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

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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

Accepted Answer

Matt J
Matt J on 23 Oct 2023
Edited: Matt J on 23 Oct 2023
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
  2 Comments
julian gaviria
julian gaviria on 23 Oct 2023
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
Matt J on 23 Oct 2023
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

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