Get TGraphErrors content from TCanvas

ftheodor · October 7, 2020, 5:19pm

Hi, I have some root files containing TCanvas that show TGraphErrors information.
I work on a python script that uses root library and copies the data to the database.
As you can see in the image there are 3 points in the canvas. I want to get the x & y dimensions from the middle one.

Could you help me on how to get the middle point information using python code? I have tried many things but nothing

Thank you!

eguiraud · October 7, 2020, 6:17pm

Hi @ftheodor,
and welcome to the ROOT forum!
I guess you can extract the TGraphErrors object from the file with (something like):

import ROOT
f = ROOT.TFile("filename.root")
graph = f.Get("nameoftgrapherror")

and then you can query the TGraphErrors for Xs and Ys of its points, e.g. with:

x = graph.GetPointX(1);
y = graph.GetPointY(1);

Cheers,
Enrico

ftheodor · October 7, 2020, 8:21pm

Hi, thank you for your reply!
Unfortunately there is no GetPointX/Y attribute on the TCanvas class ( AttributeError: ‘TCanvas’ object has no attribute ‘GetPointX’ ).

eguiraud · October 7, 2020, 8:38pm

Hi,
not the TCanvas, the TGraphErrors: the TCanvas just displays the underlying TGraphErrors, you should be able to retrieve that object (I think).

ftheodor · October 8, 2020, 10:24am

Hi, I know. I am just dont know how to retrieve the TGraphErrors using python.

Thanks for the help!

eguiraud · October 8, 2020, 10:25am

What does file.ls() print? (i.e. what’s inside the file?)

ftheodor · October 8, 2020, 10:28am

If I use this command in the root program (not pyRoot)
TGraphErrors *gre = (TGraphErrors*) _file0->Get("BPix/BPix_BmO/BPix_BmO_SEC5/BPix_BmO_SEC5_LYR1/BPix_BmO_SEC5_LYR1_LDR4H/BPix_BmO_SEC5_LYR1_LDR4H_MOD2/BPix_BmO_SEC5_LYR1_LDR4H_MOD2_ROC4")

I get the GraphErrors object. I just don’t know how to do this command in python

eguiraud · October 8, 2020, 10:30am

Perfect, then as mentioned above, from Python it should be:

import ROOT
f = ROOT.TFile("filename.root")
graph = f.Get("BPix/BPix_BmO/BPix_BmO_SEC5/BPix_BmO_SEC5_LYR1/BPix_BmO_SEC5_LYR1_LDR4H/BPix_BmO_SEC5_LYR1_LDR4H_MOD2/BPix_BmO_SEC5_LYR1_LDR4H_MOD2_ROC4")

ftheodor · October 8, 2020, 10:36am

Yes but this function still returns me a TCanvas object in Python ( <ROOT.TCanvas object (“BPix_BmO_SEC5_LYR1_LDR4H_MOD2_ROC4”) at 0x6ee2650>
)
Maybe it needs some kind of cast?

eguiraud · October 8, 2020, 10:37am

Uhm can you share the file with me please?

ftheodor · October 8, 2020, 10:41am

Sorry for not using the forum uploader but for some reason it was stucking at 15%.

eguiraud · October 8, 2020, 12:11pm

Hi,
I assumed that you saved the TGraphErrors to the file, but it looks like it really was the TCanvas object.

@couet might know how to recover the user coordinates of a point displayed by a TCanvas.

However, I suggest you save the TGraphErrors to the file rather than the TCanvas, so you have the full information in there.

Cheers,
Enrico

ftheodor · October 8, 2020, 12:29pm

Hi,
I know it is a bad way to store information, these are some old measurements and they are stored like this.
Thank you for your help!

couet · October 8, 2020, 12:55pm

The file you posted has a complex directory structure. I scanned it a bit but there is no TGraph nor TCanvas in it.

eguiraud · October 8, 2020, 1:05pm

The TCanvas is "BPix/BPix_BmO/BPix_BmO_SEC5/BPix_BmO_SEC5_LYR1/BPix_BmO_SEC5_LYR1_LDR4H/BPix_BmO_SEC5_LYR1_LDR4H_MOD2/BPix_BmO_SEC5_LYR1_LDR4H_MOD2_ROC4" (see posts above).

couet · October 8, 2020, 1:32pm

To retrieve the TGraphErrors do:

% root Iana_8.root
   ------------------------------------------------------------------
  | Welcome to ROOT 6.23/01                        https://root.cern |
  | (c) 1995-2020, The ROOT Team; conception: R. Brun, F. Rademakers |
  | Built for macosx64 on Oct 05 2020, 06:45:25                      |
  | From heads/master@v6-23-01-1421-ga5d880a434                      |
  | With Apple clang version 12.0.0 (clang-1200.0.32.2)              |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q'       |
   ------------------------------------------------------------------

root [0]
root [1] TCanvas *c = (TCanvas*) _file0->Get("BPix/BPix_BmO/BPix_BmO_SEC5/BPix_BmO_SEC5_LYR1/BPix_BmO_SEC5_LYR1_LDR4H/BPix_BmO_SEC5_LYR1_LDR4H_MOD2/BPix_BmO_SEC5_LYR1_LDR4H_MOD2_ROC4")
(TCanvas *) 0x7fa0e19081c0
root [2] TGraphErrors *gre = (TGraphErrors*)c->FindObject("Graph");
root [3] TCanvas C
root [4] gre->Draw("AL*")

ftheodor · October 8, 2020, 6:13pm

Hi and thanks for your reply!
I try to get x & y dimensions of the points but I always get 0.00000

root [29] gre->GetErrorX(1)
(double) 0.0000000
root [30] gre->GetErrorY(1)
(double) 0.0000000
root [31] gre->GetEY()[1]
(double) 0.0000000`

I tried also on pyRoot and I am still getting 0 values.

eguiraud · October 8, 2020, 7:47pm

I think you want GetPointX and GetPointY rather than GetErrorX and GetErrorY

couet · October 9, 2020, 7:21am

To get the points values you should use the methods @eguiraud suggested.
And, for info, the errors are really 0 in your TGraphErrors:

root [3] gre->Print();
x[0]=40, y[0]=12.9644, ex[0]=0, ey[0]=0
x[1]=80, y[1]=22.4258, ex[1]=0, ey[1]=0
x[2]=120, y[2]=31.0251, ex[2]=0, ey[2]=0

ftheodor · October 9, 2020, 10:39am

It works, many thanks!!
And as for the rest reading:
In python you have to use gre.GetX() or gre.GetY() function to get an array with x/y coordinates.

Thank you again!