Question

I made a publisher node that publishes Float64MultiArray message type to a topic zones. The script that contains this node constantly changes the .data portion of the message type. So for example, when it runs and I rostopic echo /zones, the .data portion of the message type looks like this [1.234,0.345](the floating point numbers constantly change). Int he terminal. So the publisher node obviously works. I'm testing to make a subscriber node to this topic zone.As seen here below

#!/usr/bin/python3
import rospy
from std_msgs.msg import Float64MultiArray
from rospy.numpy_msg import numpy_msg
import numpy as np

v=Float64MultiArray()

def callback(msg):
    global v
    v=msg


rospy.init_node("array_listener")


rospy.Subscriber("zones",Float64MultiArray, callback)

def hello():
    global v
    print(v.data[0])

hello()

when I run this script, v.data[0] is empty and gives and as a result the script gives back an index error on the terminal. v.data[0] is not supposed to be empty, it should have the first element in .data. I dont know why the subscriber does not seem to get information from the topic. I really need help on this.

Asked by distro on 2022-02-08 17:01:59 UTC

Answer 1

I think you are calling hello before callback.

Try the following.

#!/usr/bin/python3
import rospy
from std_msgs.msg import Float64MultiArray
from rospy.numpy_msg import numpy_msg
import numpy as np

v=Float64MultiArray()

def callback(msg):
    global v
    v=msg
    print(v.data[0]) # added


rospy.init_node("array_listener")


rospy.Subscriber("zones",Float64MultiArray, callback)

def hello():
    global v
    print(v.data[0])

# hello() # to comment
rospy.spin() # added

Asked by miura on 2022-02-08 17:28:09 UTC

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@miura did not work, im also not sure how im calling hello() before callback

Asked by distro on 2022-02-08 18:08:17 UTC

Thank you for trying it. I am sorry that I could not help you.

Asked by miura on 2022-02-09 18:40:30 UTC

@miura Its alright! I found the correct way thanks to someone else, check the answer above! Please don't apologize for trying to help me

Asked by distro on 2022-02-09 21:05:42 UTC

Publisher and Subscriber nodes don't seem to be communicating

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