From 4027e860576c29337617d4e6fb5cfe62324ae05a Mon Sep 17 00:00:00 2001
From: Frank Dellaert <dellaert@gmail.com>
Date: Sun, 26 Jan 2025 16:57:37 -0500
Subject: [PATCH] stream operator

---
 gtsam/discrete/DiscreteValues.cpp | 16 ++++++++++++++--
 gtsam/discrete/DiscreteValues.h   |  3 +++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/gtsam/discrete/DiscreteValues.cpp b/gtsam/discrete/DiscreteValues.cpp
index 416dfb888..3c3ed4468 100644
--- a/gtsam/discrete/DiscreteValues.cpp
+++ b/gtsam/discrete/DiscreteValues.cpp
@@ -26,12 +26,24 @@ using std::stringstream;
 
 namespace gtsam {
 
+/* ************************************************************************ */
+static void stream(std::ostream& os, const DiscreteValues& x,
+                   const KeyFormatter& keyFormatter) {
+  for (const auto& kv : x)
+    os << "(" << keyFormatter(kv.first) << ", " << kv.second << ")";
+}
+
+/* ************************************************************************ */
+std::ostream& operator<<(std::ostream& os, const DiscreteValues& x) {
+  stream(os, x, DefaultKeyFormatter);
+  return os;
+}
+
 /* ************************************************************************ */
 void DiscreteValues::print(const string& s,
                            const KeyFormatter& keyFormatter) const {
   cout << s << ": ";
-  for (auto&& kv : *this)
-    cout << "(" << keyFormatter(kv.first) << ", " << kv.second << ")";
+  stream(cout, *this, keyFormatter);
   cout << endl;
 }
 
diff --git a/gtsam/discrete/DiscreteValues.h b/gtsam/discrete/DiscreteValues.h
index fa8a8a846..df4ecdbff 100644
--- a/gtsam/discrete/DiscreteValues.h
+++ b/gtsam/discrete/DiscreteValues.h
@@ -64,6 +64,9 @@ class GTSAM_EXPORT DiscreteValues : public Assignment<Key> {
   /// @name Standard Interface
   /// @{
 
+  /// ostream operator:
+  friend std::ostream& operator<<(std::ostream& os, const DiscreteValues& x);
+
   // insert in base class;
   std::pair<iterator, bool> insert( const value_type& value ){
     return Base::insert(value);