First Steps: Files
Creating and opening ROOT files. Writing and reading objects.
Start by reading about
TFile and TKey
in the ROOT Crib which describes files and the keys used to
access the objects they contain. One very important point to take note
of: we cannot write out our Quad object to a TFile
As was explained in the
Classes
lesson, if you want to do that you must follow the instructions given in
Adding Your own Classes
to ROOT
Now we will put it into practice. You may find it
helpful to open a second browser window looking at TFile and TKey. Create
a file called:-
<![CDATA[
file_write.cxx
]]>
containing:-
<![CDATA[
{
gROOT->Reset();
TFile *MyFile = new TFile("hist.root","RECREATE");
if ( MyFile->IsOpen() ) cout << "File opened successfully" << endl;
TH1F *hist = new TH1F("hist","A histogram",100,0.,10.);
hist->Fill(1.);
hist->Fill(2.);
hist->Fill(2.);
hist->Fill(3.);
hist->Draw();
hist->Write();
MyFile->Print();
}
]]>
This creates, or recreates if already existing, the file hist.root and
checks that is was opened properly. Next a 1D histogram is created, as
explained in
Histograms
lesson. The histogram is asked to Draw itself. This is another
interesting shortcut. ROOT needs a
TCanvas
to display the histogram but we have omitted to create one. So ROOT
kindly produces a default one but warns us. Next the histogram is told
to Write itself, which it does to the current TFile. Finally the TFile
object MyFile is asked for a summary of what it contains.
Start up ROOT and run this macro. Now close the file:-
<![CDATA[
MyFile->Close();
]]>
When you do this the histogram will disappear from the TCanvas. Our histogram
was "owned" by the TFile once it was written to it, and closing the TFile
deleted it. Strictly speaking we did not need to close the TFile;
ROOT would do it automatically when it exited, but we should be tidy!
Now and exit ROOT and create the file:-
<![CDATA[
file_read.cxx
]]>
containing:-
<![CDATA[
{
gROOT->Reset();
TFile *MyFile = new TFile("hist.root","OLD");
MyFile->GetListOfKeys()->Print();
TH1F *hist = (TH1F*) MyFile->Get("hist");
hist->Draw();
}
]]>
This time we create a TFile object to connect to an existing file and
ask it for its list of keys which is then asked to print itself. This
produces something like:-
<![CDATA[
TKey Name = hist, Title = A histogram, Cycle = 1
]]>
showing that there is a
TKey
called hist. Note the cycle number, which follows the VMS concept of
version numbers. If you use the same name to write multiple objects to a
file, you don't loose any, they each get their own version number. The
TFile is asked to retrieve our histogram object by name. Recall that
the concept of access by name was discussed in the
Inheritance
lesson. Finally, to demonstrate that this really is our histogram, we
ask it to Draw itself. Of course there is nothing to stop us adding
more data to it and writing back to a TFile, a nice demonstration of how
autonomous the parts of an OO program are!
TFile
objects provide access to ROOT files.
- Objects that
inherit
from
TObject
and for which all the required CINT dictionary information has been
supplied can be written to the current (gFile) TFile by:-
<![CDATA[
my_objptr->Write();
]]>
This produces a
TKey
object associated with the TFile that holds the object's names and all
necessary access information.
- TFile objects can be asked to return a list of the keys that they
contain:-
<![CDATA[
my_fileptr->GetListOfKeys();
]]>
- Objects can be read back from a TFile by passing the object's name
to the TFile's Get message:-
<![CDATA[
My_class *my_objptr = (My_class *) my_fileptr->Get("my_objectname");
]]>
Go Back to the
The First Steps Top Page
If you have any comments about this page please send them to
Nick West