Topics_quiz: a few questions

[paedusan]

I’ve finished with 100% accuracy the Topics_quiz (yay!). But there are a few outstanding (very simple) questions in my mind:

1- how do I get from rostopic show the size of the msg.ranges array? I’ve used “msg.ranges[360]” because I’ve looked the forum discussions.

2- My robot was either too fast (and crashing), or too slow (and not moving). In the end I’ve got it running all right with move.linear.x += 0.01. How can I calculate the robot speed? What is the unit of this (m/s) ?

3- I interpreted the quiz info as if I had to write two programs, one subscriber and one publisher. That seems neat, but as I didn’t know how to do that, I’ve written a single program. What would be the best way of doing this? Independent programs for publisher and subscriber, or a single one?

Thanks in advance.

[bayodesegun]

Hi @paedusan, welcome to the community! And congratulations on completing the quiz .

I’m assuming you took the Python version of the quiz (if not, please let me know). So…

@paedusan

1- how do I get from rostopic show the size of the msg.ranges array? I’ve used “msg.ranges[360]” because I’ve looked the forum discussions.

Size of the array is 720, representing the scan points on the laser scan device, spread over the front part of the robot.

msg.ranges[360] represents the middle scan point, which is at the front-center of the robot.

Also, you can check the size of the array in code by using len(msg.ranges) (Python).

@paedusan

2- My robot was either too fast (and crashing), or too slow (and not moving). In the end I’ve got it running all right with move.linear.x += 0.01. How can I calculate the robot speed? What is the unit of this (m/s) ?

This is simulated velocity and not in m/s. Speeds btw 0.1-0.5 for both angular and linear should work fine: not too fast or slow (it did for me).

@paedusan

3- I interpreted the quiz info as if I had to write two programs, one subscriber and one publisher. That seems neat, but as I didn’t know how to do that, I’ve written a single program. What would be the best way of doing this? Independent programs for publisher and subscriber, or a single one?

You chose the best option! One program (node) is fine and easier. This a very simple program so two different programs would be overkill IMHO.

This is a good case-in-point that a single node can contain both a subscriber and a publisher.

[paedusan]

Many thanks for the excellent answers!