TF1 using formula vs. TF1 using real function

ROOT
1

Hi.
I tried to use an expression including TMath::BetaIncomplete in a TF1. For some reason the function did not behave as expected. And after some hours of debugging, I finally got this example, which illustrates what happened:

#include "TCanvas.h"
#include "TF1.h"
#include "TMath.h"

Double_t BetaFunc(Double_t *x, Double_t *p);


void functest()
{
  new TCanvas("func1","func1");
  TF1* func1 = new TF1("func1",
		       "TMath::BetaIncomplete(.5, x, 11-x )",
		       0.,10.);
  func1->Draw();

  new TCanvas("func2","func2");
  TF1* func2 = new TF1("func2",&BetaFunc,0.,10.,0);
  func2->Draw();
}

Double_t BetaFunc( Double_t *x, Double_t *par)
{
  return TMath::BetaIncomplete(.5, x[0], 11-x[0]);
}

Both functions should be the same, but func2 is corretct while func1 is just wrong.

Why can’t I use TMath::BetaIncomplete in a formula string to define a function?

Cheers, J.

2

I do not understand what you are trying to do with TMath::BetaIncomplete.
The signature of thgis function is

The first argument being the variable, a and b two parameters.

Rene

3

Hi,
this particular example is just to illustrate that there is a difference between the two way’s to declare the function. As far I can see the both methods should lead to the same function. Nevertheless if one executes the example script, the function plots (and TF1::Eval) differ for both functions. Did I miss a difference in the function definition? Why do the results differ?

Cheers, J.
functest.C (486 Bytes)

func2.jpg696×472 8.9 KB


func1.jpg696×472 8.87 KB

4

It looks like you did not read my mail carefully.

Rene

5

To answer you question precisely:
I want to draw TMath::BetaIncomplete as a function of e certain combination of parameters.

Combinations that are not too interesting from the mathematical point of view work as expected, if I define the function via the string:

TF1* func1 = new TF1("func1","TMath::BetaIncomplete(.5,x,5)",0.,20.)
func1->Draw();

or

TF1* func1 = new TF1("func1","TMath::BetaIncomplete(.5,x,x)",0.,20.)
func1->Draw();

What is the difference of my example to these examples?

Cheers, J.

6

PLEASE go back to my mail. Tmath::BetaIncomplete has 3 parameters
ONLY the first one is the variable x, the two others are parameters. It does not make any sense to declare BetaIncomplete(0.5,x,10-x)

Rene