how to put publisher and subscriber in a single Node?

edit

hello, i am writing a node which subscribes to /scan topic make some calculations and according to the calculated values it publishes different messages on the /robotState topic in the callback function.

Here is my code:

#include "ros/ros.h"
#include "sensor_msgs/LaserScan.h"
#include "std_msgs/String.h"
#include <sstream>

void switcherCallback(const sensor_msgs::LaserScan::ConstPtr& msg)
{
    ros::NodeHandle p;
    ros::Publisher state_pub = p.advertise<std_msgs::String>("robotState",1000);
    ros::Rate loop_rate(1);
    while(ros::ok())
    {
    int f=0;
    for(int i=0; i<msg->ranges.size(); i++){
        if(msg->ranges[i]<1.5f){
            f=1;
            break;
        }
    }
    if(f==1){
        std_msgs::String msg;
        std::stringstream ss;
        ss<<"1";
        msg.data = ss.str();
        state_pub.publish(msg);
    }else{
        std_msgs::String msg;
        std::stringstream ss;
        ss<<"0";
        msg.data = ss.str();
        state_pub.publish(msg);
    }
    ros::spinOnce();
    loop_rate.sleep();
    }
}

int main(int argc, char **argv)
{
    ros::init(argc, argv, "switcher");
    ros::NodeHandle n;
    ros::Subscriber sub = n.subscribe<sensor_msgs::LaserScan>("scan", 1000, switcherCallback);
    ros::spin();
    return 0;
}

But whenever i executes the code it shows the following error:

Bus error (core dumped)

i am just wandering why this error is occurred and how can i put both publisher and subscriber in a single node?