Cannot call by names the probability density functions obtained with TKDE

ROOT
,
1

Dear ROOT experts,
I want to divide two probability density functions obtained with TKDE but I cannot call the functions by name. Here I post some C++ code but I have the same problem with pyROOT.
Consider the following snippet:

    TKDE *kde1 = new TKDE(n, &data1[0], xmin, xmax);
    TKDE *kde2 = new TKDE(n, &data2[0], xmin, xmax);
    TF1 *fKde1 = kde1->GetFunction(100, xmin, xmax);
    TF1 *fKde2 = kde2->GetFunction(100, xmin, xmax);

    fKde1->SetName("f1");
    fKde2->SetName("f2");

    TF1 *f1f2 = new TF1("f1f2", "f1/f2", xmin, xmax);
    f1f2->Draw();

This gives the error

input_line_10:2:56: error: use of undeclared identifier 'f1'
Double_t TFormula____id13717184231236088683(){ return {f1}/{f2} ; }
                                                       ^
input_line_11:2:56: error: use of undeclared identifier 'f1'
Double_t TFormula____id13717184231236088683(){ return {f1}/{f2} ; }
                                                       ^
Error in <prepareMethod>: Can't compile function TFormula____id13717184231236088683 prototype with arguments 
Error in <TFormula::InputFormulaIntoCling>: Error compiling formula expression in Cling
Error in <TFormula::ProcessFormula>: "f1" has not been matched in the formula expression
Error in <TFormula::ProcessFormula>: "f2" has not been matched in the formula expression
Error in <TFormula::ProcessFormula>: Formula "f1/f2" is invalid !

However,

    TF1 *g1 = new TF1("g1", "x*x+1", 0, 10);
    TF1 *g2 = new TF1("g2", "x*x+2", 0, 10);

    TF1 *g1g2 = new TF1("g1g2", "g1/g2", 0, 10);
    g1g2->Draw();

works as expected.

I noticed that in the TF1 returned by the TKDE class there is no TFormula (f1->GetFormula() is 0), is this related to my problem?

Thanks in advance,
Daniel

ROOT Version: 6.24/06
Platform: ubuntu 20.04
Compiler: gcc 10.2.0

2

@moneta could you have a look at this, please?

3

Hi,

Calling a TF1 by name to include in another formula as above is supported only for TF1 object based on Formula, not for generic TF1 object based on some C++ code. So, what you are seeing is expected.
The workaround solution is to combine the two TF1 using C++ code, for example using a lambda function as following:

TKDE *kde1 = new TKDE(n, &data1[0], xmin, xmax);
TKDE *kde2 = new TKDE(n, &data2[0], xmin, xmax);
TF1 *fKde1 = kde1->GetFunction(100, xmin, xmax);
TF1 *fKde2 = kde2->GetFunction(100, xmin, xmax);

TF1 *f1f2 = new TF1("f1f2", [&](double *x, double *){ return fKde1->EvalPar(x,nullptr)/fKde2->EvalPar(x,nullptr); }, xmin, xmax,0);

Lorenzo

4

Hi Lorenzo,
thank you for the clear explanation, your workaround solves my problem.

Daniel