[ROOT] problem with TGraphErrors::GetHistogram() without painting the graph

From: David d'Enterria (denterria@nevis.columbia.edu)
Date: Fri Jan 10 2003 - 22:55:29 MET 

Hello Rooters,

A sequence of CINT commands showing two(?) problems:

// This works:

  const Int_t n = 5;
  Float_t y[n]  = {5.,7.,10.,14.,16.};
  Float_t x[n]  = {5.,7.,9.,13.,15.};
  TGraph *g = new TGraph(n,x,y);
  g->GetHistogram()
(const class TH1F*)0x87365c0 // non-null address. OK, no need of 
g->Draw("AP") after 3.04 release

// although:

  g->GetXaxis()
(const class TAxis*)0x0
  g->GetHistogram()->GetXaxis()
(const class TAxis*)0x8736600

// Is this above a bug or a feature ?

// Let's go on and try now with TGraphErrors

  Float_t ex[n] = {0.01,0.02,0.02,0.01,0.01};
  Float_t ey[n] = {0.,0.,0.,0.,0.};
  TGraphErrors *graph = new TGraphErrors(n,x,y,ex,ey);
  graph->GetXaxis()
(const class TAxis*)0x0
  graph->GetHistogram()
 *** Break *** segmentation violation

Relevant part of back trace from gdb:

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 1024 (LWP 994)]
0x408d2431 in TGraphErrors::ComputeRange () from /usr/root/lib/libGraf.so
(gdb) back
#0  0x408d2431 in TGraphErrors::ComputeRange () from 
/usr/root/lib/libGraf.so
#1  0x408c7a4e in TGraph::GetHistogram () from /usr/root/lib/libGraf.so
#2  0x4091f694 in G__TGraph_GetHistogram_0_3 () from 
/usr/root/lib/libGraf.so
....

Am I doing something wrong here ?

Thanks in advance,

PS: Using ROOT 3.04/02 binaries on a redhat 7.1 machine

-- 
 David D'ENTERRIA                     PostDoctoral Research Scientist
 Nevis Labs                  http://www.nevis.columbia.edu/~denterria
 Columbia University, NY          mailto:denterria@nevis.columbia.edu
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 PHENIX Group (Bldg 510C)                        tel: +1 631 344 7053
 Brookhaven National Lab, Upton, NY 11973        fax: +1 631 344 3253
 http://www.phenix.bnl.gov/~enterria          mailto:denterri@bnl.gov
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