#include "iostream"
#include "TGraphErrors.h"
#include "TH1.h"
#include "TF1.h"
#include "TLegend.h"
#include "TCanvas.h"
#include "TMinuit.h"

const Int_t n = 5;
const Float_t y[n]  = {-4.017, -2.742, -1.1478, 1.491, 6.873};
const Float_t x[n]  = {22000, 22930, 23880, 25130, 26390};
const Float_t ex[n] = {440, 470, 500, 530, 540};
const Float_t ey[n] = {0.5, 0.25, 0.08, 0.09, 1.9};

Float_t a1ForEffVar;
Float_t a2ForEffVar;

void fcn(Int_t &npar, Double_t *gin, Double_t &f, Double_t *par, Int_t iflag) {
  f = chisquare(par[0], par[1]);
}

double chisquare(Double_t alpha1, Double_t alpha2) {
  Double_t chisq = 0.;
  for (int i = 0; i < n; i++) {
    chisq += 1/aproxEffVariance(i)*pow(y[i]-alpha1*x[i]-alpha2/x[i],2);
  }
  return chisq;
}

double aproxEffVariance(Int_t i) {
  return pow(-a1ForEffVar+a2ForEffVar/pow(x[i],2),2)*pow(ex[i],2)+pow(ey[i],2);
}



void simpleEx() {

  a1ForEffVar = 0;
  a2ForEffVar = 0;

  for (int itNbr = 0; itNbr < 8; itNbr++) {
    //MINIMIZING Chi2 Using MINUIT
    TMinuit *gMinuit = new TMinuit(5);
    gMinuit->SetFCN(fcn);
    Double_t arglist[10];
    Int_t ierflg = 0;
    arglist[0] = 1;
    gMinuit->mnexcm("SET ERR", arglist ,1,ierflg);  
    static Double_t vstart[2] = {0.00098993, -589322};
    static Double_t step[2] = {0.0000001 , 22803.5};
    gMinuit->mnparm(0, "alpha1", vstart[0], step[0], 0, 0, ierflg);
    gMinuit->mnparm(1, "alpha2", vstart[1], step[1], 0, 0, ierflg);
    arglist[0] = 500;
    arglist[1] = 1.;
    gMinuit->mnexcm("MIGRAD", arglist, 2, ierflg);
    Double_t amin,edm,errdef;
    Int_t nvpar,nparx,icstat;
    gMinuit->mnstat(amin,edm,errdef,nvpar,nparx,icstat);
    gMinuit->mnprin(3,amin);
    
    
    //Try to get the result (alpha1 and alpha2) from minuit 
    Double_t params[6];
    gMinuit->GetParameters(&params[3]);    // <--- Gives: Error: Can't call TMinuit::GetParameters(&params[3])
    cout<<"params[0] "<<params[0]<<" params[1] "<<params[1]<<endl;

  }


 
}