How to interpret and use the values of the message type 'sensor_msgs/LaserScan'?

Question

Hi,

I am using the following package: stage_ros

http://wiki.ros.org/stage_ros

and the node stageros. I want to understand the topic published 'base_scan' that has a message type of 'sensor_msgs/LaserScan'

http://docs.ros.org/api/sensor_msgs/html/msg/LaserScan.html

That message has a field 'float32[] ranges' but it doesn't specify the length of the array. I found the following pdf

http://www.cim.mcgill.ca/~yiannis/417/2011/COMP-417Fall2011Assignment1.pdf

which demostrates how to iterate the array, calculating the following indexes:

unsigned int minIndex = ceil((MIN_SCAN_ANGLE_RAD - msg->angle_min) / msg->angle_increment);
unsigned int maxIndex = ceil((MAX_SCAN_ANGLE_RAD - msg->angle_min) / msg->angle_increment);

They are using the angle_min and the angle_increment to calculate the indexes that later they will use to iterate over the array. I don't understand what those operations are doing but the final values are: minIndex = 501 and maxIndex = 580 which I think are the values that are in front of the robot.

Can someone explain this to me please?

Best,

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Originally posted by andrestoga on ROS Answers with karma: 188 on 2015-04-01

Post score: 0

Answer 1

In C++, variable-length ROS message types are implemented as std::vector (source). So you should be able to do

for (int i = 0; i < msg->ranges.size(); i++)
  float r = msg->ranges[i]; 

or

for (std::vector<float>::iterator it = msg->ranges.begin(); it != ms->ranges.end(); ++it)
  float r = *it;

Disclaimer: I haven't actually tried running this exact code.

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Originally posted by Angus with karma: 438 on 2015-04-02

This answer was ACCEPTED on the original site

Post score: 4