Fit Gaus

ROOT
1

Hi

I’m trying to fit two Gaussfunctions and an Offset to a set of data (Stern-Gerlach experiment). The thing is ROOT just fits a constant. Currently I’m just doing a regular set of data, that looks like a gauss. If I fit a “gaus” it works fine, but when I try to fit “([0]/([1]sqrt(2pi)))*exp(-0.5 * ( (x-[1])/([0]) )^2)” it just does the line… I’ve attached a listing:

[code]#include “pref.h”

double findmin(const std::vector &v)
{
double min = 1900000000;
for (size_t i = 0; i < v.size(); i++)
{
min = v[i] < min ? v[i] : min;
}
return min;
}

double findmax(const std::vector &v)
{
double max = -10000000;
for (size_t i = 0; i < v.size(); i++)
{
max = v[i] > max ? v[i] : max;
}
return max;
}

int main(int argc, char** argv)
{
TApplication *t = new TApplication(“big”,&argc,argv);
char *fname = “c.csv”;
TCanvas *c = new TCanvas(fname,fname,1280,720);
//opening File
std::ifstream in;
in.open(fname);
std::string line;
std::vector sk, inten;

while(getline(in,line))
{
	try
	{
		std::stringstream sstr;
		sstr << line;
		double _sk, _inten;
		sstr >> _sk >> _inten;
		sk.push_back(_sk);
		inten.push_back(_inten);
	}catch(...){
		std::cerr << "Error reading file: "<< fname << std::endl;
	}
}
gStyle->SetOptStat("emr");
gStyle->SetOptFit(11);
size_t size = sk.size();
double min = findmin(sk);
double max = findmax(sk);
std::cout << "min: " << min << " max: " << max << "\n";
TH1F *h = new TH1F("dist", ";sk in 0.1\"; Intensity in pA", size, min, max);
//TCanvas *c = new TCanvas(fname,fname,1280,720);
for (size_t i = 0; i < size ; i++)
{
	h->SetBinContent(i, inten[i]);
	h->SetBinError(i,1);
}
//Fit
TF1 *fitfun = new TF1("fitfun", "([0]/([1]*sqrt(2*pi)))*exp(-0.5 * ( (x-[1])/([0]) )^2)", min, max); // 
h->Fit(fitfun,"");
h->Draw("E0");
fitfun->Draw("SAME");
c->Update();
c->Print("foo.png");
t->Run();
return 0;

}
[/code]

Could anyone tell me what I’m doing wrong?

2

Try:

TF1 *fitfun = new TF1("fitfun", "([0]/([1]*sqrt(2*pi)))*exp(-0.5 * ( (x-[1])/([0]) )^2)", min, max); fitfun->SetParameter(0, ((max - min) / 4.0)); // guess the peak's width fitfun->SetParameter(1, ((max + min) / 2.0)); // guess the peak's position h->Fit(fitfun,"");
Search for “Setting initial conditions” in http://root.cern.ch/root/html/TH1.html#TH1:Fit

3

Now I get a diagonal line :slight_smile: .
The thing the makes me wonder is, that in the Fit Panel it shows the function as “([0]/([1]sqrt(2pi)))exp(-(0.5sq((x-[1])/[0])))” and that is not the function I entered. I’ve read something about TF1 and it offers optimization… maybe thats it? Is there a way to turn it off?

Thanks for the help so far.

4

Try to set better initial “guess” values of the parameters (peak’s “position” and “width”).
Also, maybe try to “swap” parameters:

TF1 *fitfun = new TF1("fitfun", "([1]/([0]*sqrt(2*pi)))*exp(-0.5 * ( (x-[0])/([1]) )^2)", min, max); fitfun->SetParameter(0, ((max + min) / 2.0)); // guess the peak's position fitfun->SetParameter(1, ((max - min) / 10.0)); // guess the peak's width