gr001_simple.C

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/// \file
/// \ingroup tutorial_graphs
/// \notebook
/// \preview This tutorial demonstrates how to create simple graphs in ROOT. The example is divided into two sections:
/// - The first section plots data generated from arrays.
/// - The second section plots data read from a text file.
///
/// \macro_image
/// \macro_code
/// 
/// \author Rene Brun
 
#include<fstream> //Include the library for reading the data file
 
void gr001_simple() {
   TCanvas *c1 = new TCanvas("c1","Two simple graphs",200,10,700,500);
 
   c1->Divide(2,1); //Dividing the canvas in subpads for distinguishing the two examples, [See documentation](https://root.cern/doc/master/classTCanvas.html)
 
   //FIRST EXAMPLE (Available data)
   c1->cd(1);
 
   const Int_t n = 20; //Fill the arrays x and y with the data points
   Double_t x[n], y[n];
   for (Int_t i=0;i<n;i++) {
     x[i] = i*0.1;
     y[i] = 10*sin(x[i]+0.2);
     printf(" i %i %f %f \n",i,x[i],y[i]);
   }
   
   TGraph *gr1 = new TGraph(n,x,y); //Create a TGraph object, storing the number of data n and the x, y variables
 
   //Set the color, width and style for the markers and line
   gr1->SetLineColor(2);
   gr1->SetLineWidth(4);
   gr1->SetMarkerColor(4);
   gr1->SetMarkerStyle(21);
   gr1->SetTitle("Graph from available data"); //Choose title for the graph
   gr1->GetXaxis()->SetTitle("X title"); //Choose title for the axis
   gr1->GetYaxis()->SetTitle("Y title");
 
   //Uncomment the following line to set a custom range for the x-axis (respectively for the y-axis):
   //gr1->GetXaxis()->SetRangeUser(0, 1.8); 
 
   gr1->Draw("ACP"); //"A" draw axes, "C" = draw a smooth line through the markers (optional) and "P" = draw markers for data points 
   //Optional customization can be done on a ROOT interactive session
 
 
   //SECOND EXAMPLE (Data stored in a text file)
   c1->cd(2);
 
   const Int_t m = 20; //Known number of data points in the file
   Double_t w[m], z[m];
 
   std::ifstream file(gROOT->GetTutorialDir() + "/visualisation/graphs/data_basic.txt"); // Open the data file
 
   // Use a for loop to read the data
   for (Int_t i = 0; i < m; i++) {
       file >> w[i] >> z[i]; //Fill the arrays with the data from the file
       printf(" i %i %f %f \n",i,w[i],z[i]);
   }
 
   file.close(); //Close the file after reading
   
   TGraph *gr2 = new TGraph(m, w, z); //Create a TGraph object for the file data
   gr2->SetLineColor(4);
   gr2->SetLineWidth(2);
   gr2->SetMarkerColor(2);
   gr2->SetMarkerStyle(20);
   gr2->SetTitle("Graph from data file");
   gr2->GetXaxis()->SetTitle("W title");
   gr2->GetYaxis()->SetTitle("Z title");
 
 
   gr2->Draw("ACP");
}