Re: Fit x,y [2]
Voropaev S.G. (voropaev@inp.nsk.su)
Mon, 11 Aug 1997 12:03:58 +0700
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> From: Valery Fine <fine@mail.cern.ch>
> To: Voropaev S.G. <voropaev@inp.nsk.su>; roottalk@hpsalo.cern.ch
> Subject: Re: Fit x,y [2]
>
> > Hi,
> > I investigate my problem. New example:
> >
> > float x[20], y[20];
> > for(int i=0; i<20; i++) {x[i]=(i+1)/21.0; y[i]=0.5*x[i]+0.2;};
> > c1 = new TCanvas("c1","proba");
> > TGraph *tg = new TGraph(20,x,y);
> > tg->Draw("AP");
> > tg->Fit("pol1"); // All is OK. I see all my point and long line.
> >
> > for(int i=0; i<20; i++) {x[i]=gRandom->Rndm(1); y[i]=0.5*x[i]+0.2;};
> > delete tg; TGraph *tg = new TGraph(20,x,y); tg->Draw("AP");
> > tg->Fit("pol1");
> >
> > Now I see all my point and very short line. The X range of line is
> > 0.1-0.2!
>
> But did you see you the graphic view properly ?
> You may say "yes" but I'll insist "no". That's because of the
> shape of your function special function: 0.5*x + 0.2, Change your
> function to something else and draw it and WITH APL options to
> see the REAL its shape. (For example 0.5*x**2 + .2). No ask some
> human (not computer) to fit that "by hand".
I think the problem is in the invalid range of fit:
float x[20],y[20];
for(int i=0; i<20; i++) {x[i]=gRandom->Rndm(1); y[i]=0.5*x[i]*x[i]+0.2;};
c1 = new TCanvas("c1","Fit");
TGraph *tg = new TGraph(20,x,y);
TF1 *f1 = new TF1("f1","pol2");
tg->Draw("AP");
tg->Fit("f1","","AP"); // Fit range 0.1 - 0.2 !
f1->SetRange(0.2,1.0);
tg->Fit("f1","R","AP"); // All is OK
f1->SetRange(0.0,1.0);
tg->Fit("f1","R","AP"); // All is OK
Now, you can see that TGraph::Fit set invalid default fit range in case of
unsorted x,y data.
With best regards Voropaev S.G.