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How to publish and subscribe to the list consisting of many tuples?

asked 2022-05-12 01:04:14 -0600

Redhwan gravatar image

updated 2022-05-12 01:33:18 -0600

I tried to publish and subscribe to the list of tuples. Unfortunately, I can't.

My code for slave or talker is:

#!/usr/bin/env python

import rospy
from std_msgs.msg import Float32MultiArray

def talker():
    pub = rospy.Publisher('chatter', Float32MultiArray, queue_size=1)
    rospy.init_node('talker', anonymous=True)
    rate = rospy.Rate(10) # 10hz
    while not rospy.is_shutdown():
        hello_str = [(360, 196, 558, 460), (184, 183, 342, 477)]
        array = Float32MultiArray(data=hello_str)
        # print 'gggggggggggggggggggggggg', type(array)
        rospy.loginfo(array)
        pub.publish(array)
        rate.sleep()

if __name__ == '__main__':
    try:
        talker()
    except rospy.ROSInterruptException:
        pass

The output is :

[INFO] [1652334923.612873]: layout: 
  dim: []
  data_offset: 0
data: 
  - [360, 196, 558, 460]
  - [184, 183, 342, 477]
[INFO] [1652334923.713173]: layout: 
  dim: []
  data_offset: 0
data: 
  - [360, 196, 558, 460]
  - [184, 183, 342, 477]
[INFO] [1652334923.813342]: layout: 
  dim: []
  data_offset: 0
data: 
  - [360, 196, 558, 460]
  - [184, 183, 342, 477]
[INFO] [1652334923.913261]: layout: 
  dim: []
  data_offset: 0
data: 
  - [360, 196, 558, 460]
  - [184, 183, 342, 477]
[INFO] [1652334924.013226]: layout: 
  dim: []
  data_offset: 0
data: 
  - [360, 196, 558, 460]
  - [184, 183, 342, 477]

I need the output will be:

data : [(360, 196, 558, 460), (184, 183, 342, 477)]

The code for slave or listener is:

#!/usr/bin/env python

import rospy
from std_msgs.msg import Float32MultiArray

def callback(data):
    # rospy.loginfo(data.data)
    a = list (data.data)
    print (a)

def listener():
    rospy.init_node('listener', anonymous=True)

    rospy.Subscriber("chatter", Float32MultiArray, callback)

    rospy.spin()

if __name__ == '__main__':
    listener()

No output for listener code but when I ran it the talker code stopped and give me this message:

Traceback (most recent call last):
  File "/home/redhwan/learn1.py", line 20, in <module>
    talker()
  File "/home/redhwan/learn1.py", line 15, in talker
    pub.publish(array)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py", line 886, in publish
    raise ROSSerializationException(str(e))
rospy.exceptions.ROSSerializationException: <class 'struct.error'>: 'required argument is not a float' when writing '0'

Thank you in advance for your help.

Note: If the list like [1.1,2,3,4,5]. It is working fine.

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2 Answers

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answered 2022-05-12 10:33:08 -0600

Mike Scheutzow gravatar image

A Float32MultiArray is a ros message that wraps a multi-dimensional array. In python, given what your publisher sent, your subscriber callback msg contains a two-dimensional numpy array. If you want the values organized in some other fashion, you need to write code to read the array and create whatever it is you want.

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First of all, thank you so much for your answer. I know what you meant but I think that ROS is not designed to publish and subscribe to everything in Python for example dictionary. I am not sure about my case in this question. So, I posted my problem here asking to help.

Redhwan gravatar image Redhwan  ( 2022-05-12 19:40:01 -0600 )edit
0

answered 2022-05-12 01:51:58 -0600

ljaniec gravatar image

updated 2022-05-12 01:56:18 -0600

You want to use a list of tuples of ints, not a float32 list - the message type is incorrect. You should create a custom message for this.

Relevant questions with some hints about use of tuples and lists in ROS messages:

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Asked: 2022-05-12 01:04:14 -0600

Seen: 713 times

Last updated: May 12 '22