Hi Christian,
A TTree::GetMean,RMS could be implemented. I am however reluctant to implement
these functions. Very likely you would like a GetMean(variable, selection)
and also the full machinery with
GetMean(const char *varexp, const char *selection,Int_t nentries=1000000000,
Int_t firstentry=0);
etc for GetRMS.
This functionality is already provided by TTree::Draw as indicated by Pasha.
I believe it is more general to do
tree.Draw("var>>hist,"some selection",etc);
hist->getMean();
Rene Brun
Christian Holm Christensen wrote:
>
> 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
This archive was generated by hypermail 2b29 : Sat Jan 04 2003 - 23:50:46 MET