

I know

totalDistance third dimension is a singleton
p, the third dimension imagesList of is 3, and
other dimensions of totalDistance and imagesList match

which are all important piece of information and you must learn to clearly state when asking question for people who can understand

[~,index] = max(totalDistance,[],4);
[m,n,p,~] = size(imagesList);
x = m*n*p;
finalPixels = reshape(imagesList((1:x)'+x*(repmat(index(:)-1,p,1))),[m,n,p]);