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

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

Answer 1

[the cyclist]

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.