Real-time TTree reading

ROOT
1

Dear ROOTers,

I guess it must have been discussed somewhere, but I have troubles finding. How to access TTrees in a file that are still being written? I guess I can make a loop which will be periodically calling GetEntries() and checking if the number has changed (still, I don’t know if it can change, or do I have to close and reopen the TTree?), but maybe there are better methods, including check if the TFile was closed or not, etc.

2

From the TTree doc

   //   How to write a Tree in one process and view it from another process
   //   ===================================================================
   //   The following two scripts illustrate how to do this.
   //   The script treew.C is executed by process1, treer.C by process2
   //
   //   ----- script treew.C
   //   void treew() {
   //     TFile f("test.root","recreate");
   //     TNtuple *ntuple = new TNtuple("ntuple","Demo","px:py:pz:random:i");
   //     Float_t px, py, pz;
   //     for ( Int_t i=0; i<10000000; i++) {
   //        gRandom->Rannor(px,py);
   //        pz = px*px + py*py;
   //        Float_t random = gRandom->Rndm(1);
   //        ntuple->Fill(px,py,pz,random,i);
   //        if (i%1000 == 1) ntuple->AutoSave("SaveSelf");
   //     }
   //   }
   //
   //   ----- script treer.C
   //   void treer() {
   //      TFile f("test.root");
   //      TTree *ntuple = (TTree*)f.Get("ntuple");
   //      TCanvas c1;
   //      Int_t first = 0;
   //      while(1) {
   //         if (first == 0) ntuple->Draw("px>>hpx", "","",10000000,first);
   //         else            ntuple->Draw("px>>+hpx","","",10000000,first);
   //         first = (Int_t)ntuple->GetEntries();
   //         c1.Update();
   //         gSystem->Sleep(1000); //sleep 1 second
   //         ntuple->Refresh();
   //      }
   //   }

Cheers,
Philippe.

3

See also [Read a tfile while writing with another process

4

Thanks! That was what I needed.