fitcont¶
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*
Author: Rene Brun
This notebook tutorial was automatically generated with ROOTBOOK-izer from the macro found in the ROOT repository on Monday, March 27, 2023 at 09:47 AM.
be sure default is Minuit since we will use gMinuit
In [1]:
TVirtualFitter::SetDefaultFitter("Minuit");
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*/
gMinuit->SetErrorDef(1);
TGraph *gr1 = (TGraph*)gMinuit->Contour(80,0,2);
gr1->SetFillColor(38);
gr1->Draw("lf");
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
Draw all canvases
In [2]:
gROOT->GetListOfCanvases()->Draw()
