speed up a code, subindex for 4-d matrix?

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MementoMori 2023-4-4

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https://ww2.mathworks.cn/matlabcentral/answers/1941104-speed-up-a-code-subindex-for-4-d-matrix

回答： Rasmita 2023-4-14

Hi, in my code I need to do this operation,

for a = 1:h
    for b = 1:h
        
        ind=sub2ind(size(I), centriX+a - h/2, centriY+ b - h/2);
        A(:,:,a,b) = (I(ind));%*gamma1; 
        
    end
end

where I is a 2D matrix, "centriX" and "centriY" are 2D matrices that contains the coordinates of some centers that I am interested in.

Is there any way not to use for loops?

I have uploaded the files of "centriX" and "centriY".

Matrix "I" is too heavy, but you can take a random matrix (5200x5200). h=40

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Dyuman Joshi 2023-4-4

It is difficult to comment without the data. Can you copy-paste or attach your data? To attach, save the data as a .mat file and use the paper clip button.

MementoMori 2023-4-4

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Answer 1

Rasmita 2023-4-14

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Hi,

It is my understanding that you are trying to rewrite the above code without using loops. You can use the ‘bsxfun’ function in MATLAB to compute the required indices for all combinations of ‘a’ and ‘b’ in one go, without using loops.

In the first step, you can create the indices ‘a’ and ‘b’ relative to the center point. In the second step, you can use ‘bsxfun’ to add these indices to the corresponding elements of ‘centriX’ and ‘centriY’, resulting in a 4D array of shape ‘(numel(centriX), numel(centriY), h, h)’ containing the indices for all combinations of a and b. Finally, extract the values from matrix ‘I’ using linear indexing, then reshape the result to the desired shape, and assign it to A.

For more information, please refer to the below documentation links:

https://www.mathworks.com/help/matlab/ref/bsxfun.html

https://www.mathworks.com/help/matlab/ref/reshape.html

Hope this helps!

Regards,

Rasmita