Only sample if not provided

gtsam/discrete/DiscreteBayesNet.cpp

@@ -58,7 +58,12 @@ DiscreteValues DiscreteBayesNet::sample(DiscreteValues result) const {
   // sample each node in turn in topological sort order (parents first)
   for (auto it = std::make_reverse_iterator(end());
        it != std::make_reverse_iterator(begin()); ++it) {
-    (*it)->sampleInPlace(&result);
+    const DiscreteConditional::shared_ptr& conditional = *it;
+    // Sample the conditional only if value for j not already in result
+    const Key j = conditional->firstFrontalKey();
+    if (result.count(j) == 0) {
+      conditional->sampleInPlace(&result);
+    }
   }
   return result;
 }