I want to make a turtle pursue other turtle. And I also want to use geometry message twist. I dont know what is the problem of my code!!

edit

these are my functions that print the position of my turtles but there is a problem! when i run it the printed positions are same but in fact they are not!

void chatterCallback(const turtlesim::Pose::ConstPtr& msg)
{
 m=x;
 n=y;
 x=msg->x;
 y=msg->y;

  ROS_INFO("I heard: [%f],[%f]", x,y);
}

void chatterCallback1(const turtlesim::Pose::ConstPtr& msg)
{
 a=msg->x;
 b=msg->y;

  ROS_INFO("ey baba: [%f],[%f]",a,b);
}