Accessing a set of histograms in Roofit

Dear all,

I have a Roofit job that used to read unbinned data in the following way:
RooDataSet data = RooDataSet::read(filename, mass);
where filename is an ASCII file containing the variable mass, and I fit the unbinned distribution of this variable.
filename is declared in the following way:
void MassFit(const char
filename = “mass.dat”, const char* plotOpt = “NEU”)

Now since I have now a huge quantity of events, I would like to do an unbinned fit of the histogram of this variable. Actually I have a set of such histograms in a root file such as for instance mass.root, containing histograms “mass cond 1”, “mass cond 2”, etc…

I suppose I can still access the root file in the same way:
void MassFit(const char* filename = “mass.root”, const char* plotOpt = “NEU”)
But then I am lost. I basically know that I should use RooDataHist instead of RooDataSet, but from what I found in this forum or on various tutorials I could not figure out how to do that.

After that, if I understood correctly, the fit procedure is exactly the same. Is that right?

Can you help?

Thanks and best regards,


Hi Elizabeth

I presume you want to do a binned fit starting from ROOT histograms. In this case you need to create a RooDataHist object. There is a constructor for RooDataHist taking an histogram or a list of histograms.
See the reference documentation

Also this slide below as example.

Best Regards


Dear Lorenzo,
Thank you for your reply.
It is basically what I have done, but I think that the problem comes from the way I access the histograms, namely this statement

myHist[0] = (TH1D*)(filename->FindObjectAny(histogramName)); // get histo with name histogramName

which used to work in another case.

The diagnostic is:
histogramName Standard Z mass
Error: non class,struct,union object filename used with . or ->
*** Interpreter error recovered ***

Here after is the very short code, tried in accessing one single histogram for the moment.

Thanks and best regards,


/** \macro

using namespace std;
using namespace RooFit;

void MassFit(const char* filename = “DYJetsM50_BuildEoPhis.root”, const char* plotOpt = “NEU”) {

Float_t massMin(75), massMax(105);
RooRealVar mass(“mass”,“M(e^{+}e^{-})”, massMin, massMax,“GeV/c^{2}”);
Int_t nBins(150);

TH1D *myHist[50];
char hist = new char[50];
std::ostringstream os1;
os1 << “Standard Z mass”;
TString histogramName = os1.str(); // build histo namefor individual histo access
cout << " histogramName " << histogramName << endl;
myHist[0] = (TH1D
)(filename->FindObjectAny(histogramName)); // get histo with name histogramName

// Create a datahist that imports contents of all TH1 mapped by index category c
RooDataHist dh(“mass”,“mass”,mass,Import(*myHist[0])) ;

// RooPlot* plot = mass.frame(Range(massMin,massMax),Bins(nBins));
// dh->plotOn(plot);
// plot->Draw();



You can try to do :

myHist[0] = (TH1*) filename->Get(histogramName); 


Hi Lorenzo,

It seems to work, at least for histograms having a name without any space in it. I will rebuild the root file in order to suppress spaces.
Then it is not the end of the story as I have to read all the histograms and to fit. Let’s hope it will go smoothly…

Thanks and best regards,


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