Wouter Hulsbergen wrote:
>
> Dear ROOT people,
>
> I have written the following code
>
> {
> #include <math.h>
>
> float distribution(float pt, float pbar)
> {
> float pzbar = sqrt(pbar*pbar-pt*pt) ;
> float Norm = 1. ;
> return (pbar*pt/2* (pbar*pzbar + pt*pt*log(pbar+pzbar))
> - pt/3 * pzbar*pzbar*pzbar - pt*pt*pt * pzbar) /Norm ;
> };
>
> int main()
> {
> float pbar = 4. ;
> TF1 func("func","distribution(x,pbar)",0,pbar) ;
> func.Draw();
> }
> }
>
> which I want to use to plot the `distribution' function, using
> root> .L distrib.C ;
> root> main();
>
> However, this doesn't work because
> Unknown name : "distribution(x,pbar)"
> although the function `distribution' is known. Can I use user
> defined functions in a TF1 or TFormula definition?
> Regards, Wouter.
The class TF1 supports several constructors:
- constructor where you pass the function as one single string.
examples:
"sin(x)/x" , "[0]*x/sin([1]*x)"
- constructor where you pass a pointer to a function
of type double(double*x, double*params)
I have modified your macro to use the second and more general form:
{
// macro wouter
double distribution(double *x, double *par)
{
double pbar = par[0];
double pt = x[0];
double pzbar = sqrt(pbar*pbar-pt*pt) ;
float Norm = 1. ;
return (pbar*pt/2* (pbar*pzbar + pt*pt*log(pbar+pzbar))
- pt/3 * pzbar*pzbar*pzbar - pt*pt*pt * pzbar) /Norm ;
}
int main()
{
double pbar = 4. ;
TF1 *func = new TF1("func",distribution,0,pbar,1) ;
func->SetParameter(0,pbar);
func->Draw();
}
}
To execute this macro:
Root > .L wouter.C
Root > main()
Note that in your original macro, you add an additional problem>
Your TF1 object func was automatically deleted when exiting
from the scope of main(). You should create the TF1 object via new.
Rene Brun
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