Treegetval

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.

Author: Olivier Couet
This notebook tutorial was automatically generated with ROOTBOOK-izer (Beta) from the macro found in the ROOT repository on Tuesday, January 17, 2017 at 02:44 PM.

Create a simple ttree with 5 branches



In [1]:

    
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



In [2]:

    
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);









    



The arrays' dimension is 5000

Retrieve variables 5 et 6



In [3]:

    
Double_t *vxs = T->GetVal(5);
Double_t *vxc = T->GetVal(6);

Draw with option goff and generate only one variable



In [4]:

    
T->Draw("x","Run==1","goff");

Retrieve variable 0



In [5]:

    
Double_t *vx  = T->GetVal(0);

Create and draw graphs



In [6]:

    
TGraph *gs = new TGraph(n,vx,vxs);
TGraph *gc = new TGraph(n,vx,vxc);
gs->Draw("ap");
gc->Draw("p");









    



Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1

Draw all canvases



In [7]:

    
gROOT->GetListOfCanvases()->Draw()