How to save a user-defined function

ROOT
1

I would like to save a canvas, that contains a user defined function, as a .C file.

I’ve tried creating the TF1 using a CLING function or a general C function.

When I try to open the canvas .C file, I get either the errors:

Error in TF1::TF1: No function found with the signature myTF1(Double_t,Double_t*)
Error in TClingCallFunc::IFacePtr(kind): Attempt to get interface while invalid.
Error in TClingCallFunc::ExecT: Called with no wrapper, not implemented!

or

Processing exampleCanvas.C…
input_line_45:2:66: error: use of undeclared identifier ‘customFunction’
Double_t TFormula____id16614074293475595597(Double_t x){ return customFunction(x[0]) ; }
^
Error in TFormula::Eval: Can’t find TFormula____id16614074293475595597 function prototype with arguments Double_t
Error in TFormula::ProcessFormula: Could not find customFunction function with 1 argument(s)
Error in TFormula::ProcessFormula: Formula “customFunction(x)” is invalid !
Error in TFormula::Eval: Formula is invalid and not ready to execute
customFunction is unknown.

The only work around I can find is using the GetHistogram method of TF1, using that to draw and save the function instead. However, this does not look nice as the histogram drops to 0 at the end of the functions range.example.C (473 Bytes)

The attached macro demonstrates what I am trying to do: run the macro, then try to open the canvas saved as a .C file.

Could someone please explain what I should do?

Thanks in advance

ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

2

Function defined as a C function can be saved in a C file.
@moneta may explain.

3

I have the same issue. Calling TCanvas::SaveAs(“file.C”) when the plot contains a user defined function produces “file.C” that does not work in ROOT6. This process works in ROOT5 though. Is this a feature or a bug?

A workaround is to use TGraph::TGraph(TF1 *):
TGraph *g = new TGraph(myTF1);
g->Draw(“a,L”);
myCanvas->SaveAs(“exampleCanvas.C”);

Then
root exampleCanvas.C
works and reproduces the plot.

4

Can you post the reproducer file.C ?

5

@couet is there a way to delay this topic being closed after 14 days please? @moneta is yet to reply

6

Hi James, you just did! :smiley: Topics are only closed after 14 days of inactivity, pinging us does delay closing the topic

7

Ok, I have created a simplified version that illustrates what I was trying to do, it’s called example_modified.C (478 Bytes). Running
root -b -q example_modified.C produces two files:
exampleCanvas_does_not_work.C
exampleCanvas_works_fine.C

root exampleCanvas_works_fine.C produces a nice TGraph version of the function

root exampleCanvas_does_not_work.C , on the other hand, says
Error in TF1::TF1: No function found with the signature myTF1(Double_t,Double_t*)
and then it proceeds to say
Error in TClingCallFunc::IFacePtr(kind): Attempt to get interface while invalid.
Error in TClingCallFunc::ExecT: Called with no wrapper, not implemented!
…
~400 times

So my example is different from that of the OP in the sense that it’s implementing the custom function
as Double_t f(Double_t*, Double_t*), where the first argument is the coordinates and the second argument is the parameters. Then the canvas saves the plot using “TF1::SetSavedPoint” method. This used to work fine in ROOT5 but stopped working in ROOT6.

8

Hi,

Thank you for providing this example. It is true this feature does not work anymore. It would be great if you could open a JIRA open issue for this and we will try to restore it as soon as possible

Best regards

Lorenzo

9

This problem should be fixed by this PR

Thank you for reporting the issue. It was broken since 6.04

10

Thanks, I didn’t expect you would get to this so quickly.

11

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