Fitting multiple sub ranges

ROOT
1

Hi,

I’m trying to fit multiple sub ranges, but I just don’t succeed… I"m trying to stick to the example given in the ROOT user guide, but I guess I don’t understand how it’s supposed to work…

The histogram which I’d like to fit is defined in the following way:
TH1F *to_all = new TH1F(“to_all”,“turn on EM_HI all”,80,0.,80.);

Then I’m basically doing the followin (compare to the user guide, page 65/66):

TF1 *t1 = new TF1(“t1”,turnon_func,0,20);
t1->SetParameters(12.0,1.0,0.2); //halfpoint,slope,plateau
TF1 *t2 = new TF1(“t2”,turnon_func,20,40);
t2->SetParameters(25.0,1.0,1.0); //halfpoint,slope,plateau
TF1 *total = new TF1(“total”,turnon_func+turnon_func,5,40,6); <-THIS LINE IS THE PROBLEM

Double_t par[6];
to_all->Fit(“t1”,“R”);
to_all->Fit(“t2”,“R+”);
t1->GetParameters(&par[0]);
t2->GetParameters(&par[3]);
total->SetParameters(par);
to_all->Fit(total,“R+”);

The function I’d like to fit (“turnon_func”) is defined in the following way:
Double_t turnon_func(Double_t *x, Double_t *par)
{
double halfpoint = par[0];
double slope = par[1];
double plateau = par[2];
double offset = 0;

double pt = TMath::Max(x[0],0.000001);

double arg = 0;
arg = (pt - halfpoint)/(TMath::Sqrt(pt)slope);
double fitval = offset+0.5plateau*(1+TMath::Erf(arg));
return fitval;
}

Above I’ve marked the line where I’m running into trouble. I can compile the program, but when running I get the error message that “total” cannot be compiled. Any ideas what I have to change to make it work?

Cheers,
Carsten

2

Carsten,

See the documentation of class TF1, in particular:

…
TF1 objects can reference other TF1 objects (thanks John Odonnell)
of type A or B defined above.This excludes CINT interpreted functions
and compiled functions.
…

Proceed like in tutorial FittingDemo.C

Rene

3

Hi Rene,

I’ve tried to do combine functions as described, however the results are not very encouraging. Maybe I’m doing something wrong, but the resulting fit doesn’t look as expected.

What I want to do is to fit a turn on curve using TMath::ERG function. However now I have an overlap of two turn-on curves, hence I’ll just define a new function which is returns turnon(x,par)+turnon(x,&par[3]) (as described in the FittingDemo.C example). However I don’t see the first turn-on starting and going into saturation, and then the second turn-on kicking in, but just one turn-on curve (which of course doesn’t describe the data). I’ve tried setting a couple of parameters without any success, and finally I set all six parameters hoping that I would get a good fit now. Also without success.

So what’s the secret in doing a good fit? Am I doind something wrong? Do I just have to be lucky by setting the right parameters?

Cheers,
Carsten

4

Hi Carsten,

In your combination function, you actually probably need to take in consideration the range. Instead of

shouldn’t you have something similar to:

if (x<20) return turnon(x,par); else return turnon(x,&(par[3]));

Cheers,
Philippe.