MATLAB: How to get the diagonal and a subdiagonal in the same matrix

diagmatrixmatrix manipulation

I'm trying to get matrix that only has the diagonal and the first subdiagonal values only. Say I have a 4×4 matrix
A=[ 1  2  3  4;
    5  6  7  8;
    9 10 11 12;
   13 14 15 16]
I want to get a matrix
B=[1 2  0  0;
   0 6  7  0;
   0 0 11 12;
   0 0  0 16].
I know using
diag(diag(A))
will give me a 4×4 matrix with the diagonal but the part I'm hung up on is getting the subdiagonal in the form of a 4×4 matrix, I used
diag(diag(A,1))
and it gave me a 3×3 matrix of the subdiagonal. To conclude, is there a way of getting a subdiagonal in the original matrix dimensions?

Best Answer

A=[ 1  2  3  4;
    5  6  7  8;
    9 10 11 12;
   13 14 15 16]
B=triu(A)-triu(A,2)
B =
       1     2     0     0
       0     6     7     0
       0     0    11    12
       0     0     0    16
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