Hi,
You have several concept that are mis-used.
If a branch is named 'NELE', it's name is a string that is
only usable when going through the TTree interface. Naming a
variable with the same name is insufficient to connect it to
the data in the TTree.
Also you never 'read' the TTree. You do not loop over the entries.
TTree::Draw (and TTree::Process) handles this looping for you.
If you need to call TH1F::Fill directly you have to do the looping.
Your code could look like what I appended at the end. However there
are simplier alternative (for the simple cases), for example read the
documentation for TTree::Draw and you will see that you can fill an
histogram that you have allocated your self by naming it in the Draw
command.
fChain = ... some initilization is missing here ...
TFile f("filename.root","new");
TH1F *ne_pointer = new TH1F("ne", "Num electrons",80,0,20);
fChain->Draw("NELE>>ne");
f.Write();
After writing the code below, I have a feeling that you actually extracted
this code from your original code and that your code was extracted from a
more complex and complete example.
It looks like you extracted this code from the result of running MakeClass
and or MakeSelector on your TTree. If this is the case, note that you have
not extracted some essential part that do the proper definiton, setting and
looping. In the future, please provide a full example so that we could be
more helpful.
Cheers,
Philippe.
// your code updated:
{
// create an empty root file first
TFile f("filename.root","new");
// create a histogram and fill it from a tre
TH1F *ne = new TH1F("ne", "Num electrons",80,0,20);
TTree *fChain; //!pointer to the analyzed TTree or TChain
Int_t fCurrent; //!current Tree number in a TChain
fChain = ... something is missing here ...
Int_t NELE_var; //define the type of NELE (the name of a leave
in the tree)
fChain->SetBranchAddress("NELE",&NELE_var);
Int_t nentries = Int_t(fChain->GetEntriesFast());
for (Int_t entryInChain=0; entryInChain<nentries; entryInChain++) {
Int_t entryInTree = LoadTree(entryInChain);
if (entryInTree < 0) break;
fChain->GetEntry(entryInChain);
ne->Fill(NELE); //fill data from a tree to a histogram
}
ne->Write(); //write the histogram into the empty root file created
}
PS. I strongly recommend that you read the full User's Guide and take a look
at the file in $ROOTSYS/tutorials.
-----Original Message-----
From: owner-roottalk@pcroot.cern.ch
[mailto:owner-roottalk@pcroot.cern.ch]On Behalf Of tcli
Sent: Tuesday, August 05, 2003 5:08 PM
To: roottalk@pcroot.cern.ch
Subject: [ROOT] how to create histograms using info from TTree
Dear root users,
The simplest way to histogram data from a tree is TTree::Draw.
But I'd like to
create histograms and then fill them with the leaves of a tree. I tried
with the following macro:
{
// create an empty root file first
TFile f("filename.root","new");
// create a histogram and fill it from a tree
TH1F *ne = new TH1F("ne", "Num electrons",80,0,20);
TTree *fChain; //!pointer to the analyzed TTree or TChain
Int_t fCurrent; //!current Tree number in a TChain
Int_t NELE; //define the type of NELE (the name of a leave
in the tree)
ne->Fill(NELE); //fill data from a tree to a histogram
ne->Write(); //write the histogram into the empty root
file
created
}
However, the above commands give me the wrong histogram with only one
entry (supposed to be 1000 entries) when I open the histogram with
TBrowser.
So is there any suggestion?
Thnaks in advance!
tcli
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