Subscriber callback function isn't invoked

edit

Two publishers publish array of integers to two different topics (Topic_1 and Topic_2) and a third node subscribes to these two topics.

  ros::Publisher publisher = nh.advertise<std_msgs::Int32MultiArray>("Topic_1",1000);
    std_msgs::Int32MultiArray vec;

      ROS_INFO("Array elements from file [array1.txt] are published to Topic_1:\n");
      ifstream infile("array1.txt");

      while(infile >> temp)
          vec.data.push_back(temp);

      publisher.publish(vec);
      ros::spin();
      return 0;
    }

Here is the code for second publisher

...
...
 ros::Publisher publisher = nh.advertise<std_msgs::Int32MultiArray>("Topic_2",1000);

    std_msgs::Int32MultiArray vec2;
    ROS_INFO("Array elements from file [array2.txt] are published to Topic_2:\n");

  ifstream infile("array2.txt");
  while(infile >> temp)
      vec2.data.push_back(temp);

  publisher.publish(vec2);
  ros::spin();

  return 0;
}

Here is the one for subscriber:

#include "ros/ros.h"
#include<bits/stdc++.h>
#include<std_msgs/Int32MultiArray.h>

void sort();

void callback_1(const std_msgs::Int32MultiArray::ConstPtr& vec1)
{ 
  ROS_INFO("Vector recieved! over Topic_1\n");
}

void callback_2(const std_msgs::Int32MultiArray::ConstPtr& vec2)
{
  ROS_INFO("Vector recieved! over Topic_2\n");
}


int main(int argc, char **argv)
{

  ros::init(argc, argv, "listener");

  ROS_INFO("Reading the vectors from both the topics");

  ros::NodeHandle nh;

  ros::Subscriber sub_1 = nh.subscribe("Topic_1",1000,callback_1);
  ros::Subscriber sub_2 = nh.subscribe("Topic_2",1000,callback_2);

  ros::spin();
  return 0;
}

But ROS_INFO("Vector recieved! over Topic_1\n");statements are not at all executed. I even wrote a launch file to start these three nodes but the above statement is not executed. But when I run rosnode list, all the three nodes are listed and even both the topics are displayed when I run rostopic list What could probably be going wrong?