Ah, okay. In ?Tridiagonal it is stated that the tridiagonal matrix can be converted into a regular matrix by applying Array(). Also, in the context of that code, one shouldn’t even fill the rest of the matrices (see article comments), so one should just remove full.

As far as I know, you might consider Matrix(x) to be the replacement for full(x). I am not positive, but I think full was removed in 1.0 in favor of Matrix, which is clearer. To be clear, Array also works, just sharing Matrix as somebody who often wants to convert special structures into regular dense matrices while debugging and stuff.