How to update fit parameters?

ROOT
1

Hello!

I want to fit my data by nonlinear function, so I must define start parameters.
For example:

Double_t fitFunction(Double_t *x,Double_t *par) 
{ 
	//just for example
	return Exp(x[0]*par[0]) + par[1] ;
}

void Fit ()
{
vector<double> xv;
vector<double> yv;

//fill vectors ...

TGraph * gr = new TGraph(xv.size(), &xv[0], &yv[0]);
TF1 *fitFcn = new TF1("fitFcn", fitFunction, 0, 1, 2);

fitFcn->SetParameter(0, 0.5);
fitFcn->SetParLimits(0, 0, 1);

fitFcn->SetParameter(1, 0.5);
fitFcn->SetParLimits(1, 0, 1);

gr->Fit("fitFcn", "R");
gr->Draw("AP");
}

My fit is bad, so I want to define new start parameters.
I try to do this in the following manner:

void Fit ()
{
// old code 
vector<double> xv;
vector<double> yv;

//fill vectors ...

TGraph * gr = new TGraph(xv.size(), &xv[0], &yv[0]);
TF1 *fitFcn = new TF1("fitFcn", fitFunction, 0, 1, 2);

fitFcn->SetParameter(0, 0.5);
fitFcn->SetParLimits(0, 0, 1);

fitFcn->SetParameter(1, 0.5);
fitFcn->SetParLimits(1, 0, 1);

gr->Fit("fitFcn", "R");

//new code
if( fitFcn->GetChisquare() / fitFcn->GetNDF() > 1)
{
	fitFcn->SetParameter(0, 0.2);//new start value
	fitFcn->SetParLimits(0, 0, 1);

	fitFcn->SetParameter(1, 0.8);////new start value
	fitFcn->SetParLimits(1, 0, 1);

	gr->Fit("fitFcn", "R");
}

gr->Draw("AP");
}

Unfortunately, this method does not work and my fit the same.
But I don’t know for sure: my new code is wrong or my code is right and minimiser found the same minimum.

How to define new parameters properly?

2

Hi,

the procedure is correct. What is the output of the minimiser in the two cases?