TGraph GetPoint

ROOT
1

Hi,

I am trying to read a TGraphErrors in from a file and extract the points X and Y which i then want to store and use later.
I have managed to obtain the graph and redraw it, however when i try to extract the points and print the values to the screen have a problem. The code compiles, but i get a bizzare output.
The code segment is:

//get the graph
ostringstream myname;
myname<<“CorrectedElectron_”<<ID<<"_"<<RunNo;
cout<<"Getting Graph "<<myname.str().c_str()<<endl;

  TGraphErrors * h1 = (TGraphErrors*)f1->Get(myname.str().c_str());
  h1->Draw();

  //works till here

  Double_t myX , myY;
  //    Int_t Num = h1->GetN();
  h1->GetX();
  h1->GetY();

for(int j = 0; j< 200; j++){
  
  Double_t mynum1 = h1->GetPoint(j,myX,myY);    ///Why doesn't this work how i want it to??
     cout<<j<<"  "<<myX<<"  "<<myY<<"  "<<mynum1<<endl;
  
}

The output i get is:

Getting Graph CorrectedElectron_1431_143
j myX myY mynum1
0 0 6.95335e-310 8.41928e+07
1 0 6.95335e-310 8.41928e+07
2 0 6.95335e-310 8.41928e+07
3 0 6.95335e-310 8.41928e+07
4 0 6.95335e-310 8.41928e+07
5 0 6.95335e-310 8.41928e+07
6 0 6.95335e-310 8.41928e+07
7 0 6.95335e-310 8.41928e+07
8 0 6.95335e-310 8.41928e+07
9 0 6.95335e-310 8.41928e+07

etc. for each of the 200 points.

Can anyone see where my mistake is?
Thanks

2

It works for me:

{ Int_t i; Double_t xp, yp; Double_t x[10], y[10]; for (i=0;i<10;i++) { x[i] = i*0.1; y[i] = 10*sin(x[i]+0.2); } gr = new TGraph(10,x,y); for (i=0;i<10;i++) { gr->GetPoint(i,xp,yp); printf("xp[%d] = %g - yp[%d] = %g\n",i,xp,i,yp); } }
It gives:

xp[0] = 0 - yp[0] = 1.98669
xp[1] = 0.1 - yp[1] = 2.9552
xp[2] = 0.2 - yp[2] = 3.89418
xp[3] = 0.3 - yp[3] = 4.79426
xp[4] = 0.4 - yp[4] = 5.64642
xp[5] = 0.5 - yp[5] = 6.44218
xp[6] = 0.6 - yp[6] = 7.17356
xp[7] = 0.7 - yp[7] = 7.83327
xp[8] = 0.8 - yp[8] = 8.41471
xp[9] = 0.9 - yp[9] = 8.91207
3

Hi,

Thanks for your quick reply. I have tried saving your graph to a file and reading it back in before extracting the points:

void func() {
  cout<<"Hello World"<<endl;

 TFile * f2 = new TFile("testpoints.root","RECREATE");

  Int_t i;
  Double_t xp, yp;
  Double_t x[10], y[10];

  for (i=0;i<10;i++) {
    x[i] = i*0.1;
    y[i] = 10*sin(x[i]+0.2);
  }

  TCanvas * c1 = new TCanvas("c1","c1");
  TGraph * gr = new TGraph(10,x,y);
  gr->SetTitle("mygraph");
  gr->Draw("AP");
  c1->Write();

  f2->Write();
  f2->Close();

  delete gr;
  delete c1;

  cout<<"Now get the points"<<endl;

  TFile * f3 = new TFile("testpoints.root","READ");

  TGraph * g1 = (TGraph*)f3->Get("c1");
  if(!g1){
    cout<<"Graph not found"<<endl;
    return;
  }

  g1->Draw();

  for (i=0;i<10;i++) {
    g1->GetPoint(i,xp,yp);
    printf("xp[%d] = %g - yp[%d] = %g\n",i,xp,i,yp);
  }

  cout<<"finished "<<endl;
}

I now get:
xp[0] = 0 - yp[0] = 0
xp[1] = 0 - yp[1] = 0
xp[2] = 0 - yp[2] = 0
xp[3] = 0 - yp[3] = 0
xp[4] = 0 - yp[4] = 0
xp[5] = 0 - yp[5] = 0
xp[6] = 0 - yp[6] = 0
xp[7] = 0 - yp[7] = 0
xp[8] = 0 - yp[8] = 0
xp[9] = 0 - yp[9] = 0

So is it something in the reading in procedure? When i draw the graph it looks the same as the original though. I am very confused.

4

Try:

{
   cout<<"Hello World"<<endl;

   TFile * f2 = new TFile("testpoints.root","RECREATE");

   Int_t i;
   Double_t xp, yp;
   Double_t x[10], y[10];

   for (i=0;i<10;i++) {
      x[i] = i*0.1;
      y[i] = 10*sin(x[i]+0.2);
   }

   TCanvas * c1 = new TCanvas("c1","c1");
   TGraph * gr = new TGraph(10,x,y);
   gr->SetName("mygraph");
   f2->cd();
   gr->Write();
   f2->Close();

   delete gr;
   delete c1;

   cout<<"Now get the points"<<endl;

   TFile * f3 = new TFile("testpoints.root","READ");

   TGraph * g1 = (TGraph*)f3->Get("mygraph");
   if(!g1){
   cout<<"Graph not found"<<endl;
   return;
   }

   for (i=0;i<10;i++) {
   g1->GetPoint(i,xp,yp);
   printf("xp[%d] = %g - yp[%d] = %g\n",i,xp,i,yp);
   }

   cout<<"finished "<<endl;
}
5

hi, pls how did you resolve the issue?

6

Have you tried my last suggestion ?

7

I created another function inplace of the GetPoint. I was unable to use the ypoints that i derived from the getpoint.

thanks

8

Ok. So you solved your problem ?

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