looping over multiple root files

Hi,

I have 4 root files (in the future n files) that contain TGraphs with the same name, I would like to make some overlayed plots with the contents of the files, my plan is to loop over the first file TGraphs, then get the TGraphs for each on of the other files and then plot them overlayed.

I'm having problem when I open more than one file. Can anybody please give me some suggestions about the correct procedure to do this?

The error message looks something like this R__unzip: error in header
Error: illegal pointer to class object obj 0x0 3 FILE:macros/overlay_performance_plots.C LINE:45

here is a piece of the code I'm using

  TString file0 = "file0.root";
  TString file1 = "file1.root";
  TString file2 = "file2.root";
  TString file3 = "file3.root";

  TFile * f0 = new TFile(file0);
// TFile * f1 = new TFile(file1);
// TFile * f2 = new TFile(file2);
// TFile * f3 = new TFile(file3);

  TIter next(f0->GetListOfKeys());
  TKey *key;
  while ((key=(TKey*)next())) {

    TObject *obj = key->ReadObj();

    if ( obj->IsA()->InheritsFrom( "TGraph" ) ) {

      printf("key: %s \n", key->GetName());
      TMultiGraph *mg = new TMultiGraph();
      mg->SetTitle("Overlayed Plots");

      TObject *obj = key->ReadObj();
      TGraph *g0 = (TGraph*)obj;
      mg->Add(g0);

/// everything fine up to here and when only one file is open, as soon as I open an extra file it complains
      TGraph *g1 = f1->GetListOfKeys()->FindObject(h1->GetName());
      mg->Draw("AC");

    }
 }

Thank you for your help,

Jorge A. Received on Thu May 01 2008 - 12:11:10 CEST