Fixes bug when reporting number of positions/velocities for discrete …

…state context (RobotLocomotion#9524)
ChristopherSweeney · Sep 26, 2018 · 0addfac · 0addfac
1 parent 2fe8fe2
commit 0addfac
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Showing 2 changed files with 9 additions and 2 deletions.
diff --git a/multibody/multibody_tree/multibody_plant/test/multibody_plant_test.cc b/multibody/multibody_tree/multibody_plant/test/multibody_plant_test.cc
@@ -320,6 +320,13 @@ class AcrobotPlantTests : public ::testing::Test {
     discrete_context_->FixInputPort(
         discrete_plant_->get_actuation_input_port().get_index(),
         Vector1<double>(0.0));
+
+    const auto& mbt_context =
+        dynamic_cast<MultibodyTreeContext<double>&>(*discrete_context_);
+    ASSERT_EQ(mbt_context.num_positions(), 2);
+    ASSERT_EQ(mbt_context.num_velocities(), 2);
+    ASSERT_EQ(mbt_context.get_positions().size(), 2);
+    ASSERT_EQ(mbt_context.get_velocities().size(), 2);
   }
 
   // Computes the vector of generalized forces due to gravity.

diff --git a/multibody/multibody_tree/multibody_tree_context.h b/multibody/multibody_tree/multibody_tree_context.h
@@ -98,12 +98,12 @@ class MultibodyTreeContext: public systems::LeafContext<T> {
 
   /// Returns the size of the generalized positions vector.
   int num_positions() const {
-    return this->get_continuous_state().get_generalized_position().size();
+    return topology_.num_positions();
   }
 
   /// Returns the size of the generalized velocities vector.
   int num_velocities() const {
-    return this->get_continuous_state().get_generalized_velocity().size();
+    return topology_.num_velocities();
   }
 
   /// Returns `true` if the state is discrete and `false` if the state is