MATLAB: Creating a diagonalmatrix with diag and add a value to it

Question

hi,

I'm trying to create a diagonal matrix, with two minor diagonals and one major diagonal. I already created the two minor, but I don't know how to create the main one.

Code:

%diagonalmatrix 
D = diag((-1)*k_i,-1) + diag(k_i+k_(i+1)) + diag((-1)*k_i, 1);  

diag((-1)*k_i,-1) and diag((-1)*k_i, 1) already work.

How to change diag(k_i+k_(i+1)) ?

Must I create a new matrix with the addition already included for the major diagonal? How to?

Thanks!

I included the concept in the attachment.

Answer 1

Try this. It shows an example with symbolic variables to show that it creates the correct matrix. You can try it with k as a numeric vector

k = sym('k', [1, 8]);
A = diag(k(2:end-1),1) + diag(k(2:end-1),-1) + diag(k(1:end-1)+k(2:end));

Result

>> A
A =
[ k1 + k2,      k2,       0,       0,       0,       0,       0]
[      k2, k2 + k3,      k3,       0,       0,       0,       0]
[       0,      k3, k3 + k4,      k4,       0,       0,       0]
[       0,       0,      k4, k4 + k5,      k5,       0,       0]
[       0,       0,       0,      k5, k5 + k6,      k6,       0]
[       0,       0,       0,       0,      k6, k6 + k7,      k7]
[       0,       0,       0,       0,       0,      k7, k7 + k8]