treegetval.C

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/// \file
/// \ingroup tutorial_tree
/// \notebook
/// Illustrates how to retrieve  TTree variables in arrays.
///
/// This example:
///   - creates a simple TTree,
///   - generates TTree variables thanks to the `Draw` method with `goff` option,
///   - retrieves some of them in arrays thanks to `GetVal`,
///   - generates and draw graphs with these arrays.
///
/// The option `goff` in `TTree::Draw` behaves like any other drawing option except
/// that, at the end, no graphics is produced ( `goff`= graphics off). This allows
/// to generate as many TTree variables as needed. All the graphics options
/// (except `para` and `candle`) are limited to four variables only. And `para`
/// and `candle` need at least two variables.
///
/// Note that by default TTree::Draw creates the arrays obtained
/// with GetVal with a length corresponding to the parameter `fEstimate`.
/// By default fEstimate=1000000 and can be modified
/// via TTree::SetEstimate. To keep in memory all the results use:
/// ~~~{.cpp}
///   tree->SetEstimate(-1);
/// ~~~
/// SetEstimate should be called if the expected number of selected rows
/// is greater than 1000000.
///
/// \macro_image
/// \macro_output
/// \macro_code
///
/// \author Olivier Couet

void treegetval() {
   // create a simple TTree with 5 branches
   Int_t run, evt;
   Float_t x,y,z;
   TTree *T = new TTree("T","test friend trees");
   T->Branch("Run",&run,"Run/I");
   T->Branch("Event",&evt,"Event/I");
   T->Branch("x",&x,"x/F");
   T->Branch("y",&y,"y/F");
   T->Branch("z",&z,"z/F");
   TRandom r;
   for (Int_t i=0;i<10000;i++) {
      if (i < 5000) run = 1;
      else          run = 2;
      evt = i;
      x = r.Gaus(10,1);
      y = r.Gaus(20,2);
      z = r.Landau(2,1);
      T->Fill();
   }

   // Draw with option goff and generate seven variables
   Int_t n = T->Draw("x:y:z:Run:Event:sin(x):cos(x)","Run==1","goff");
   printf("The arrays' dimension is %d\n",n);

   // Retrieve variables 0, 5 et 6
   Double_t *vx  = T->GetVal(0);
   Double_t *vxs = T->GetVal(5);
   Double_t *vxc = T->GetVal(6);

   // Create and draw graphs
   TGraph *gs = new TGraph(n,vx,vxs);
   TGraph *gc = new TGraph(n,vx,vxc);
   gs->Draw("ap");
   gc->Draw("p");
}