Create a TFile with TTrees, then update the TFile with more TTrees #askROOT

ROOT tutorial #1

Download the C codes and run the following commands:

$ alias rot='root -l -b -q'
$ rot create_trees.C+
$ rot update_trees.C+

Tested in ROOT 6.06/00

create_trees.C

#include "TFile.h"
#include "TTree.h"
#include "TRandom.h"

void create_trees() {

    TFile * f1 = TFile::Open("giallo.root", "RECREATE");
    gRandom->SetSeed(1);

    TTree * t1 = new TTree("tree1", "tree1");
    double var1;
    t1->Branch("var1", &var1);
    for (int i=0; i<100; i++) {
        var1 = gRandom->Rndm();
        t1->Fill();
    }

    TTree * t2 = new TTree("tree2", "tree2");
    double var2;
    t2->Branch("var2", &var2);
    for (int i=0; i<100; i++) {
        var2 = gRandom->Rndm();
        t2->Fill();
    }

    t1->Write();
    t2->Write();
    f1->Close();

}

update_trees.C

#include "TFile.h"
#include "TTree.h"
#include "TRandom.h"

void update_trees() {

    TFile * f1 = TFile::Open("giallo.root", "UPDATE");
    gRandom->SetSeed(2);

    TTree * t3 = new TTree("tree3", "tree3");
    double var3;
    t3->Branch("var3", &var3);
    for (int i=0; i<100; i++) {
        var3 = gRandom->Rndm();
        t3->Fill();
    }

    TTree * t4 = new TTree("tree4", "tree4");
    double var4;
    t4->Branch("var4", &var4);
    for (int i=0; i<100; i++) {
        var4 = gRandom->Rndm();
        t4->Fill();
    }

    t3->Write();
    t4->Write();
    f1->Close();

}