add comments

gtsam/discrete/TableFactor.cpp

@@ -252,6 +252,11 @@ DecisionTreeFactor TableFactor::operator*(const DecisionTreeFactor& f) const {
 DecisionTreeFactor TableFactor::toDecisionTreeFactor() const {
   DiscreteKeys dkeys = discreteKeys();
 
+  // Record key assignment and value pairs in pair_table.
+  // The assignments are stored in descending order of keys so that the order of
+  // the values matches what is expected by a DecisionTree.
+  // This is why we reverse the keys and then
+  // query for the key value/assignment.
   DiscreteKeys rdkeys(dkeys.rbegin(), dkeys.rend());
   std::vector<std::pair<uint64_t, double>> pair_table;
   for (auto i = 0; i < sparse_table_.size(); i++) {
@@ -265,13 +270,16 @@ DecisionTreeFactor TableFactor::toDecisionTreeFactor() const {
     pair_table.push_back(std::make_pair(k, sparse_table_.coeff(i)));
   }
 
-  // Sort based on key assignment so we get values in reverse key order.
+  // Sort the pair_table (of assignment-value pairs) based on assignment so we
+  // get values in reverse key order.
   std::sort(
       pair_table.begin(), pair_table.end(),
       [](const std::pair<uint64_t, double>& a,
          const std::pair<uint64_t, double>& b) { return a.first < b.first; });
 
-  // Create the table vector
+  // Create the table vector by extracting the values from pair_table.
+  // The pair_table has already been sorted in the desired order,
+  // so the values will be in descending key order.
   std::vector<double> table;
   std::for_each(pair_table.begin(), pair_table.end(),
                 [&table](const std::pair<uint64_t, double>& pair) {