codes for gain measurement. The fitting doesnt work properly.

gain_measurement.cpp

+void gain_measurement()
+{
+  ifstream file_255;
+  
+  file_255.open("255_255.dat");
+  
+  cout<<"The data file 255_255 is opened..."<<std::endl;
+  
+  TCanvas *c1 = new TCanvas();
+  TTree *tree = new TTree("tree", "tree");
+  TH1D *hist_255 = new TH1D("hist_255"," Threshold gain measurement",100,900,1300);
+
+  
+  tree->ReadFile("255_255.dat", "x/D:y/D");
+  tree->SetEstimate(-1);//Set the #entries to estimate variable limits. n=-1, estimate is set to current maximum of the tree.
+  
+  tree->Draw("x>>hist_255","y","HIST");
+  
+  hist_255->GetXaxis()->SetTitle("Threshold");
+  hist_255->GetYaxis()->SetTitle("# signals above the threshold");
+
+  //fitting the histogram with modified error function
+
+  TF1 *modified_erf = new TF1("modified_erf","[0]*(1-TMath::Erf((x-[1])/(sqrt(2)*[2])))",1000,1250);//Not sure how to give the fit range
+  
+  modified_erf->SetParameter(0,100);
+  modified_erf->SetParameter(1,1115);
+  modified_erf->SetParameter(2,0);//Not sure how to give the initial value
+
+  modified_erf->SetParNames("constant","mean","noise");
+
+  hist_255->Fit("modified_erf");
+  hist_255->Draw();
+						
+  //file_255.close();
+
+  cout<<"Code succesfull..."<<std::endl;
+}		  
+										  
+										  
+