Matrix Dimension mis-match problem

I have a very quick question. I am working through a finite difference problem (1D steady-state heat transfer) and am having trouble getting the coefficient matrix A, shown below:

n = length(dL);
A = diag(ones(n-1),-1)-2*eye(n)+diag(ones(n-1),1);

I am getting the error:

Error using  - 
Matrix dimensions must agree.

Error in final (line 85)
A = diag(ones(n-1),-1)-2*eye(n)+diag(ones(n-1),1);

Can someone please tell me where my error is? I feel like this is extremely straight-forward and should be working.

Answer 1

[Star Strider]

You need to create a vector of ones for the off-diagonals, not a matrix of them, so ones has to have 2 arguments in the diag calls:

A = diag(ones(n-1,1),-1)-2*eye(n)+diag(ones(n-1,1),1);

[lsutiger1]

Yep, just figured that out a couple of minutes ago! Thank you for the response.

[Star Strider]

My pleasure!