Plotting ASCII files in ROOT

Question

I am trying to write a small macro that reads data from an ASCII file that has 4 columns. But I want to graph only the second the third columns as (x, y).

Answer 1

The constructor of TGraph can directly take a CSV file, see the documentation.

TGraph g("data.csv", "%*lg %lg %lg %*lg", ",");

The first argument is the filename, and the second argument a format string. Skipped columns are denoted with a *; to skip the last column you could actually just omit it from the format string,

%*lg %lg %lg

The third argument is the column separator which might be , for your flavor of CSV.

Answer 2

follow this example: https://root.cern.ch/root/html534/tutorials/tree/basic.C.html and instead of filling a histogram make a graph.

Answer 3

You can use the "CSV Contructor" as shown above or simply generate an empty TGraph (or TGraphErrors), loop over your CSV file and add points. Example with some constant error bars given in the first line/header:

    ifstream infile("input.csv");

    TGraphErrors *g1 = new TGraphErrors();
    g1->SetName("name_for_graph");
    g1->SetTitle("Title for your Graph;x values [x unit];y values [y unit]");

    Int_t pt=0, nv=0;
    Double_t e_vc=0.;
    Double_t vs=0., vc=0.;

    infile >> e_vs >> e_vc;

    while (1) {
            if(!infile.good()) break;

            infile >> vs >> vc;

            g1->SetPoint(pt, vs, vc);
            g1->SetPointError(pt, e_vs, e_vc);
            pt++;
    }
    infile.close();

...

    g1->Draw("APX");