Function readMessages(from ROS toolbox) can't read message type "nmea_msgs/Sentence".

To reproduce this BUG:

>> bag = rosbag('any bag contains nmea sentence')

>> nmea = select(bag,"MessageType","nmea_msgs/Sentence")

>> readMessages(nmea,3)

When using readMessages, no matter which row you try to read, you will get this error:

Reference to non-existent field 'Sentence_'.

Error in ros.BagSelection/copyFromStruct (line 661)

cobj.(fc) = objArray(:);

Error in ros.BagSelection/getClassFromStruct (line 630)

ros1msg{k,1} = obj.copyFromStruct(ros1msg{k,1},msgStructs{k});

Error in ros.BagSelection/readMessages (line 275)

msgs = obj.getClassFromStruct(msgStructs);

Answer 1

[Karthik Reddy Vennapureddy]

Hello Kevin Tarn,

Thanks for reporting the issue. May I know which version of MATLAB are you using? Here are the quick workarounds, which can circumvent this error.

workaround 1: you can try to use 'DataFormat', 'struct' options to read messages from the bag file. I would recommend changing the following line

readMessages(nmea,3)

to

readMessages(nmea, 3, 'DataFormat', 'struct')

This reads messages much faster compared to reading class objects.

workaround 2: If you do not want to use 'DataFormat','struct' options for reading messages, please reach out to Tech Support team.

Thanks,

Karthik Reddy

[Kevin Tarn]

I'm using 2020b.

And actually I make a workaround myself:

1. Open "C:\Program Files\MATLAB\R2020b\toolbox\ros\mlroscpp\+ros\BagSelection.m" in text editor with privilege.
2. Go to line 661: cobj.(fc) = strObj.(fc);
3. Replace it with:

if fc(end)=='_'

cobj.(fc) = strObj.(fc(1:end-1));

else

cobj.(fc) = strObj.(fc);

end

This works fine so far (shrug