Read Graph Points from File

In this Chapter we will learn how to exploit some of the functionalities ROOT provides to display data exploiting the class TGraphErrors, which you already got to know previously.

Read Graph Points from File[edit | edit source]

The fastest way in which you can fill a graph with experimental data is to use the constructor which reads data points and their errors from an ASCII file (i.e. standard text) format:

TGraphErrors(const char *filename,
const char *format="%lg %lg %lg %lg", Option_t *option="");

The format string can be:

  • "%lg %lg" read only 2 first columns into X,Y
  • "%lg %lg %lg" read only 3 first columns into X,Y and EY
  • "%lg %lg %lg %lg" read only 4 first columns into X,Y,EX,EY

This approach has the nice feature of allowing the user to reuse the macro for many different data sets. Here is an example of an input file. The nice graphic result shown is produced by the macro below, which reads two such input files and uses different options to display the data points.

# Measurement of Friday 26 March
# Experiment 2 Physics Lab

1   6   5
2   12  5
3   14  4.7
4   20  4.5
5   22  4.2
6   24  5.1
7   35  2.9
8   45  4.1
9   44  4.8
10  53  5.43

Graph with expectation.png

// Reads the points from a file and produces a simple graph.
int macro2(){
    auto c=new TCanvas();c->SetGrid();

    TGraphErrors graph_expected("./macro2_input_expected.txt",
                                "%lg %lg %lg");
       "Measurement XYZ and Expectation;"
       "lenght [cm];"
    graph_expected.DrawClone("E3AL"); // E3 draws the band

    TGraphErrors graph("./macro2_input.txt","%lg %lg %lg");

    // Draw the Legend
    TLegend leg(.1,.7,.3,.9,"Lab. Lesson 2");
    leg.AddEntry(&graph_expected,"Expected Points");
    leg.AddEntry(&graph,"Measured Points");

    return 0;

In addition to the inspection of the plot, you can check the actual contents of the graph with the TGraph::Print() method at any time, obtaining a printout of the coordinates of data points on screen. The macro also shows us how to print a coloured band around a graph instead of error bars, quite useful for example to represent the errors of a theoretical prediction.