Subscribe to a custom message and publish in the same node

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Hi all,

I have looked in the forum for days trying to find a solution to my problem, but I can't find it. I'm trying to use a custom message that I have created in order to subscribe to that message, do some computation on it and then publish a standard message.

My custom message is called TractionData.msg and is:

Header header
int8 state
int8 mode
int16 speed
int16 vel_l
int16 vel_r
int16 throttle
int16 brake
int8 reverse_flag
int32 odo_travel

My node is based on this tutorial and is:

#include <ros/ros.h>
#include <tf/transform_broadcaster.h>
#include <nav_msgs/Odometry.h>
#include "std_msgs/String.h"
#include <furbot_msgs/TractionData.h>

furbot_msgs::TractionData msg;
msg.vel_l = 0;
msg.vel_r = 0;

void odometryCallback(const std_msgs::String::ConstPtr& msg)
{
    msg.vel_r = *msg->data.vel_r;
    msg.vel_l = *msg->data.vel_l;
    ROS_INFO("Rear wheels angular speed is: [%s]RL [%s]RR", msg->data.vel_l, msg->data.vel_r);
}

int main(int argc, char** argv){
    ros::init(argc, argv, "odometry_publisher");

    ros::NodeHandle n;
    ros::Publisher odom_pub = n.advertise<nav_msgs::Odometry>("odom", 50);
    tf::TransformBroadcaster odom_broadcaster;

    //getting traction data subscribing to TractionData message
    ros::Subscriber sub = n.subscribe("TractionData", 1000, odometryCallback);

    //furbot specification
    double radius = 0.3; //wheel radius
    double L = 0.6425; //distance between wheel and origin of robot frame
    double pi = 3.1415926535;


    //v = 2πr × RPM × (60/1000) km/hr

    double x = 0.0;
    double y = 0.0;
    double th = 0.0;

    double vx = 0.1;
    double vy = -0.1;
    double vth = 0.1;


    //assuming furbot works in rad for angles

    ros::Time current_time, last_time;
    current_time = ros::Time::now();
    last_time = ros::Time::now();

    ros::Rate r(1.0);
    while(n.ok()){

    ros::spinOnce();               // check for incoming messages
    current_time = ros::Time::now();

    /*
    //compute odometry in a typical way given the velocities of the robot
    double dt = (current_time - last_time).toSec();
    double delta_x = (vx * cos(th) - vy * sin(th)) * dt;
    double delta_y = (vx * sin(th) + vy * cos(th)) * dt;
    double delta_th = vth * dt;
    */

    //converting rear speeds in linear velocities (from RPM to m/s)
    // m/s = 60*RPM/(4*pi^2*r)
    double vel_r_lin = (60*msg.vel_r)/(4*pi*pi*radius);
    double vel_l_lin = (60*msg.vel_l)/(4*pi*pi*radius);

    //compute v and w using matlab formulas (challenge emaro days)
    double v = (vel_r_lin + vel_l_lin)/2;
    double w = (vel_l_lin - vel_r_lin)/L;

    /*
    //furbot [v, w] computation
    double v = th * r; //phi1*r/2 + phi2*r/2
    double w = th * r / L;
    */

    //furbot odometry equations
    double dt = (current_time - last_time).toSec();
    double x_new = x + v*dt*cos(th);
    double y_new = y + v*dt*sin(th);
    double th_new = th + w*dt;

    x = x_new;
    y = y_new;
    th = th_new;

    //since all odometry is 6DOF we'll need a quaternion created from yaw
    geometry_msgs::Quaternion odom_quat = tf::createQuaternionMsgFromYaw(th);

    //first, we'll publish the transform over tf
    geometry_msgs::TransformStamped odom_trans;
    odom_trans.header.stamp = current_time;
    odom_trans.header.frame_id = "odom";
    odom_trans.child_frame_id = "base_link";

    odom_trans.transform.translation.x = x;
    odom_trans.transform.translation.y = y;
    odom_trans.transform.translation.z = 0.0;
    odom_trans.transform.rotation = odom_quat;

    //send the transform
    odom_broadcaster ...