倫倫的小blog

Ubuntu 14.04 + ROOT v.5.34

Goal: learn the basic I/O ( Tfile class)

we will mostly use CINT to demonstrate.

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//--------create a pointer that can reference to TFile class---
TFile* f = new TFile("demo.root")
f->close()

//--------create instance from TFile class-----
TFile f("demo.root","recreate");     // to overwrite
TFile f("demo.root",);                     // to read
TFile f("demo.root", "update");      // to append
f.Close()
   
   
f.ls() // to list what in side this TFile object “f”

tip:   use tab to auto complete the input.
tip:   ROOT file is independent of .root file extension.
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Example 1: save objects to root file

TFile* f = new TFile ("test.root","recreate");  

f->ls()

TH1F h0("h0","histro0",100,-10,10)
TH1F h1("h1","histro1",100,-10,10)

h0.FillRandom("gaus",100)
h1.FillRandom("gaus",100)

h0.Write()
h1.Write()

f->ls()
// you can see two new histograms in the test.root file.

// let update h0
h0.FillRandom("gaus",200)
h0.Draw()
h0.Write()
f->ls()

f->close()




 CINT retrieves the one with the highest cycle number.

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TFile:: mkdir("char")     // to mkdir
TFile:: cd()           // to change dir
TFile:: delete()     // to delete element or dir
TFile:: ls("-m")    // to see objects in memory.
gDirectory->pwd() // to show the current path.

Example 2, to make a dir
TFile* f1 = new TFile ("test1.root","recreate");  
TFile* f2 = new TFile ("test2.root","recreate");  
gDirectory->pwd()
f1->cd()
f1->ls()
f1->mkdir("new_dir")
f1->ls()
f1->cd("new_dir”)
TH1F h0("h0","histro0",100,-10,10)
h0.FillRandom("gaus",100)
h0.Write()
f1->ls()
new TBrowser
f1->Close()

All histograms and trees are created in the current director




ROOT File provides sequential access and direct access.
 
Example 3, to retrieve objects.
TFile f("demo.root")
f.GetListOfKeys()->Print()
f.ls()