To mirror (i.e, to copy) the upper matrix to the lower half in matrices arranged in 3D

Question

julian gaviria 2023-10-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2037361-to-mirror-i-e-to-copy-the-upper-matrix-to-the-lower-half-in-matrices-arranged-in-3d

评论： Matt J 2023-10-23

M.mat

These two lines mirror (i.e, copy) the upper matrix to the lower half in matrix M, and replace the diagonal elements of the same M with NaN values

M2=triu(M)+triu(M,1)';
%2. To replace the diagonal elements of A with NaN values
M2(eye(size(M2))==1) = nan;

How can I implement them in a 3D array? (e.g., M_3D=374x374x5). The code below does not work.

ns = size(M_3D,3);
M_3D_ = M_3D;
for s = 1:ns
    thisM = M_3D(:,:,s);
    thisM = triu(M_3D(:,:,s))+triu(M_3D,1)';
    M_3D_(:,:,s) = thisM;
    
    M3D_(eye(size(M_3D_(:,:,s)))==1) = nan;
end

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Answer 1

Matt J 2023-10-23

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2037361-to-mirror-i-e-to-copy-the-upper-matrix-to-the-lower-half-in-matrices-arranged-in-3d#answer_1339026

编辑：Matt J 2023-10-23

在 MATLAB Online 中打开

M_3D=randi(100, 4,4,3); %fake input
N=size(M_3D,1);
mask=triu(ones(N)); 
mask(1:N+1:end)=nan;
M_3D=M_3D.*mask;
M_3D=M_3D+pagetranspose(M_3D)
M_3D = 
M_3D(:,:,1) =

   NaN   100    71    68
   100   NaN    68    46
    71    68   NaN    65
    68    46    65   NaN


M_3D(:,:,2) =

   NaN    68    99    23
    68   NaN    50    29
    99    50   NaN    45
    23    29    45   NaN


M_3D(:,:,3) =

   NaN    73    46     4
    73   NaN    57     2
    46    57   NaN    56
     4     2    56   NaN

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julian gaviria 2023-10-23

在 MATLAB Online 中打开

It nicely works. Thanks @Matt J

One last question. Do you mind to describe what you did in

mask(1:N+1:end)=nan;
M_3D=M_3D.*mask;
M_3D=M_3D+pagetranspose(M_3D)

Matt J 2023-10-23

在 MATLAB Online 中打开

It may help to look at the mask,

N=8;
mask=triu(ones(N)); 
mask(1:N+1:end)=nan
mask = 8×8
   NaN     1     1     1     1     1     1     1
     0   NaN     1     1     1     1     1     1
     0     0   NaN     1     1     1     1     1
     0     0     0   NaN     1     1     1     1
     0     0     0     0   NaN     1     1     1
     0     0     0     0     0   NaN     1     1
     0     0     0     0     0     0   NaN     1
     0     0     0     0     0     0     0   NaN