Detailed Description

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:

tree->SetEstimate(-1);

SetEstimate should be called if the expected number of selected rows is greater than 1000000.

Processing /mnt/build/workspace/root-makedoc-v608/rootspi/rdoc/src/v6-08-00-patches/tutorials/tree/treegetval.C...
The arrays' dimension is 5000

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 5 et 6
   Double_t *vxs = T->GetVal(5);
   Double_t *vxc = T->GetVal(6);
   // Draw with option goff and generate only one variable
   T->Draw("x","Run==1","goff");
   // Retrieve variable 0
   Double_t *vx  = T->GetVal(0);
   // Create and draw graphs
   TGraph *gs = new TGraph(n,vx,vxs);
   TGraph *gc = new TGraph(n,vx,vxc);
   gs->Draw("ap");
   gc->Draw("p");
}

Author: Olivier Couet

Definition in file treegetval.C.