How to use diag() once and change values after certain element?

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arthurk
arthurk 2020-2-24
编辑： the cyclist 2020-2-24
For example:
for i = 1:3
matrix(i,i) = diag(2);
matrix(i,i+1) = diag(1);
matrix(i+1,i) = diag(-1);
end
matrix(4,4) = 2;
This outputs
2 1 0 0
-1 2 1 0
0 -1 2 1
0 0 -1 2
So a simpler way of doing the above, use diag() to give a n x n matrix (n being something you choose to be), where the 0th diagonal has 2s, the 1st diagonal has 1s and the -1st diagonal has -1s
For the latter half of my question.
Can you tell where diag() to stop at ?
Basically do diag(2,0) until cell (2,2) and then after that do diag(1,0) for the same matrix. So end result is:
2 0 0 0
0 2 0 0
0 0 1 0
0 0 0 1

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the cyclist
the cyclist 2020-2-24
编辑：the cyclist 2020-2-24
A simple way to construct the exact matrix in your first part is
matrix = 2*eye(4) + diag([-1 -1 -1],-1) + diag([1 1 1],1)
How you want to generalize this will depend on the specifics of your problem.
I think the blkdiag command will be helpful for the second part.

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