Fix confusion between parents and frontals

gtsam/discrete/DiscreteConditional.h

@@ -90,6 +90,22 @@ public:
   /// GTSAM-style equals
   bool equals(const DiscreteFactor& other, double tol = 1e-9) const;
 
+  /// @}
+  /// @name Parent keys are stored *first* in a DiscreteConditional, so re-jigger:
+  /// @{
+
+  /** Iterator pointing to first frontal key. */
+  typename DecisionTreeFactor::const_iterator beginFrontals() const { return endParents(); }
+
+  /** Iterator pointing past the last frontal key. */
+  typename DecisionTreeFactor::const_iterator endFrontals() const { return end(); }
+
+  /** Iterator pointing to the first parent key. */
+  typename DecisionTreeFactor::const_iterator beginParents() const { return begin(); }
+
+  /** Iterator pointing past the last parent key. */
+  typename DecisionTreeFactor::const_iterator endParents() const { return end() - nrFrontals_; }
+
   /// @}
   /// @name Standard Interface
   /// @{

gtsam/discrete/tests/testDiscreteBayesNet.cpp

@@ -18,8 +18,10 @@
 
 #include <gtsam/discrete/DiscreteBayesNet.h>
 #include <gtsam/discrete/DiscreteFactorGraph.h>
-#include <gtsam/base/Testable.h>
+#include <gtsam/discrete/DiscreteMarginals.h>
 #include <gtsam/base/debug.h>
+#include <gtsam/base/Testable.h>
+#include <gtsam/base/Vector.h>
 
 #include <CppUnitLite/TestHarness.h>
 
@@ -29,10 +31,42 @@
 using namespace boost::assign;
 
 #include <iostream>
+#include <string>
+#include <vector>
 
 using namespace std;
 using namespace gtsam;
 
+/* ************************************************************************* */
+TEST(DiscreteBayesNet, bayesNet) {
+  DiscreteBayesNet bayesNet;
+  DiscreteKey Parent(0, 2), Child(1, 2);
+
+  auto prior = boost::make_shared<DiscreteConditional>(Parent % "6/4");
+  CHECK(assert_equal(Potentials::ADT({Parent}, "0.6 0.4"),
+                     (Potentials::ADT)*prior));
+  bayesNet.push_back(prior);
+
+  auto conditional =
+      boost::make_shared<DiscreteConditional>(Child | Parent = "7/3 8/2");
+  EXPECT_LONGS_EQUAL(1, *(conditional->beginFrontals()));
+  Potentials::ADT expected(Child & Parent, "0.7 0.8 0.3 0.2");
+  CHECK(assert_equal(expected, (Potentials::ADT)*conditional));
+  bayesNet.push_back(conditional);
+
+  DiscreteFactorGraph fg(bayesNet);
+  LONGS_EQUAL(2, fg.back()->size());
+
+  // Check the marginals
+  const double expectedMarginal[2]{0.4, 0.6 * 0.3 + 0.4 * 0.2};
+  DiscreteMarginals marginals(fg);
+  for (size_t j = 0; j < 2; j++) {
+    Vector FT = marginals.marginalProbabilities(DiscreteKey(j, 2));
+    EXPECT_DOUBLES_EQUAL(expectedMarginal[j], FT[1], 1e-3);
+    EXPECT_DOUBLES_EQUAL(FT[0], 1.0 - FT[1], 1e-9);
+  }
+}
+
 /* ************************************************************************* */
 TEST(DiscreteBayesNet, Asia)
 {

gtsam/discrete/tests/testDiscreteConditional.cpp

@@ -36,6 +36,11 @@ TEST( DiscreteConditional, constructors)
   DiscreteConditional::shared_ptr expected1 = //
       boost::make_shared<DiscreteConditional>(X | Y = "1/1 2/3 1/4");
   EXPECT(expected1);
+  EXPECT_LONGS_EQUAL(0, *(expected1->beginFrontals()));
+  EXPECT_LONGS_EQUAL(2, *(expected1->beginParents()));
+  EXPECT(expected1->endParents() == expected1->beginFrontals());
+  EXPECT(expected1->endFrontals() == expected1->end());
+  
   DecisionTreeFactor f1(X & Y, "0.5 0.4 0.2 0.5 0.6 0.8");
   DiscreteConditional actual1(1, f1);
   EXPECT(assert_equal(*expected1, actual1, 1e-9));