Hi Daniel,
I have implemented your suggestion in the CVS head.
However, I am curious to know why you were calling TF1::SetRange
in your loop calling GetRandom.
Rene Brun
On Wed, 15
Jan 2003,
Daniel De Marco wrote:
> * Daniel De Marco <ddm@ge.infn.it> [15/01/2003 11:02]:
> > I'm using TF1::GetRandom() to get random numbers distributed as a blackbody
> > spectrum. I need random numbers above some threshold so I do:
> >
> > f->SetRange(min, max); // assume f is a TF1* properly declared
> > // min and max are the bounds for the random number
> > f->GetRandom();
> >
> > to 20 times slower using GetRandom. Do I do something wrong? There's a way
> > to speed up the generation?
>
> I found that calling SetRange the internal arrays that TF1 uses to compute
> the random numbers are deleted and need to be recalculated at the next call
> to GetRandom. I overcome this problem changing TF1 and adding a method
> Double_t TF1::GetRandom(Double_t min, Double_t max)
> that generates a random number between min and max.
> The routine is equal to Double_t TF1::GetRandom() until the end where after
> the line "// return random number" becomes:
>
> // return random number
> Double_t dx = (fXmax-fXmin)/fNpx;
> Int_t nbinmin=(min-fXmin)/dx;
> Int_t nbinmax=(max-fXmin)/dx+1;
>
> Double_t pmin=fIntegral[nbinmin];
> Double_t pmax=fIntegral[nbinmax];
>
> Double_t x;
> do {
> Double_t r = gRandom->Uniform(pmin,pmax);
>
> bin = TMath::BinarySearch(fNpx,fIntegral,r);
> rr = r - fIntegral[bin];
>
> if(fGamma[bin])
> xx = (-fBeta[bin] +
> TMath::Sqrt(fBeta[bin]*fBeta[bin]+2*fGamma[bin]*rr))/fGamma[bin];
> else
> xx = rr/fBeta[bin];
> x = fAlpha[bin] + xx;
> } while(x<min || x>max);
> return x;
>
>
> Maybe this can be useful to someone else so I posted it here.
>
> Ciao, Daniel.
>
> --ddm
>
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