NaN unless I print it out?

[unmovingcastle]

Hi Rooters,

I have here a script that counts some doubles.

double count_passed_events_v2(TString file_prefix, int num_jobs){

  TChain *my_chain = new TChain("passTree");

  for(int i=1; i<=num_jobs; i++){

    TString file_name ("");
    file_name.Form(file_prefix + "/run%d/IceFinal_%d_passTree0.root", i ,i);

    my_chain->Add(file_name); 
  }
  
  TTreeReader reader(my_chain);

  TTreeReaderValue<double_t> w1(reader, "event.nicemc::EventSummary.neutrino.path.weight");
  TTreeReaderValue<double_t> w2(reader, "event.nicemc::EventSummary.loop.positionWeight");
  TTreeReaderValue<double_t> w3(reader, "event.nicemc::EventSummary.loop.directionWeight");

  double weighted_passed_event_count;
  while (reader.Next()) {
    weighted_passed_event_count += *w1/(*w2 * *w3); 
  }

  
  delete my_chain;
  return weighted_passed_event_count;
}

As it is, it does not work. I’ve found that it only works when I also tell the script to print out something, anything. For example, if I add
std::cout << "done with " << file_prefix << std::endl;
then the macro runs just fine.

Could someone please tell me why this is the case?
I am fairly new to ROOT and C++ in general, and I don’t really know what to even search for when I google this problem.

ROOT Version: 6.30/02
Platform: Fedora
Compiler: Build for linuxx8664gcc

[yus]

Hi,

you probably have an undefined behaviour somewhere. Meaning that anything may happen, even your script appearing to be working with cout added.

[unmovingcastle]

Hi yus,

Thanks for the help!
Could you give me some examples of undefined behavior? I am not familiar with the concept.

[yus]

This article explains the concept of UB:
https://en.cppreference.com/w/cpp/language/ub