Organize the logic to transform given matrix into required:
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Givien[1 2 3
4 5 6
7 8 9]
Reqried[-1 2 3
0 -1 6
0 0 -1]
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Voss
le 23 Jan 2022
Modifié(e) : Voss
le 23 Jan 2022
A = reshape(1:9,3,[]).'
B = triu(A);
B(1:size(A,1)+1:end) = -1
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Voss
le 23 Jan 2022
Modifié(e) : Voss
le 23 Jan 2022
That statement sets the elements along the diagonal of B to be -1.
C = magic(6)
C(1:size(C,1)+1:end)
C(1:size(C,1)+1:end) = 1000
It is using linear indexing, which in MATLAB goes down the columns first. Starting with index 1 (the upper-left), incrementing by one more than the number of rows gives you the index of each element along the diagonal.
C = magic(3)
C(1:end) % all elements by linear index
C([1 5 9]) % diagonal elements
C(1:4:end) % same
You could also use eye(), the identity matrix function:
eye(3)
C(logical(eye(3)))
C(logical(eye(3))) = -1
John D'Errico
le 23 Jan 2022
You could also have done in a slightly simpler way:
A = reshape(1:9,3,[]).'
B = triu(A,1) - eye(size(A))
Thus triu (and tril) with a second argument, allows you to control which diagonal to go to.
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