Accessing a subscribers data at a different rate

edit

Hello, thanks for any and all responses, but simply:

I am running a control loop that generates and publishes new velocities at 100hz, every-time it calculates a new velocity it needs the current motor shaft position data. However, this position data is subscribed To and arriving at a much higher rate than the control is running.

How can I/best method for accessing the current data from this subscribed to topic at the moment I need it in the control loop?

An example:

ros::Subscriber pointSub;
ros::Subscriber posSub;

float phi = 0.0;     /// a global variable that gets updated 


void controlLoop(const geometry_msgs::Point & currentPoint) {
  //RUNS ONCE!!! triggered by arriving x,y,z point data

  ros::Rate rate(100)
  for(int i = 0 ; i < 100; i++)
  {

      //HERE I WANT THE UPDATED, most recent, PHI VALUE

      ros::spinOnce();
      rate.sleep();
  }

} //END OF control loop

void fastPositionCb(const std_msgs::Float64 currPos) {
  //Gets data published faster than 100hz .... say 1kHz

  phi = CurrPos.data;

}

int main(int argc, char * argv[]) {
  ros::init(argc, argv, "example");
  ros::NodeHandle nh;

  pointSub = nh.subscribe("testPoint", 1, controlLoop);  // happens once
  posSub = nh.subscribe("phiPos", 1, fastPositionCb);   // happens very fast

  // Spin
  ros::spin();
}