Fitting function works

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);
+  TH1D *hist_255 = new TH1D("hist_255"," Threshold gain measurement",500,850,1750);
 
   
   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->SetMarkerStyle(3);
+  tree->Draw("x>>hist_255","y","HIST P");
   
   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
+  TF1 *modified_erf = new TF1("modified_erf","([0]/2)*(1-TMath::Erf((x-[1])/(sqrt(2)*[2])))",1220,1700);
 
-  modified_erf->SetParNames("constant","mean","noise");
 
-  hist_255->Fit("modified_erf");
-  hist_255->Draw();
-						
-  //file_255.close();
+  modified_erf->SetParNames("constant","mean","noise");
+  modified_erf->SetParameter(1,hist_255->GetMean());
+  modified_erf->SetParameter(2,hist_255->GetRMS());
+  modified_erf->SetParLimits(0,200,250.0);
+  modified_erf->SetLineWidth(2);
+  modified_erf->SetLineColor(2);
+   
 
+  hist_255->Fit("modified_erf","R");
+  hist_255->GetListOfFunctions()->FindObject("modified_erf")->Draw("same");
+  
   cout<<"Code succesfull..."<<std::endl;
 }