Assign values to arrow marker starting position

edit

Hello,

I am trying to publish an arrow marker and want to display this arrow in rviz. I want the starting point of the arrow to be at a point of a sensor_msgs/PointCloud message. When I am defining a starting and end point of the arrow marker like the following, it works:

visualization_msgs::Marker markers_msg;
markers_msg.points.resize(2);
markers_msg.points[0].x = 0.0f;
markers_msg.points[0].y = 0.0f;
markers_msg.points[0].z = 0.0f;
markers_msg.points[1].x = 1.0f;
markers_msg.points[1].y = 1.0f;
markers_msg.points[1].z = 1.0f;

Now I am trying to assign a specific point of the point cloud to be the starting point of the arrow marker, but it is not working. When I am echoing the cloud.points[] message values are output.

sensor_msgs::PointCloud cloud;
visualization_msgs::Marker markers_msg;
markers_msg.points.resize(2);
markers_msg.points[0].x = cloud.points[...].x;
markers_msg.points[0].y = cloud.points[...].y;
markers_msg.points[0].z = cloud.points[...].z;
markers_msg.points[1].x = 1.0f;
markers_msg.points[1].y = 1.0f;
markers_msg.points[1].z = 1.0f;

I get the error message: "Segmentation fault (core dumped)"

Can someone may help me?

Thank you very much in advance!

Kind regards,

Raphael