Embed TSpline3 in TF1 in pyroot

ROOT
1

_ROOT Version: 6, python2,3
_Platform: Fedora
_Compiler: gcc

Hello,

I do simple TSpline to TGraph points in pyroot like,

spline = ROOT.TSpline3(“name”, Graph)

and then easily get values for some x:

for x in LIST: spline.Eval(x)

But now I want this TSpline as TF1 object so I can integrate and do other stuff;

The simple

fit = TF1( “fit”, spline, 0.1, 2.95)
Graph.Fit(“fit”)

Doesn’t work for me; I know the number of parameters is missing in fit, but this wouldn’t mean anything since its a spline not a user-defined function!

Any ideas, thanks

2

Hi,

a fit is a procedure with which you extract the “best” value for the parameters of your model as well as their uncertainties. In your case you are trying to fit a spline to your data, without having parameters. In some sense the procedure is not well defined.
What are you really trying to achieve?

Cheers,
D

3

import ROOT
import json
from sys import argv, exit

def main():

fdata = ROOT.TFile(“graph_file.root”, “READ”);

PT_DATA = [1,2,3,4,5,6…]
Graph = ROOT.TGraph()
fdata.GetObject(“Graph”, Graph)

#use a cubic spline to smooth the graph

s = ROOT.TSpline3(“name”, Graph)
Graph.Write()

EVAL_VALUES = []

for i in PT_DATA:

  temp = round(s.Eval(i), 8)
  EVAL_VALUES.append(temp)

main()

if one runs this in pyroot this works; TSpline fits the TGraph and one can get then any point from Eval; Now, you can not Integrate the TSpline but I need here a TF1 object

4

Hi @Damir_Devetak

With PyROOT, you can construct TF1 objects from Python callables. In C++, one of the overloads of the TF1 constructor has this signature:
TF1 (const char *name, Double_t(*fcn)(Double_t *, Double_t *), Double_t xmin=0, Double_t xmax=1, Int_t npar=0, Int_t ndim=1, EAddToList addToGlobList=EAddToList::kDefault)

So the function it expects has two parameters which are double arrays. See for instance this example:


and here:

What you could do is wrap your TSpline3 in a python function def mywrapper(x, p): ... that uses the TSpline3.

Enric

5

thanks this should work!

6

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