How to access histogram of TF1 drawn with DrawCopy()

[canigia]

Hi,

in order to draw a TF1 with different parameter settings to differend TPads in a TCanvas, I used the methode DrawCopy(), since using Draw() would update all pads to the latest parameter setting when updating the TCanvas. Unfortunately it seems that I cannot access the histogram of the TF1 by using TF1::GetHistogram() to configure the style, like it is possible after using Draw().

I already tried as well with gPad->GetListOfPrimitives()->At(…), but also it didn’t work.

Can somebody tell me how to access the histogram, perhaps via the TPad?

Best regards and thanks,
Klaus

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ROOT Version: 6.18-04
Platform: Not Provided
Compiler: Not Provided

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[etejedor]

Hi,

DrawCopy returns the new TF1 that is created (the copy), what happens if you invoke GetHistogram on that copy?

Otherwise @couet can give a hand when he is back next week.

[canigia]

Hi,

thanks for the answer! Accessing the pointer to the copy seems like a good idea I somehow overlooked. However, when I type in ROOT prompt:

root [1] TF1 *f1 = new TF1("f1","gaus",0,10);f1->SetParameters(1,5,0.3)
root [2] f1->Draw()
Info in <TCanvas::MakeDefCanvas>:  created default TCanvas with name c1
root [3] f1->GetHistogram()->Draw()
root [4] TF1 *f2 = (TF1*)f1->DrawCopy()
(TF1 *) 0x4dc55e0
root [5] f2->Draw()

 *** Break *** segmentation violation

Something seems to be wrong with the returned pointer to the copy…

Best,
Klaus

[etejedor]

Hi,

If you do f2->Draw(), that copy is already drawn in DrawCopy, I don’t know whether that matters, @couet ?

Does it work to do f2->GetHistogram()->Draw() ?

[canigia]

f2->GetHistogram()->Draw() also gives a segfault.

Funny thing, when I do e.g. f2->GetHistogram()->SetTitle("a;b;c") then the titles are updated on the plot, and f2->GetHistogram()->GetEntries() prints 200.000. But f2->Draw() or drawing the histogram directly crashes.

I think some style changes indeed work (SetLabelSize, SetNdivisions,…), since those things are being immediately updated, while others like axis ranges are not.

[etejedor]

I see, @couet is back on Monday, so I suggest we wait for him to explain this behaviour better.

[couet]

I see the crash with:

{
   TF1 *f1 = new TF1("f1","gaus",0,10);f1->SetParameters(1,5,0.3);
   TF1 *f2 = new TF1();
   f2 = (TF1*)f1->DrawCopy();
   f2->Print();
   f2->Draw();
}

Print() is ok … though … weird.

[moneta]

The errors looks to be when drawing again in the same pad. This works:

{
   TF1 *f1 = new TF1("f1","gaus",0,10);f1->SetParameters(1,5,0.3);
   TF1 * f2 = (TF1*)f1->DrawCopy();
   f2->Print();
   new TCanvas(); f2->Draw();
}

Lorenzo