CVXR: subsetting certain variables

Question

library("CVXR");
A <- Variable(3,3);
D <- matrix(c(A[1,1],0,0,0,A[2,2],0,0,0,A[3,3]),nrow=3);
A-D

Here, we have a matrix of variables, A (I am not sure if this is how to call the object correctly), and D as a matrix of variables, in which the diagonal consists of the diagonal of A, and everything else is 0.00. I would like to be able to set constraints such as (A-D)%*%x == y, as one would normally do, e.g. A%*%x == y would work fine. However, R does not seem to like A-D -- "s4 object is not subtractable". How to go about this?

Answer 1

I think you can do:

B <- Variable(3,3)

and add the constraints B[1,1] == 0, B[2,2] == 0, B[3,3] == 0, B %*% x == y.