Get histograms from two different files and plot on same canvas

[KatR]

I am 100% sure I am making an extremely stupid mistake here. I have a macro that is supposed to get one histogram from each file and plot them on the same canvas. what it actually does is plot them separately:

void compare(){

TFile *f1 = TFile::Open("hfile.root");

TFile *f2 = TFile::Open("PythiaWithTrackSystematicsCombined.root");

TFile *output = new TFile("output.root","Recreate");

gROOT -> cd();

output->cd();

TCanvas *c = new TCanvas("c");

c->cd();

TH1F *pTNominal = new TH1F("pTNominal","pT Nominal",100,50,7000);

if(f1){pTNominal = (TH1F*)f1->Get("h");

}

TH1F *pTSystematics1 = new TH1F("pTSystematics1","pT Systematics Variation 1",100,50,7000);

pTSystematics1 = (TH1F*)f2->Get("h1_central_rjet_pt_TRACKING1");

if(pTNominal){pTNominal->Draw();

std::cout<<"histogram exists"<<endl;}

pTNominal->SetLineColor(kBlue);

std::cout<<"in the loop"<<endl;

pTNominal->Write();

pTSystematics1->Draw("Same");

pTSystematics1->SetLineColor(kRed);

pTSystematics1->Write();

std::cout<<"in second loop"<<endl;

c->Update();
output->Write();
output->Close();

The output is two empty histograms with the name pTSystematics1 and pTNominal, and two filled histograms with the names they had in the original file, i.e. h and h1_central_rjet_pt_TRACKING1. What am I missing? Thanks for any help!

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[yus]

Hi,

here is how I would do this:

TCanvas *c = new TCanvas("c");

TFile *f1 = TFile::Open("hfile.root");
if (!f1 || f1->IsZombie())
{
    printf("Can't open f1!\n");
    return;
}
TH1F* pTNominal = static_cast<TH1F*>(f1->Get("h"));
if (!pTNominal)
{
    printf("Can't find 'h' in f1!\n");
    return;
}
pTNominal->SetDirectory(gROOT);
pTNominal->SetLineColor(kBlue);
pTNominal->Draw("hist"); // without "hist" the points will be drawn, and you seem to want a line, otherwise why set line color?
f1->Close();

TFile *f2 = TFile::Open("PythiaWithTrackSystematicsCombined.root");
if (!f2 || f2->IsZombie())
{
    printf("Can't open f2!\n");
    return;
}
TH1F* pTSystematics1 = static_cast<TH1F*>(f2->Get("h1_central_rjet_pt_TRACKING1"));
if (!pTSystematics1)
{
    printf("Can't find 'h1_central_rjet_pt_TRACKING1' in f2!\n");
    return;
}
pTSystematics1->SetDirectory(gROOT);
pTSystematics1->SetLineColor(kRed);
pTSystematics1->Draw("hist same");
f2->Close();

TFile *output = new TFile("output.root", "Recreate");
pTNominal->Write();
pTSystematics1->Write();
output->Write();
output->Close();
return;

But I don’t have your root files so I can’t test this…

[KatR]

Hi! Thank you, that removes the empty histograms! They still plot in separate histograms though

[yus]

What do you mean by separate histograms? Can you post a screenshot maybe?

[KatR]

Contents of the file and one of the two histograms. It’s plotted, but I’d like both on the same axes in the same plot.

[yus]

If you want to have two superimposed histograms stored in the file you need a different approach. Sorry, I thought you wanted to have these superimposed histograms plotted on screen and to write two separate histograms in the file. You probably want to take a look at the THStack class in this case.