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

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

Answer 1

[Matt J]

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

[julian gaviria]

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]

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