Dear Rooters,
I performed histogram multiplication by 0.1,
using the member function "Multiply(TF1* h1, Double_t c1 = 1)".
The same operation could be done in two ways as follows.
constant function * constant,
1) 1.0 * 0.1
2) 0.1 * 1.0
The bin contents of histos after both methods are same,
but the errors are not same.
Why?
ROOT Version 3.03/08 was used.
A simple script is listed below.
{
//////////////////////////////////////////////////
//<Prepare two same histogram with errors>
TH1F* h1 = new TH1F("h1","h1",5,0,5);
for(int i=0;i<5;i++) {
h1->SetBinContent(i,i*100.);
}
TH1F* h2=h1->Clone();
h2->SetNameTitle("h2","h2");
h1->SetLineColor(3);
h1->SetFillColor(3);
h2->SetLineColor(2);
h2->SetFillColor(2);
h1->Sumw2();
h2->Sumw2();
//////////////////////////////////////////////////
//<Performe multiplication>
// fuction * constant
// 1) 1 * 0.1
// 2) 0.1 * 1.0
TF1* f1 = new TF1("f1","1.0",0,5);
TF1* f2 = new TF1("f2","0.1",0,5);
h1->Multiply(f1,0.1);
h2->Multiply(f2,1.0);
//////////////////////////////////////////////////
//<Draw hist and show bin error>
h1->Draw("E2");
h2->Draw("E same");
cout << h1->GetBinError(1) << endl;
cout << h2->GetBinError(1) << endl;
}
Best Regards,
Hajime
Hajime Nanjo
International Center for Elementary Particle Physics,
University of Tokyo
E-mail: nanjyo@icepp.s.u-tokyo.ac.jp
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