use auto for map for-loop

1. reserve vector size in DecisionTreeFactor::apply
2. use auto in range-base for-loop to avoid implictly conversion in VectorValues and DecisionTreeFactor. Some format issues are address, too (add spaces).

gtsam/discrete/DecisionTreeFactor.cpp

@@ -94,7 +94,10 @@ namespace gtsam {
     for (Key j : f.keys()) cs[j] = f.cardinality(j);
     // Convert map into keys
     DiscreteKeys keys;
-    for (const std::pair<const Key, size_t>& key : cs) keys.push_back(key);
+    keys.reserve(cs.size());
+    for (const auto& key : cs) {
+      keys.emplace_back(key);
+    }
     // apply operand
     ADT result = ADT::apply(f, op);
     // Make a new factor

gtsam/linear/VectorValues.cpp

@@ -79,7 +79,7 @@ namespace gtsam {
 
   /* ************************************************************************ */
   VectorValues::iterator VectorValues::insert(const std::pair<Key, Vector>& key_value) {
-    std::pair<iterator, bool> result = values_.insert(key_value);
+    const std::pair<iterator, bool> result = values_.insert(key_value);
     if(!result.second)
       throw std::invalid_argument(
       "Requested to insert variable '" + DefaultKeyFormatter(key_value.first)
@@ -344,14 +344,13 @@ namespace gtsam {
   }
 
   /* ************************************************************************ */
-  VectorValues operator*(const double a, const VectorValues &v)
+  VectorValues operator*(const double a, const VectorValues& c) {
-  {
     VectorValues result;
-    for(const VectorValues::KeyValuePair& key_v: v)
+    for (const auto& [key, value] : c)
 #ifdef TBB_GREATER_EQUAL_2020
-      result.values_.emplace(key_v.first, a * key_v.second);
+      result.values_.emplace(key, a * value);
 #else
-      result.values_.insert({key_v.first, a * key_v.second});
+      result.values_.insert({key, a * value});
 #endif
     return result;
   }