Re: Bug report: TGraph::Eval()

Hi Alexandr,

Thanks for reporting this problem in TGraph::Eval. I agree with your simple correction and it is now in CVS.

Rene Brun

On Tue, 26 Apr
2005,
Alexandr Malusek wrote:

> Hi,
>
> There seems to be a bug in the TGraph::Eval() routine:
>
> Test code:
>
> {
> const Int_t dim = 5;
> Double_t x[dim] = {1.0, 2.0, 3.0, 4.0, 5.0};
> Double_t y[dim] = {1.0, 2.0, 3.0, 4.0, 5.0};
> TGraph *g = new TGraph(dim, x, y);
> cout << "y(4.99) = " << g->Eval(4.99) << endl;
> cout << "y(5.00) = " << g->Eval(5.00) << endl;
> cout << "y(5.01) = " << g->Eval(5.01) << endl;
> }
>
> Output:
>
> y(4.99) = 4.99 # correct
> y(5.00) = 1.01246e+07 # wrong
> y(5.01) = 1.01449e+07 # wrong
>
> I think the code in root/graf/src/TGraph.cxx:
>
> Double_t TGraph::Eval(Double_t x, TSpline *spline, Option_t *option) const
>
> ...
>
> //linear interpolation
> //find point in graph immediatly below x
> //In case x is < fX[0] or > fX[fNpoints-1] return the extrapolated point
> Int_t up, low = TMath::BinarySearch(fNpoints,fX,x);
> if (low == fNpoints-1) {up=low; low = up+1;}
> if (low == -1) {low=0; up=1;}
> if (fX[low] == fX[low+1]) return fY[low];
> Double_t yn = x*(fY[low]-fY[low+1]) +fX[low]*fY[low+1] - fX[low+1]*fY[low];
> return yn/(fX[low]-fX[low+1]);
>
> should be
>
> //linear interpolation
> //find point in graph immediately below x
> //In case x is < fX[0] or > fX[fNpoints-1] return the extrapolated point
> Int_t low = TMath::BinarySearch(fNpoints,fX,x);
> Int_t up = low+1;
> if (low == fNpoints-1) {up=low; low = up-1;}
> if (low == -1) {low=0; up=1;}
> if (fX[low] == fX[up]) return fY[low];
> Double_t yn = x*(fY[low]-fY[up]) +fX[low]*fY[up] - fX[up]*fY[low];
> return yn/(fX[low]-fX[up]);
>
>
> The problem is with
>
> if (low == fNpoints-1) {up=low; low = up+1;}
>
> the rest are cosmetic changes to utilize the variable "up". The
> modified code gives correct results:
>
> y(4.99) = 4.990000e+00
> y(5.00) = 5.000000e+00
> y(5.01) = 5.010000e+00
>
> The problem was observed with root-4.03/02 and root-4.03/04 on Linux,
> other versions may also be affected.
>
> Regards,
> Alexandr
>
Received on Tue Apr 26 2005 - 16:34:16 MEST