How to subscribe to an OccupancyGrid map and publish a converted array from it? [closed]

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Hello everyone I'm trying to set up a frontier-navigation system in Gazebo.Now I am able to build the map using SLAM. Now I have come to the stage that I need to determine the frontier grids in the map and calculate some distances and utilities.I have noticed that the OccupancyGrid message holds only a 1 dimension array as stated in OG msg

In order to do further processing,I need to convert the 1 dimention data into a 2 dimention array(exactly like the real-time map). Now I'm trying to write a subscriber and publisher node.As I followed the tutorials I have written the following codes for simple test but it just doesn't work.

#include "ros/ros.h"
#include "std_msgs/String.h"
#include <sstream>
#include "nav_msgs/OccupancyGrid.h"

void mapConvert(const nav_msgs::OccupancyGrid::ConstPtr& msg)
{
  ROS_INFO("I heard: [%d]",msg->info.width);
  map_pub.publish(msg->info.width);
}

int main(int argc, char **argv)
{
  ros::init(argc, argv, "map_converter");
  ros::NodeHandle n;
  ros::Subscriber sub = n.subscribe("map", 1000, mapConvert);
  ros::Publisher map_pub = n.advertise<int>("mapconverted", 1000);
  ros::Rate loop_rate(10);

 int count = 0;
  while (ros::ok())
  {
    ros::spinOnce();
    loop_rate.sleep();
  }
  ros::spin();
  return 0;
}

and when I use catkin_make it shows

  /home/edmond/rosws/src/myrobot_gazebo/src/mapconvert.cpp: In function ‘void mapConvert(const ConstPtr&)’:
  /home/edmond/rosws/src/myrobot_gazebo/src/mapconvert.cpp:9:7: error: ‘map_pub’ was not declared in this scope
         map_pub.publish(msg->info.width);
         ^
  In file included from /opt/ros/indigo/include/ros/serialization.h:37:0,
                   from /opt/ros/indigo/include/ros/publisher.h:34,
                   from /opt/ros/indigo/include/ros/node_handle.h:32,
                   from /opt/ros/indigo/include/ros/ros.h:45,
                   from /home/edmond/rosws/src/myrobot_gazebo/src/mapconvert.cpp:1:
  /opt/ros/indigo/include/ros/message_traits.h: In instantiation of ‘static const char* ros::message_traits::MD5Sum<M>::value() [with M = int]’:
  /opt/ros/indigo/include/ros/message_traits.h:228:103:   required from ‘const char* ros::message_traits::md5sum() [with M = int]’
  /opt/ros/indigo/include/ros/advertise_options.h:92:40:   required from ‘void ros::AdvertiseOptions::init(const string&, uint32_t, const SubscriberStatusCallback&, const SubscriberStatusCallback&) [with M = int; std::string = std::basic_string<char>; uint32_t = unsigned int; ros::SubscriberStatusCallback = boost::function<void(const ros::SingleSubscriberPublisher&)>]’
  /opt/ros/indigo/include/ros/node_handle.h:239:7:   required from ‘ros::Publisher ros::NodeHandle::advertise(const string&, uint32_t, bool) [with M = int; std::string = std::basic_string<char>; uint32_t = unsigned int]’
  /home/edmond/rosws/src/myrobot_gazebo/src/mapconvert.cpp:17:69:   required from here
  /opt/ros/indigo/include/ros/message_traits.h:121:29: error: ‘__s_getMD5Sum’ is not a member of ‘int’
       return M::__s_getMD5Sum().c_str();
                               ^
  /opt/ros/indigo/include/ros/message_traits.h: In instantiation of ‘static const char* ros::message_traits::DataType<M>::value() [with M = int]’:
  /opt/ros/indigo/include/ros/message_traits.h:237:105:   required from ‘const char* ros::message_traits::datatype() [with M = int]’
  /opt/ros/indigo/include/ros/advertise_options.h:93:44:   required from ‘void ros::AdvertiseOptions::init(const string&, uint32_t, const ...