ignore rate calculate when last time is zero

first time when self.get_last_printed_tn is zero, the rate value is not correct, just ignore when last time is zero.

Signed-off-by: Chris Ye <[email protected]>

ros2topic/ros2topic/verb/hz.py

@@ -214,10 +214,13 @@ def get_hz(self, topic=None):
         """
         if not self.get_times(topic=topic):
             return
+        elif self.get_last_printed_tn(topic=topic) == 0:
+            self.set_last_printed_tn(self.get_msg_tn(topic=topic), topic=topic)
+            return
         elif self.get_msg_tn(topic=topic) < self.get_last_printed_tn(topic=topic) + 1:
             return
         with self.lock:
-            # frequency
+            # Get frequency every one minute
             n = len(self.get_times(topic=topic))
             mean = sum(self.get_times(topic=topic)) / n
             rate = 1. / mean if mean > 0. else 0