We needed to create a histogram from some output of a python program. Therefore I wrote a minimal root program to read in the data from a file and save the histogram as a pdf.

The datafile should contain one entry per row. Save the following source code as e.g. histogram.c and then start root with

root -b -q 'histogram.C("filename")'

Here is the sourcecode:

#include <TCanvas.h>
#include <TH1.h>
#include <iostream.h>
#include <Riostream.h>
#include "Math/PdfFuncMathCore.h"

int main(){
  histogram("test")
}

void histogram(char* fileName) {

  gROOT->Reset();
  TCanvas *c1 = new TCanvas("c1","c1",800,800);
  TH1I *histo = readDataToHist(fileName);
  c1->cd(1);
  histo->Draw();
  c1->Update();
  c1->Print("histogram.pdf","Portrait pdf");
}

TH1I* readDataToHist(char* fileName)
{
  ifstream in;
  in.open(fileName);
  Double_t bin;
  Int_t nlines = 0;

  Int_t x_min = 0;
  Int_t x_max = 10;
  TH1I *histo = new TH1I("h1","Histogram Title", 100,x_min,x_max);

  while(1)
  {
    in >> bin;
    if (!in.good()) break;
    //cout << "bin: " << bin << '\n';
    histo->Fill(bin);
    nlines++;
  }
  in.close();
  return histo;
}