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

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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/h...

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...

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,