Converting a TGraph to a TH1D nearly works

Newbie
1

ROOT Version: 6.16
Platform: 20.04
Compiler: linuxx8664gcc

Dear co-rooters,

I am trying to convert a TGraph to a TH1D. This is convenient because i can use the TH1 functions later on.

For that I created a function, that accepts a TGraph object, and based on that it creates a binning. Note the the x points of the TGraph are not necessarily equidistant.
Then in my main code I am creating the the TGraph based on an ascii file and I am using the function to construct a TH1D histogram.

The trouble is that in some bins the BinCenter of the histogram is not exactly the same as the x of the TGraph. See the following image for instance.

Here is my code that I compile using .L macro.C++ and execute using oscillations(). Note that I upoaded the .dat as a .zip file.

Any idea on how to fix that?

Thanks in advance!

#include "TH1D.h"
#include "TGraph.h"
#include "TCanvas.h"

#include <vector>
#include <fstream>
#include <iostream>

using namespace std;

TH1D* TGraph_to_TH1D(TGraph* graph) {
    if (!graph) {
        std::cerr << "Error: Null TGraph pointer!" << std::endl;
        return nullptr;
    }
    
    
    int n = graph->GetN();
    double *x = new double[(n+1)];
    
    x[0] = graph->GetX()[0] - (graph->GetX()[1] - graph->GetX()[0]) / 2.0;
    x[n] = graph->GetX()[n-1] - (graph->GetX()[n-1] - graph->GetX()[n-2]) / 2.0;
    
    for (int i=1; i<n; ++i){
    	x[i] = (graph->GetX()[(i-1)] + graph->GetX()[i]) / 2.0;
    }
    
    TH1D *h = new TH1D("h", "h", n, x);
    
    for (int i=0; i<n; ++i) {
    	h->SetBinContent(i+1, graph->GetY()[i]);
	}

    return h;
}


void oscillations() {
    // Open the .dat file
    std::ifstream infile("235U_MT102_eV.dat");
    if (!infile) {
        std::cerr << "Error opening file!" << std::endl;
        return;
    }

    // Vectors to store data
    std::vector<double> energies, cross_sections;

    double energy, cross_section;
    while (infile >> energy >> cross_section) {
        energies.push_back(energy);
        cross_sections.push_back(cross_section);
    }
    infile.close();

    int nPoints = energies.size();
    if (nPoints == 0) {
        std::cerr << "No data found in file!" << std::endl;
        return;
    }

    // Create a TGraph
    TGraph *graph = new TGraph(nPoints, energies.data(), cross_sections.data());
    graph->SetTitle("Cross Section vs Energy;Energy (eV);Cross Section (barns)");
    
    TH1D *hist =(TH1D*) TGraph_to_TH1D(graph);

    // Draw the histogram and the graph
    hist->Draw("histo");

    graph->SetMarkerStyle(4);
    graph->SetMarkerSize(1);
    graph->Draw("PLsame"); // Overlay TGraph on histogram

}

235U_MT102_eV.dat.zip (260.1 KB)

Screenshot from 2025-03-04 12-43-52
Screenshot from 2025-03-04 12-43-521092×923 9.09 KB

2

If the points are not equidistant in x, as you said and already saw, you have to be more careful when you calculate the bin edges or centres.

1 Like
3

Which TH1 functions do you need which are not present in TGraph ?

4

Functions like Rebin, Divide(with other histograms), Scale.

5

Ok. Function like Rebin make sense only on binned data. Your data set is initially unbinned. I guess it is not possible to convert any graph into an histogram which will graphical exactly match the graph points. For that you need to make sure that the X value of each point correspond exactly to the center of each bins, Which not always possible.