Problem to fit a straight line with tminuit

[fabiokopp]

I,m trying to fit a simple straight line (y=ax+b) using the class tminuit. However, the coefficients results are different from that obtained through gnuplot.
In gnuplot, I have obtained:
a=3.0 ;
b=-0.001

in root, I get
a=2.22E-16
b=-3.0

root macro:

#include<iostream>
#include<TMinuit.h>
#include<TApplication.h>
#include"TF1.h"
#include"TF2.h"
#include"TMath.h"


using namespace std;


Float_t x[6],y[6];

Double_t func(float x,float y, Double_t *par)
{
 Double_t value=(   par[1] + par[0]*x    ) ;
 return value;
}

//______________________________________________________________________________
void fcn(Int_t &npar, Double_t *gin, Double_t &f, Double_t *par, Int_t iflag)
{
   const Int_t nbins = 6;
   Int_t i;


   for (i=0;i<nbins; i++) {
     f  = func(x[i],y[i],par) ;
    
 }
    

}
  
void run(){      
// the x values 
        x[0]=0.0;
        x[1]=0.5;
        x[2]=1.0;
        x[3]=1.5;
        x[4]=2.0;
        x[5]=2.5;
// the y values
        y[0]=0.0;
        y[1]=1.5;
        y[2]=3.0;
        y[3]=4.5;
        y[4]=6.0;
	y[5]=7.5;
 
  const int npar=2;
  TMinuit minuit(npar);
  minuit.SetFCN(fcn);

  Double_t arglist[2];
  Int_t ierflg = 0;

  double par[npar];
  double stepSize[npar];
  double minVal[npar];
  double maxVal[npar];
  string parName[npar];

  par[0]=2.0;
  stepSize[0]=0.01;
  minVal[0]=0.0;
  maxVal[0]=4.0;
  parName[0]="x";

  par[1]=-2.0;
  stepSize[1]=0.01;
  minVal[1]=-3.0;
  maxVal[1]=3.0;
  parName[1]="y";


   arglist[0] = 500.0;
   arglist[1] = 1.0;
   minuit.mnexcm("MIGRAD", arglist ,1,ierflg);


for(int i=0; i<npar; i++){
   minuit.DefineParameter(i, parName[i].c_str(),
   par[i],stepSize[i],minVal[i],maxVal[i]);
}

minuit.Migrad();
double outpar[npar],err[npar];
for(int i=0; i<npar; i++){
   minuit.GetParameter(i,outpar[i],err[i]);
   cout<<"Fitted parameter is:" << i << 
"   "<<outpar[i]<<"  "<< "+/-" <<  
"   " << err[i] <<endl;
 
}

}

[Wile_E_Coyote]

See: ${ROOTSYS}/tutorials/fit/Ifit.C
Try: [code]Double_t func(Double_t x, Double_t *par) { return (par[0] * x + par[1]); }

void fcn(Int_t &npar, Double_t *gin, Double_t &f, Double_t *par, Int_t iflag)
{
const Int_t nbins = sizeof(x) / sizeof(x[0]);
f = 0.0;
for (Int_t i = 0; i < nbins; i++) {
Double_t dy = y[i] - func(x[i], par);
f += dy * dy;
}
}[/code]

[fabiokopp]

Thank you, Wile. You solved my simple problem.