[ROOT] FPE in TMinuit

From: Jeff Hartnell (jeffrey.hartnell@physics.ox.ac.uk)
Date: Tue Jul 29 2003 - 14:43:59 MEST 

Hi,
I have discovered that TMinuit will cause a Floating Point Exception
(FPE) with a rare combination of numbers.

I am using root 3.05/05 on SuSE 8.2 Linux but I believe it is a generic
problem.

In order to see this FPE you have to set the flag (sigfpe?) that causes
a program to crash when it happens (the safest option!).

Once you have turned on FPE trapping then the macro pasted below will
crash.

Here is the trace from gdb:

Program received signal SIGFPE, Arithmetic exception.
[Switching to Thread 16384 (LWP 22186)]
0x428cdbb4 in TMinuit::mnmigr() () from
/usr/local/root_3_05_05/root/lib/libMinuit.so
(gdb) up
#1  0x428c5c44 in TMinuit::mnexcm(char const*, double*, int, int&) ()
   from /usr/local/root_3_05_05/root/lib/libMinuit.so
(gdb) up
#2  0x428bc5cf in TFitter::ExecuteCommand(char const*, double*, int) ()
   from /usr/local/root_3_05_05/root/lib/libMinuit.so
(gdb)
#3  0x41cb8f94 in TGraph::Fit(TF1*, char const*, char const*, double,
double) ()
   from /usr/local/root_3_05_05/root/lib/libGraf.so
(gdb)

Hope you can fix it!

Thanks,
Jeff.

Here is the macro:
{

//there are two steps to this
//1.) slightly modify the values used to make fit not fpe
//2.) use the values that cause the fpe

  float oddplanes[30] = {0};  
  float oddstrips[30] = {0};
  int oddcnt = 0;

  //set up a particular senario
  oddcnt=4;
  oddplanes[0]=15;
  oddplanes[1]=17;
  oddplanes[2]=19;
  oddplanes[3]=21;

  oddstrips[0]=1;
  oddstrips[1]=3;
  oddstrips[2]=6;
  oddstrips[3]=9;

  TF1* form=new TF1("form","pol1",0,60);

  for (Int_t i=0;i<oddcnt;i++){
    cout<<"oddcnt="<<oddcnt
	<<", plane="<<oddplanes[i]
	<<", strip="<<oddstrips[i]<<endl;

    //hack to make it work
    oddplanes[i]+=0.00001;
    oddstrips[i]+=0.00001;
    cout<<"After hack: oddcnt="<<oddcnt
	<<", plane="<<oddplanes[i]
	<<", strip="<<oddstrips[i]<<endl; 
  }
  TGraph* oddgr=new TGraph(oddcnt,oddplanes,oddstrips);
  cout<<"Created graph"<<endl;
  oddgr->Fit(form);  
  cout<<"Fitted graph"<<endl<<endl;

  //set up the particular senario again
  oddcnt=4;
  oddplanes[0]=15;
  oddplanes[1]=17;
  oddplanes[2]=19;
  oddplanes[3]=21;

  oddstrips[0]=1;
  oddstrips[1]=3;
  oddstrips[2]=6;
  oddstrips[3]=9;

  for (Int_t i=0;i<oddcnt;i++){
    cout<<"oddcnt="<<oddcnt
	<<", plane="<<oddplanes[i]
	<<", strip="<<oddstrips[i]<<endl;
    //oddplanes[i]+=0.0001;
    //oddstrips[i]+=0.0001;
    cout<<"oddcnt="<<oddcnt
	<<", plane="<<oddplanes[i]
	<<", strip="<<oddstrips[i]<<endl; 
  }

  TGraph* oddgr=new TGraph(oddcnt,oddplanes,oddstrips);
  cout<<"Created graph"<<endl;

  cout<<"I will FPE here..."<<endl;

  oddgr->Fit(form);  
  cout<<"Fitted graph"<<endl;

  cout<<"Finished macro"<<endl;
}

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