How to get list element from rosparam parameter from the cmd? - ros

Question

I loaded this yaml file:
num_boxes: 1
boxes: [[x: 0.349, y: 0.213, z: 0.117]]
with rosparam load my_config.yaml then I can do rosparam get boxes and get:
- - {x: 0.349}
- {y: 0.213}
- {z: 0.117}
But how can I access only the first list or elements in the second list? I tried boxes[0], boxes(0) and boxes{0} but nothing worked.

Answer 1

This is an old question, but I'm answering it because it's still useful to put an answer here.
Using the example given, boxes is a list of lists (YAML syntax). From the rosparam command line tool, we can't process rosparam get /boxes any further (excepting grep and regex). In common use, we can access the Parameter Server in C++ / Python.
# Python
>>> import rospy
>>> boxes = rospy.get_param("/boxes"); boxes
[[{'x': 0.349}, {'y': 0.213}, {'z': 0.117}]]
# Boxes is a python list/array of list of dicts
>>> boxes[0]
[{'x': 0.349}, {'y': 0.213}, {'z': 0.117}]
>>> boxes[0][0]
{'x': 0.349}
// C++
#include <vector>
#include <ros/ros.h>
// yaml_list: [1, 2]
ros::NodeHandle *nh;
std::vector<int> yaml_list;
double x = 0.0;
int main(int argc, char** argv){
ros::init(argc, argv, "get_params_node");
nh = new ros::NodeHandle("");
nh->getParam("/yaml_list", yaml_list);
x = yaml_list[0];
// ...
return 0;
}
To have deeper data structures, such as vectors of vectors of maps, you have to use the xmlrpcpp (.h/.cpp) interface.
// C++
#include <vector>
#include <map>
#include <string>
#include <ros/ros.h>
#include <xmlrpcpp/XmlRpcValue.h> // catkin component
ros::NodeHandle *nh;
XmlRpc::XmlRpcValue boxes; // std::vector<std::vector<std::map<std::string,double>>>
double x = 0.0;
int i = 0;
double point[3] = {0};
int main(int argc, char** argv){
ros::init(argc, argv, "get_params_node");
nh = new ros::NodeHandle("");
nh->getParam("/boxes", boxes);
if(boxes.getType() == XmlRpc::XmlRpcValue::Type::TypeArray && boxes.size() > 0){
// boxes[0] is a 'TypeArray' aka vector
if(boxes[0].getType() == XmlRpc::XmlRpcValue::Type::TypeArray && boxes[0].size() > 0){
// boxes[0][0] is a 'TypeStruct' aka map
if(boxes[0][0].getType() == XmlRpc::XmlRpcValue::Type::TypeStruct && boxes[0][0].hasMember("x")){
x = double(boxes[0][0]["x"]);
for(XmlRpc::XmlRpcValue::iterator it = boxes[0][0].begin(); it != boxes[0][0].end(); ++it){
point[i++] = double(*it);
}
}
}
}
// ...
return 0;
}
In standard use, the <rosparam> XML tag keeps the same YAML or rosparam command line syntax.