numjac on a subset of variables

I want to calculate the numerical Jacobian of my system using numjac.

It works fine with the regular syntax:

[Jac, FAC] = numjac(@(t,y)model(t,y,X,VAR),ttot,ytot',dydt,jac_tol,FAC);

However I don't need the full jacobian, but only the derivatives with respect to some y's. I can calculate the full one then select the needed lines, but it is a waste of computational time. How can I calculate the jacobian only where I need it?

Answer 1

[Vinh Nguyen]

I think you can do that by providing the Jacobian Structure. You can set the Jacobian Structure element 0 or 1. 1 for the derivatives respective to your variables you need.