Hi Rooters, Browsing the documentation, it's not clear to me how I should do the following (not unusual) thing: Starting from a TTree (a converted Ntuple) with lots of events and variables, I want to compute a few complex variables for each of a few selected events, and store them to be able to use them many times (for example to do an unbinned maximum likelihood fit). With Fortran/Hbook I would read the ntuple, select the events, and put the few complex variables for each of a few selected events in a common. Then my MINUIT FCN would use this common. With Root, I imagine I can do exactly the same thing, create a new TTree with only the variables I need and the events I need. Then my MINUIT FCN would use this TTree. But I would have lost all links to the original event data. But since I was told in C++ course that I have to think completely different, I thought I could do the following: add a selection flag as a leaf to all events (not even sure if and how it is doable), add my complex variables as new leaves to the event which are selected. Then my MINUIT FCN would read first the event flag, then the complex variables only for the good events (as explained in the web pages). So, what is the right way ? Note: the examples on the web are in the style: I loop once on the events and fill histograms. I would like to do (but maybe I shouldn't): I loop on the events, and add information to be used later. Thanks a lot David Rousseau
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