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How do I insert multiple vectors as diagonals in the same matrix?

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I'm trying to use a forward-difference method to solve a parabolic PDE. I need a matrix with the main diagonal and both diagonals one spot off from the main diagonal to be defined and the rest 0. My current code is this:
v = lambda*ones(1,m-2);
v1 = (1-2*lambda)*ones(1,m-1);
A = diag(v,1);
A = diag(v1,0);
A = diag(v,-1);
lambda is just a scalar. This is what my intuition told me to do, but after each time I do A=diag, it overwrites the previous matrix. I need the vector v to be on the off diagonals -1 and 1, and the vector v1 to be on the main diagonal. This is probably pretty easy but I am just missing it. Any help would be appreciated.

Accepted Answer

Kelly Kearney
Kelly Kearney on 16 Apr 2019
You're close... you just need to add the matrices together:
A = diag(v,1) + diag(v,-1) + diag(v1,0)

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