Read vectors as variables for a TGraph

ROOT
1

Hi,
I’m quite unexperienced with ROOT…
I store data (x,y and error of y values) in vectors and want to draw “y : x” with TGraph. Somehow it seems that TGraph doesn’t recognize my vectors.

As a second step I want to add a second graph in my plot with TGraphAsymmErrors; that’s why I use TGraph and not simply a histogram.

#include <fstream>
#include <string>
#include <iostream>
#include <vector>
#include <TGraph.h>
#include <TGraphAsymmErrors.h>
#include <TFile.h>
#include <TCanvas.h>
#include <TH1.h>

using namespace std;
using std::ifstream;

vector<double> xp;
vector<double> F2;
vector<double> errF2;

void plotting(){
//make filenames
	string name,file_mod,file_dat,file_root;
	
		name = "Q1100b394";
		file_mod = "model/" + name + ".dat";

//read model
		int i = 0;
		double mtest;

		ifstream modfile(file_mod.c_str());
		
		while (modfile >> mtest){
			i++;
			if (i%3 == 2){
				F2.push_back(mtest);
			}
			else if (i%3 == 0){
				errF2.push_back(mtest);
			}
			else{
				xp.push_back(mtest);
			}
		}		
		modfile.close();

//TCanvas
	 	gROOT ->SetStyle("Plain");
		TCanvas *c1 = new TCanvas("c1",name.c_str(),200,10,700,500);
	 	gPad->SetLogx(1);
	
// draw a frame to define the range
		TH1F *h = c1->DrawFrame(0.0001,0,0.01,1);

//TGraph
		TGraph *g_mod = new TGraph(xp.size(),xp,F2);

		g_mod ->SetLineColor(1);
	 	g_mod ->SetLineWidth(2);
	 	g_mod ->SetMarkerColor(1);
	 	g_mod ->SetMarkerStyle(1);
	 	g_mod ->GetXaxis()->SetTitle("x_{P}");
	 	g_mod ->GetYaxis()->SetTitle("F2");
	 	g_mod ->GetXaxis()->CenterTitle();
	 	g_mod ->GetYaxis()->CenterTitle();
	 	g_mod ->Draw("CP");

		delete xp;
		delete F2;
		delete errF2;
}

This is my error message:

Error: Can't call TGraph::TGraph(xp.size(),xp,F2) in current scope plotting.cpp:49:
Possible candidates are...
(in TGraph)
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(void);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(Int_t n);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(Int_t n,const Int_t* x,const Int_t* y);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(Int_t n,const Float_t* x,const Float_t* y);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(Int_t n,const Double_t* x,const Double_t* y);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(const TGraph& gr);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(const TVectorF& vx,const TVectorF& vy);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(const TVectorD& vx,const TVectorD& vy);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(const TH1* h);
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(const TF1* f,Option_t* option="");
/Users/***/Programs/root/lib/libHist.so  -1:-1   0 public: TGraph TGraph::TGraph(const char* filename,const char* format="%lg %lg",Option_t* option="");

Thanks a lot for help!
Boginja

2

Hi,

There is no TGraph constructor taking vectors as arguments. Try this syntax:

   TGraph *g_mod = new TGraph(xp.size(), &xp[0], &F2[0]);

Cheers, Bertrand.

3

That works perfectly fine Bertrand, thank you!
Can you explain me why?

Boginja

4

Hi Boginja,

The STL vector is designed to give direct access to its internal array of type (double in this case) by using &vec[0], which is similar to a double *

Cheers, Bertrand.

7

Should be fixed now

1 Like
8

@bellenot

I have similar problem. I know we don’t have a graph constructor for a vector. I attach the code here.

AlphaData_Si.txt (7.9 KB)

teststatement.C (4.1 KB)

I want to choose only the energy range between 1 and 4 together with 4 and 10 from the text file I read. First column in the text represents the alpha particle energy, and the another column I read is the range.

Problematic part:

vector x_AlphaCalculated_Si;
vector y_AlphaCalculated_Si;

for(int i=0; i<number2 ; i++){
    
    if(x_AlphaData_Si[i] > 1 && x_AlphaData_Si[i] <=4 )      {
        x_AlphaCalculated_Si.push_back(x_AlphaData_Si[i]) ;
        y_AlphaCalculated_Si.push_back( exp(1.61*sqrt(x_AlphaData_Si[i] )) )   ; }
    
    else if(x_AlphaData_Si[i] >4 && x_AlphaData_Si[i] <= 10) {
        x_AlphaCalculated_Si.push_back(x_AlphaData_Si[i]) ;
        y_AlphaCalculated_Si.push_back( ( (0.05*x_AlphaData_Si[i] ) + 2.85)* pow(x_AlphaData_Si[i], 1.5) ) ; }
    
    else cout << "sucks!" << endl;
    
    cout << "x: "<< x_AlphaData_Si[i] << "  y_data:  " << y_AlphaData_Si[i] << "  y_calculated:  " << y_AlphaCalculated_Si[i]  <<endl;
    
}

TGraph *gr18 = new TGraph(x_AlphaCalculated_Si.size(), &x_AlphaCalculated_Si[0], &y_AlphaCalculated_Si[0]);
//?????  TGraph (const TVectorD &vx, const TVectorD &vy)
9

And what is “problematic”?

You could simplify it (no need for any vectors):

    TGraph *gr18 = new TGraph();
    for (int i = 0; i < number2; i++) {
      if (x_AlphaData_Si[i] > 1. && x_AlphaData_Si[i] <= 4.)
        gr18->SetPoint(gr18->GetN(), x_AlphaData_Si[i], exp(1.61 * sqrt(x_AlphaData_Si[i])));
      else if (x_AlphaData_Si[i] > 4. && x_AlphaData_Si[i] <= 10.)
        gr18->SetPoint(gr18->GetN(), x_AlphaData_Si[i], (0.05 * x_AlphaData_Si[i] + 2.85) * pow(x_AlphaData_Si[i], 1.5));
      else cout << i << " sucks!" << endl;
    }