Subcriber callback within a while loop?

edit

Is it possible for a subscriber to make a callback within a while loop?... if so how then..

In my case i am Subscribing to topic which giving me a position of a robot. I have a array of 8 positions.

double position[8] = {2.0 , 3, 3.0, ...}

And in a while in a loop, I make to robot walk to those position, one after one. i

nt  i = 0
while(i<8)
{
    robotposition = position[i];
   if(real_position == robotposition){
    i++;
   }
}

Here is the real_position, the real position of the robot, which I receive from a callback function.. The problem here is though that this value doesn't get updated when it is in the while loop.. and i don't why.. how do i do that?