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How to get the diagonal and a subdiagonal in the same matrix?

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Jose
Jose 2014-10-15
评论： Mohammad Abouali 2014-10-15
I'm trying to get matrix that only has the diagonal and the first subdiagonal values only. Say I have a 4x4 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 4x4 matrix with the diagonal but the part I'm hung up on is getting the subdiagonal in the form of a 4x4 matrix, I used
diag(diag(A,1))
and it gave me a 3x3 matrix of the subdiagonal. To conclude, is there a way of getting a subdiagonal in the original matrix dimensions?

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Mohammad Abouali
Mohammad Abouali 2014-10-15
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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