The TF1 class (derived from TFormula) supports 3 kinds of functions:
1- simple formula that can be expressed in one line. The formula
can refer to other formula with parameters. An example is given
in the tutorial "fillrandom".
2- Interpreted C++ functions as illustrated in the macro
myfit.C below.This is the most convenient way for typical
minimization
problems. The function may be multi-dimensional with
many parameters
3- Compiled C++ functions. Note that case2 can call a function
of case3.
Rene Brun
//-----------------macro myfit.C--------------------------------
// This macro gets in memory an histogram from a root file
// and fits a user defined function.
// Note that a user defined function must always be defined
// as in this example:
// - first parameter: array of variables (in this example only
1-dimension)
// - second parameter: array of parameters
// Note also that in case of user defined functions, one must set
// an initial value for each parameter.
Double_t fitf(Double_t *x, Double_t *par)
{
Double_t arg = 0;
if (par[2]) arg = (x[0] - par[1])/par[2];
Double_t fitval = par[0]*TMath::Exp(-0.5*arg*arg);
return fitval;
}
void myfit()
{
TFile *f = new TFile("hsimple.root");
TCanvas *c1 = new TCanvas("c1","the fit canvas",500,400);
TH1F *hpx = (TH1F*)f->Get("hpx");
// Creates a Root function based on function fitf above
TF1 *func = new TF1("fitf",fitf,-2,2,3);
// Sets initial values and parameter names
func->SetParameters(100,0,1);
func->SetParNames("Constant","Mean_value","Sigma");
// Fit histogram in range defined by function
hpx->Fit("fitf","r");
// Gets integral of function between fit limits
printf("Integral of function = %g\n",func->Integral(-2,2));
}