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