How to calculate sensor_msgs/LaserScan data ?

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As @loguna have mentioned, but you should notice that the angle increment play a role in it. In @loguna example, the angle_increment is 1 degree, assumed that the measure unit of angle_increment is degree(but it's rad)

@loguna: I have 3 additional questions: The "range" in your example is it the field_range ? range_min or range_max ? I use RViz: How I can set the angle in rad there ? and How I can get these Range: [2.0 2.0 2.0 0.0 0.0 0.0 ....] ?

Its in Field range. So the lidar spins 360 degree, it collects signals and place them into an array in meters. So lets say the Lidar collect a signal per degree (Check with your Lidar manufacturer), your array should contain [360] meters values. To obtain the signal, import the LaserScan from sensor_msgs.msg. Than try printing "LaserScan.ranges".

If you wish to detect obstacle at a specific direction: The way i usually do is place an object in front of the Lidar. Next I set a random value like LaserScan.range[180] to see if the lidar did pick up the object. If it didnt, i change the "value" of LaserScan.range[value] until i get a hit.

N usually 360 degree Lidar is use more towards mapping and not really to detect obstacles at a specific direction. Hence in RVIZ, it will show all the points in 360 degree.