Logo ROOT   6.12/07
Reference Guide
fitcont.C File Reference

Detailed Description

View in nbviewer Open in SWAN Example illustrating how to draw the n-sigma contour of a Minuit fit.

To get the n-sigma contour the ERRDEF parameter in Minuit has to set to n^2. The fcn function has to be set before the routine is called.

WARNING!!! This test works only with TMinuit

The TGraph object is created via the interpreter. The user must cast it to a TGraph*

pict1_fitcont.C.png
pict2_fitcont.C.png
Processing /mnt/build/workspace/root-makedoc-v612/rootspi/rdoc/src/v6-12-00-patches/tutorials/fit/fitcont.C...
FCN=96.6163 FROM MIGRAD STATUS=CONVERGED 62 CALLS 63 TOTAL
EDM=2.07594e-08 STRATEGY= 1 ERROR MATRIX ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 Constant 1.43454e+02 2.29666e+00 9.00732e-03 -6.49594e-05
2 Mean 1.60911e-02 1.30402e-02 6.28520e-05 3.57224e-03
3 Sigma 9.87683e-01 9.45637e-03 1.24479e-05 2.14428e-02
#include "TMinuit.h"
void fitcont()
{
//be sure default is Minuit since we will use gMinuit
TCanvas *c1 = new TCanvas("c1");
TH1F *h = new TH1F("h","My histogram",100,-3,3);
h->FillRandom("gaus",6000);
h->Fit("gaus");
c1->Update();
TCanvas *c2 = new TCanvas("c2","contours",10,10,600,800);
c2->Divide(1,2);
c2->cd(1);
/*get first contour for parameter 1 versus parameter 2*/
TGraph *gr12 = (TGraph*)gMinuit->Contour(40,1,2);
gr12->Draw("alp");
c2->cd(2);
/*Get contour for parameter 0 versus parameter 2 for ERRDEF=2*/
gMinuit->SetErrorDef(4); //note 4 and not 2!
TGraph *gr2 = (TGraph*)gMinuit->Contour(80,0,2);
gr2->SetFillColor(42);
gr2->Draw("alf");
/*Get contour for parameter 0 versus parameter 2 for ERRDEF=1*/
TGraph *gr1 = (TGraph*)gMinuit->Contour(80,0,2);
gr1->SetFillColor(38);
gr1->Draw("lf");
}
Author
Rene Brun

Definition in file fitcont.C.