Hi Rene, I would in principle be happy to implement this. Getting the errors correct is very tricky, however. Assumptions have to be made about the distributions from which the graphs are derived, and any correlations between them. For many problems, the errors can not be calculated correctly without knowing the actual histograms from which the graphs were created. An example would be the ratio of two efficiency curves, which share a common histogram as a denominator. I do not suggest implementing a 'deafult' error calculation which assumes Poisson-distributed uncorrelated data, since many users would not realize that this formula is incorrect for their particular situation. So, I think Tancredi really should use a loop, as it's not much code anyways. Cheers, Andy. --- Rene Brun <Rene.Brun@cern.ch> wrote: > Hi Tancredi, > > We do not have a TGraphErrors::Divide function. However, after some > gymnastic, > you should be able to do the job via TGraphAsymmErrors::BayesDivide > or the corresponding TGraphAsymmErrors constructor. > See the description of this function at: > http://root.cern.ch/root/htmldoc/TGraphAsymmErrors.html#TGraphAsymmErrors:BayesDivide > > You will have to transform your TGraphErrors to histograms before > using this > function. > A direct interface TGraphAsymmErrors::Divide taking two TGraphs as > input > could be implemented. > I am ccying this mail to Andy Haas the author of BayesDivide. > Andy, would you like to implement TGraphAsymmErrors::Divide ? > > Rene Brun > > Tancredi Carli wrote: > > > > Hello, > > > > I have two objects of type TGraphErrors, let's say TG1 and TG2 > > and I would like to divide them, e.g. TG3=TG1/TG2 and get > > the correct error treatment. Is there an function for this > > operation which I overlooked ? > > > > ..or do I have to get the values at each point by GetPoint > > make the division, calculate the error, create new objects > > and set the points ? > > > > Thank you for you help. > > > > Tancredi Carli > > > > -- > > > > > +-------------------------------------------------------------------+ > > | Tancredi Carli tancredi.carli@cern.ch > | > > | CERN, Division EP phone ++41 22 767 9606 > | > > | CH-1211 Geneva 23 Room 40-1-C06 > | > > > +-------------------------------------------------------------------+ >
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