rostopic echo of a field of an array in ROS Indigo

asked 2015-11-04 08:50:35 -0600

157 ●14 ●20 ●23 http://www.roboprotos.com

Hi, I have an issue trying to display a field of an array in a message in ROS Indigo with the command rostopic echo. I'm trying to display the position of an object detected with the Ork package, using this command :

rostopic echo /recognized_object_array/objects[0]/pose/pose/pose/position

But I got this traceback :

Traceback (most recent call last):
  File "/opt/ros/indigo/bin/rostopic", line 35, in <module>
    rostopic.rostopicmain()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rostopic/__init__.py", line 1765, in rostopicmain
    _rostopic_cmd_echo(argv)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rostopic/__init__.py", line 1136, in _rostopic_cmd_echo
    _rostopic_echo(topic, callback_echo, bag_file=options.bag)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rostopic/__init__.py", line 788, in _rostopic_echo
    index = submsg_class.__slots__.index(field)
AttributeError: 'NoneType' object has no attribute '__slots__'

But it works if I only do :

rostopic echo /recognized_object_array/objects[0]

This is just an example, I have the same issue with other custom messages using arrays. Is there a specific syntax to display the field of an element of an array ?

answered 2016-02-15 08:28:56 -0600

peci1

1366 ●27 ●69 ●60

It should work in Indigo now (it didn't use to work in older Indigo releases, but now I tested it and it works).

The

AttributeError: 'NoneType' object has no attribute '__slots__'

means you reference an invalid field in your topic path definition. Make sure you correctly type in all parts of the path and "dereference" all arrays.

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Comments

It works for me in Indigo:

rostopic echo /namespace/ball_tracking/ball_states/ball_states[0]/position

However if I would like to visualize that in rviz as pointstamped it does not work: Error subscribing: Character [[] at element [49] is not valid in Graph... Any ideas how to sole this?

Markus ( 2018-05-07 02:18:12 -0600 )edit

Is your topic .../ball_states[0]/position of type PointStamped and is element 49 continuously published ? Just a guess, if you track a lot of balls, it's likely that your array has a different size each frame and so element 49 might not always exist. However I would suggest to open a new question.

Cyril Jourdan ( 2018-05-07 03:17:22 -0600 )edit

hm no the problem is that I am not allowed to used these brackets [ in rviz as topic name. And I do not know how to select the first element of an array otherwise ..... :( thats the problem here.

Markus ( 2018-05-07 03:47:12 -0600 )edit

Ok I see, you could to write a simple node that does the translation to a MarkerArray of spheres which is resized dynamically and display your balls in rviz ?

Cyril Jourdan ( 2018-05-07 04:29:25 -0600 )edit

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