Re: TMatrix and TGraph

Rene Brun (Rene.Brun@cern.ch)
Tue, 24 Mar 1998 13:12:07 +0100

Krieg, M. wrote:
>
> Hi Roots,
>
> I have the following problem concerning TGraph and the Linear Algebra
> package:
>
> I want to plot several lineplots in a single graph using the TGraph class.
> All plots use the same x-values stored in a float pointer *x but have (of
> course) different y-values. The y-values are stored in a Matrix nK(i,j) of
> the TMatrix class, each line of the Matrix corresponds to the y-values of
> one plot.
> Now the problem is, that TGraph needs 2 float pointers pointing at the x and
> y values of the plot. So if I want to use TGraph I have to set float
> pointers pointing on each line of the TMatrix Object.
> I would like to know how one does that with a real good programming style
> using the linear algebra package of ROOT! Has anyone an idea?
>
> Thanks a lot in advance!
> Martina

A possible solution is illustrated in the example below

Rene Brun

// macro gramat.C. Using matrix and graphs
{
gROOT->Reset();
const Int_t ngraphs = 10;
const Int_t npoints = 15;
Float_t a[ngraphs] =
{0.2,0.3,0.42,0.48,0.62,0.71,0.91,1.03,1.14,1.35};
Float_t b[ngraphs] = {1,2,3,4,5,6,7,8,9,10};
Float_t x[npoints] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15};

TCanvas *c1 = new TCanvas("c1","test Matrix and Graphs");
c1->SetGrid();
c1->DrawFrame(0,0,16,32);

TMatrix mat(npoints,ngraphs);
for (Int_t gr=0;gr<ngraphs;gr++) {
for (Int_t point=0;point<npoints;point++) {
mat(point,gr) = b[gr] + a[gr]*x[point] + gRandom->Gaus(0,0.3);
}
TGraph *graph = new TGraph(npoints,x,&mat(0,gr));
graph->SetMarkerStyle(20+gr);
graph->Fit("pol1","q0");
graph->Draw("p");
}
mat.Print();
}