Remove nroll argument from rollout

doc/changelog.rst

@@ -15,6 +15,7 @@ Python bindings
 - Provide prebuilt wheels for Python 3.13.
 - Added ``bind`` method and removed id attribute from :ref:`mjSpec` objects. Using ids is error prone in scenarios of repeated attachment and
   detachment. Python users are encouraged to use names for unique identification of model elements.
+- Removed ``nroll`` argument from :ref:`rollout<PyRollout>` because its value can always be inferred.
 
 Bug fixes
 ^^^^^^^^^

python/mujoco/rollout.cc

@@ -39,7 +39,6 @@ Roll out open-loop trajectories from initial states, get resulting states and se
   input arguments (required):
     model              instance of MjModel
     data               associated instance of MjData
-    nroll              integer, number of initial states from which to roll out trajectories
     nstep              integer, number of steps to be taken for each trajectory
     control_spec       specification of controls, ncontrol = mj_stateSize(m, control_spec)
     state0             (nroll x nstate) nroll initial state vectors,
@@ -190,7 +189,7 @@ PYBIND11_MODULE(_rollout, pymodule) {
   pymodule.def(
       "rollout",
       [](const MjModelWrapper& m, MjDataWrapper& d,
-         int nroll, int nstep, unsigned int control_spec,
+         int nstep, unsigned int control_spec,
          const PyCArray state0,
          std::optional<const PyCArray> warmstart0,
          std::optional<const PyCArray> control,
@@ -201,13 +200,14 @@ PYBIND11_MODULE(_rollout, pymodule) {
         raw::MjData* data = d.get();
 
         // check that some steps need to be taken, return if not
-        if (nroll < 1 || nstep < 1) {
+        if (nstep < 1) {
           return;
         }
 
         // get sizes
         int nstate = mj_stateSize(model, mjSTATE_FULLPHYSICS);
         int ncontrol = mj_stateSize(model, control_spec);
+        int nroll = state0.shape(0);
 
         // get raw pointers
         mjtNum* state0_ptr = get_array_ptr(state0, "state0", nroll, 1, nstate);
@@ -232,7 +232,6 @@ PYBIND11_MODULE(_rollout, pymodule) {
       },
       py::arg("model"),
       py::arg("data"),
-      py::arg("nroll"),
       py::arg("nstep"),
       py::arg("control_spec"),
       py::arg("state0"),

python/mujoco/rollout.py

@@ -29,7 +29,6 @@ def rollout(model: mujoco.MjModel,
             *,  # require subsequent arguments to be named
             control_spec: int = mujoco.mjtState.mjSTATE_CTRL.value,
             skip_checks: bool = False,
-            nroll: Optional[int] = None,
             nstep: Optional[int] = None,
             initial_warmstart: Optional[npt.ArrayLike] = None,
             state: Optional[npt.ArrayLike] = None,
@@ -50,7 +49,6 @@ def rollout(model: mujoco.MjModel,
       ([nroll or 1] x [nstep or 1] x ncontrol)
     control_spec: mjtState specification of control vectors.
     skip_checks: Whether to skip internal shape and type checks.
-    nroll: Number of rollouts (inferred if unspecified).
     nstep: Number of steps in rollouts (inferred if unspecified).
     initial_warmstart: Initial qfrc_warmstart array (optional).
       ([nroll or 1] x nv)
@@ -74,7 +72,7 @@ def rollout(model: mujoco.MjModel,
   #   don't allocate output arrays
   #   just call rollout and return
   if skip_checks:
-    _rollout.rollout(model, data, nroll, nstep, control_spec, initial_state,
+    _rollout.rollout(model, data, nstep, control_spec, initial_state,
                      initial_warmstart, control, state, sensordata)
     return state, sensordata
 
@@ -83,8 +81,6 @@ def rollout(model: mujoco.MjModel,
     raise ValueError('control_spec can only contain bits in mjSTATE_USER')
 
   # check types
-  if nroll and not isinstance(nroll, int):
-    raise ValueError('nroll must be an integer')
   if nstep and not isinstance(nstep, int):
     raise ValueError('nstep must be an integer')
   _check_must_be_numeric(
@@ -121,7 +117,7 @@ def rollout(model: mujoco.MjModel,
   _check_trailing_dimension(model.nsensordata, sensordata=sensordata)
 
   # infer nroll, check for incompatibilities
-  nroll = _infer_dimension(0, nroll or 1,
+  nroll = _infer_dimension(0, 1,
                            initial_state=initial_state,
                            initial_warmstart=initial_warmstart,
                            control=control,
@@ -146,7 +142,7 @@ def rollout(model: mujoco.MjModel,
     sensordata = np.empty((nroll, nstep, model.nsensordata))
 
   # call rollout
-  _rollout.rollout(model, data, nroll, nstep, control_spec, initial_state,
+  _rollout.rollout(model, data, nstep, control_spec, initial_state,
                    initial_warmstart, control, state, sensordata)
 
   # return outputs

python/mujoco/rollout_test.py

@@ -192,6 +192,110 @@ def test_multi_step(self, model_name):
     np.testing.assert_array_equal(state, py_state)
     np.testing.assert_array_equal(sensordata, py_sensordata)
 
+  @parameterized.parameters(ALL_MODELS.keys())
+  def test_infer_nroll_initial_state(self, model_name):
+    model = mujoco.MjModel.from_xml_string(ALL_MODELS[model_name])
+    nstate = mujoco.mj_stateSize(model, mujoco.mjtState.mjSTATE_FULLPHYSICS)
+    data = mujoco.MjData(model)
+
+    nroll = 5  # number of rollouts
+    nstep = 1  # number of steps
+
+    initial_state = np.random.randn(nroll, nstate)
+    control = np.random.randn(nstep, model.nu)
+    state, sensordata = rollout.rollout(model, data, initial_state, control)
+
+    mujoco.mj_resetData(model, data)
+    control = np.tile(control, (nroll, 1, 1))
+    py_state, py_sensordata = py_rollout(model, data, initial_state, control)
+    np.testing.assert_array_equal(state, py_state)
+    np.testing.assert_array_equal(sensordata, py_sensordata)
+
+  @parameterized.parameters(ALL_MODELS.keys())
+  def test_infer_nroll_control(self, model_name):
+    model = mujoco.MjModel.from_xml_string(ALL_MODELS[model_name])
+    nstate = mujoco.mj_stateSize(model, mujoco.mjtState.mjSTATE_FULLPHYSICS)
+    data = mujoco.MjData(model)
+
+    nroll = 5  # number of rollouts
+    nstep = 1  # number of steps
+
+    initial_state = np.random.randn(nstate)
+    control = np.random.randn(nroll, nstep, model.nu)
+    state, sensordata = rollout.rollout(model, data, initial_state, control)
+
+    mujoco.mj_resetData(model, data)
+    initial_state = np.tile(initial_state, (nroll, 1))
+    py_state, py_sensordata = py_rollout(model, data, initial_state, control)
+    np.testing.assert_array_equal(state, py_state)
+    np.testing.assert_array_equal(sensordata, py_sensordata)
+
+  @parameterized.parameters(ALL_MODELS.keys())
+  def test_infer_nroll_warmstart(self, model_name):
+    model = mujoco.MjModel.from_xml_string(ALL_MODELS[model_name])
+    nstate = mujoco.mj_stateSize(model, mujoco.mjtState.mjSTATE_FULLPHYSICS)
+    data = mujoco.MjData(model)
+
+    nroll = 5  # number of rollouts
+    nstep = 1  # number of steps
+
+    initial_state = np.random.randn(nstate)
+    control = np.random.randn(nstep, model.nu)
+    initial_warmstart = np.tile(data.qacc_warmstart.copy(), (nroll, 1))
+    state, sensordata = rollout.rollout(model, data, initial_state, control,
+                                        initial_warmstart=initial_warmstart)
+
+    mujoco.mj_resetData(model, data)
+    initial_state = np.tile(initial_state, (nroll, 1))
+    control = np.tile(control, (nroll, 1, 1))
+    py_state, py_sensordata = py_rollout(model, data, initial_state, control)
+    np.testing.assert_array_equal(state, py_state)
+    np.testing.assert_array_equal(sensordata, py_sensordata)
+
+  @parameterized.parameters(ALL_MODELS.keys())
+  def test_infer_nroll_state(self, model_name):
+    model = mujoco.MjModel.from_xml_string(ALL_MODELS[model_name])
+    nstate = mujoco.mj_stateSize(model, mujoco.mjtState.mjSTATE_FULLPHYSICS)
+    data = mujoco.MjData(model)
+
+    nroll = 5  # number of rollouts
+    nstep = 1  # number of steps
+
+    initial_state = np.random.randn(nstate)
+    control = np.random.randn(nstep, model.nu)
+    state = np.empty((nroll, nstep, nstate))
+    state, sensordata = rollout.rollout(model, data, initial_state, control,
+                                        state=state)
+
+    mujoco.mj_resetData(model, data)
+    initial_state = np.tile(initial_state, (nroll, 1))
+    control = np.tile(control, (nroll, 1, 1))
+    py_state, py_sensordata = py_rollout(model, data, initial_state, control)
+    np.testing.assert_array_equal(state, py_state)
+    np.testing.assert_array_equal(sensordata, py_sensordata)
+
+  @parameterized.parameters(ALL_MODELS.keys())
+  def test_infer_nroll_sensordata(self, model_name):
+    model = mujoco.MjModel.from_xml_string(ALL_MODELS[model_name])
+    nstate = mujoco.mj_stateSize(model, mujoco.mjtState.mjSTATE_FULLPHYSICS)
+    data = mujoco.MjData(model)
+
+    nroll = 5  # number of rollouts
+    nstep = 1  # number of steps
+
+    initial_state = np.random.randn(nstate)
+    control = np.random.randn(nstep, model.nu)
+    sensordata = np.empty((nroll, nstep, model.nsensordata))
+    state, sensordata = rollout.rollout(model, data, initial_state, control,
+                                        sensordata=sensordata)
+
+    mujoco.mj_resetData(model, data)
+    initial_state = np.tile(initial_state, (nroll, 1))
+    control = np.tile(control, (nroll, 1, 1))
+    py_state, py_sensordata = py_rollout(model, data, initial_state, control)
+    np.testing.assert_array_equal(state, py_state)
+    np.testing.assert_array_equal(sensordata, py_sensordata)
+
   @parameterized.parameters(ALL_MODELS.keys())
   def test_one_rollout_fixed_ctrl(self, model_name):
     model = mujoco.MjModel.from_xml_string(ALL_MODELS[model_name])
@@ -328,7 +432,7 @@ def thread_initializer():
 
     def call_rollout(initial_state, control, state, sensordata):
       rollout.rollout(model, thread_local.data, initial_state, control,
-                      skip_checks=True, nroll=initial_state.shape[0],
+                      skip_checks=True,
                       nstep=nstep, state=state, sensordata=sensordata)
 
     n = nroll // num_workers  # integer division