How to draw min/max value from ntuple variables?

ROOT
1

Hi,

I’m quite a new ROOT user, trying to get to grips with it still, so sorry if this question is a bit basic.

I have an ntuple root file with x, y, t variables, with multiple values of t for specific x, y coordinates. I would like to draw this to a 3D graph where only the minimum value of t per coordinate is drawn, ignoring all other values of t.

Is there any way to do this? I’m not sure where to even start.

Thanks in advance!

2

Hi,

let’s divide the problem in steps:

  1. Reading a TTree
  2. Getting the maximum of a collection
  3. Plotting values in a 3D graphic

Now, ROOT can help you in each of the steps!

  1. See root.cern.ch/doc/master/hsimpleReader_8C.html and modify according to your needs.
  2. This one depends on how you stored the t values. In general, the STL library offers a way to get the min element of a collection en.cppreference.com/w/cpp/algorithm/min_element
  3. Here you find the ROOT implementation of a 3D graph: root.cern.ch/doc/master/graph2d … it_8C.html

Cheers,
Danilo

3

Thanks for the reply!

I stupidly failed to mention that I’m forced to run ROOT version 5 as I’m using a windows machine, which doesn’t have TTreeReader. Is there an equivalent for ver 5?

4
5 
{

   Float_t x,y,t;
   TTree *T = new TTree("T","test tree");
   T->Branch("x",&x,"x/F");
   T->Branch("y",&y,"y/F");
   T->Branch("t",&t,"t/F");
   TRandom r;
   for (Int_t i=0;i<5000;i++) {
      x = r.Gaus(10,1);
      y = r.Gaus(20,2);
      if (i<100) t = -100;
      else t = r.Landau(2,1);
      T->Fill();
   }

   Int_t n = T->Draw("t","","goff");

   Double_t *vt = T->GetVal(0);
   Double_t min = 99999;
   for (int i=0; i<n;i++)  {if (vt[i] < min) min = vt[i];}
   T->SetMarkerStyle(20);
   T->Draw("x:y:t", Form("t==%f",min));
}