Setter analogues of `MultibodyTree::get_positions_from_array()` and `MultibodyTree::get_velocities_from_array()`

Question

There do not seem to be analogous setters. If I want to fill in the relevant pieces of an array, is there an interface for doing that?

Answer 1

Three ways of doing this:

1. Using the joint specific APIs as in the example here.
2. For free bodies you can use the plant's API as here.
3. If you know what you are doing, you can play with the state vector as done here.

Answer 2

I added methods to provide exactly this functionality in this PR: https://github.com/RobotLocomotion/drake/pull/9503