getting values from a TTree

From: Roger Mason <rmason_at_esd.mun.ca>
Date: Fri, 3 Oct 2008 16:58:42 -0230

Hello,

I'm trying to read values (energy) from a branch of a TTree into an array ( e[i] = (Double_t*)*b_energy->GetEntry(i); ) in the code below:

#include "TFile.h"
#include "TTree.h"
#include "TBrowser.h"
#include "TH2.h"
#include "TRandom.h"
#include "TCanvas.h"

void read_tree_minimal()
{

   static Double_t energy;
   Double_t e[1200];

   TFile *f = new TFile("Ca2Mn46d.topaz.20080506.160756.root");    TTree *tr = (TTree*)f->Get("t");
   TBranch *b_energy = tr->GetBranch("energy");    b_energy->SetAddress(&energy);
   TH1F *henergy = new TH1F("henergy","energy in eV",10,-2143.62,-2143.54);

   Int_t nentries = (Int_t)tr->GetEntries();    printf("There are %i entries\n",(int*)nentries);    for (Int_t i=0;i<nentries;i++) {

      b_energy->GetEntry(i);
      e[i] = (Double_t*)b_energy->GetEntry(i);
      henergy->Fill(energy);

   }

   printf("Energy # %i = %d\n",1,(float*)e[1]);    henergy->Draw();

   return;
}

Trying to de-reference the pointer b_energy in this way does not work:

root [3] .x read_tree_minimal.C
There are 1200 entries
Error: Illegal pointer operation (tovalue) read_tree_minimal.C:23:

(Double_t)8.00000000000000000e+00

*** Interpreter error recovered ***

Can someone please tell me how to do this?

Thanks,
Roger Received on Fri Oct 03 2008 - 21:37:06 CEST