Hi Pietro,
The reason for your program not running is that you must draw the
function before trying to get the axis. Otherwise, the axis object do
not even exist.
Best regards,
Alberto
Fabrizio Pierella wrote:
>
> Hi Pietro,
>
> > When I use it as a sequence of prompt commands everything works
> properly,
> > but if I try to let ROOT execute them as a macro I get this error:
>
> I cannot reproduce what you say, I get the error in both cases
> (both as a sequence of prompt commands and macro, ROOT 3.03/06);
> the reason is that
>
> root [3] TGraph *gr = new TGraph (5,x,y);
> root [4] cout << gr->GetXaxis() << endl;
> (nil)
>
> which is related to TGraph painting.
> the following seems to me well working
>
> void TGraph_example()
> {
> gROOT->Reset();
> c1 = new TCanvas("c1","Pietro Graph",200,10,700,500);
>
> Double_t x[5];
> Double_t y[5];
> for (Int_t i=0; i<5; i++) {x[i]=0.1*i;y[i]=10*sin(x[i]+0.2);}
> TGraph *gr = new TGraph (5,x,y);
> gr->Draw("AC*");
> c1->Update();
> gr->GetXaxis()->SetRangeUser(0.2,0.4);
> c1->Modified();
> }
>
> Hoping it is usefull, regards
>
> Fabrizio
>
> +=======================================================+
> || Fabrizio Pierella ||
> || E-mail: Fabrizio.Pierella@cern.ch ||
> || pierella@bo.infn.it ||
> || Tel. CERN (Bld.29-R-019) +41 (22) 7677767 ||
> || in Bologna (off.203) +39 051 2091103 ||
> +=======================================================+
>
> On Tue, 9 Jul 2002, Pietro Govoni wrote:
>
> > Dear rooters,
> > I'm trying to draw Tgraph in a certain range, narrower than the
> > definition's one.
> > The code I'm using is a simple example:
> >
> > void TGraph_example()
> > {
> > Double_t x[5];
> > Double_t y[5];
> > for (Int_t i=0; i<5; i++) {x[i]=0.1*i;y[i]=10*sin(x[i]+0.2);}
> > TGraph *gr = new TGraph (5,x,y);
> > gr->GetXaxis()->SetRangeUser(0.2,0.4);
> > gr->Draw("AC*");
> > }
> >
> > When I use it as a sequence of prompt commands everything works properly,
> > but if I try to let ROOT execute them as a macro I get this error:
> >
> > Processing TGraph_example.C...
> > Error: illegal pointer to class object GetXaxis() 0x0 126
> > FILE:TGraph_example.C LINE:9
> > *** Interpreter error recovered ***
> >
> > This is my version of root:
> >
> > * Version 3.03/05 1 May 2002 *
> >
> > Is there anything that I forgot?
> > Thank you for your attention, regards
> >
> > pietro
> >
> >
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