Hi,
On Wed, 13 Mar 2002 13:19:55 -0800
"Aron, Navneet" <navneet@SLAC.stanford.edu> wrote
concerning "[ROOT] Getting mean of Ntuple variables":
>
> Hi,
> Can any one tell me how do we get the mean , rms values of the various ntuple variables in the root file.
> My try is as follows:
>
> {
> gROOT->Reset();
> TFile *r = new TFile("g1onaxis.root");
> TTree *tree = (TTree*)r->Get("TKR_Hits_In_Lyr_0");
> Float_t mean = tree->GetMean();
> }
While the suggestion given by Pasha is indeed the right way to go, it
would be nice to have methods like
Double_t TTree::GetMean(const Char_t* leaf)
Double_t TTree::GetRMS(const Char_t* leaf)
(should not be const, since the tree should cache the calculation),
and maybe higher moments too.
I once made a small class TStatistics, which can help you do low-level
statistics (average, variance, covariance) on a N-dimensional sample.
The advantage of this class is, that it minimizes the rounding errors
due to finite machine precision, _and_ you only need to loop over the
data once to make the (co)variance. The algorithm is the same as the
one used in TPrincipal. It also has a service (static) method for a
1D sample, so that one can easily use it to caculate the average and
variance of a leaf variable:
TTree* tree = (TTree*)gDirectory->Get("Tree");
Double_t x;
Double_t mean = 0;
Double_t rms = 0;
tree->SetBranchAddress("X", &x);
Ssiz_t n = tree->GetEntries();
for (Int_t i = 0; i < Int_t(n); i++) {
tree->GetEntry(i);
TStatistics::AddPoint(x, i, mean, rms);
}
Notice, that the average and variance is calculated unbinned, unlike
in a histogram.
You can get the TStatistics class from my web site [1].
The tar-ball is standalone, but also demonstrates how to use Autotools
with ROOT.
Yours,
Christian Holm Christensen -------------------------------------------
Address: Sankt Hansgade 23, 1. th. Phone: (+45) 35 35 96 91
DK-2200 Copenhagen N Cell: (+45) 28 82 16 23
Denmark Office: (+45) 353 25 305
Email: cholm@nbi.dk Web: www.nbi.dk/~cholm
[1] http://cholm.home.cern.ch/cholm/root/#tstatistics
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