How to get laser points in a specific time of a bag

edit

I have some code that get the messages and break after there is something inside, but laser ranges data change every time I execute my code. This happens probably cause I get the msg in different periodo of time. I want set a specific time to get it, I mean get the ranges always at the same time.

This how I'm doing now.

while(ros::ok()) {
    for (i = 0; i < 540; i++) {
      if (!clustersScan.laserMsg.ranges.empty()) {
        RANGES[i] = clustersScan.laserMsg.ranges[i];
      }
    }

    if(!clustersScan.laserMsg.ranges.empty()) break;

    ros::spinOnce();
  }