Best way to evaluate value from string

Hi fellow rooters,

I’m trying to write a DAQ interface to display some basic histograms for incoming data in real-time. I’d like users to be able to determine what to plot with a simple string statement that would be interpreted as though they’d put it into a TTree::Draw command. We have an Event class with a dictionary, which is the only branch in our analyzed trees, so statemens would all be things like “event.pulse_integral / event.pulse_amplitude”, etc.

The best way I can think of right now is to use TTreeFormulas, so something like:

void RealTimeSpectrum::Initialize(const char* string_to_plot) { fTree = new TTree("tree","tree"); Event* dummy = new Event; fTree.Branch("event",&dummy); delete dummy; fFormula = new TTreeFomula("myformula", string_to_plot, fTree); fFormula->SetQuickLoad(true); //should I call this? } void RealTimeSpectrum::Process(Event* evt) { fTree->SetBranchAddress("event", &evt); for(int i=0 ; i < fFormula->GetNdata(); ++i) fHisto->Fill( fFormula->EvalInstance(i) ); }

I’m reasonably certain something like this will work, but is it the “right” way to go about it?
Should I call the SetQuickLoad method?
Is having to call SetBranchAddress or use an interpreted function like this going to be a huge performance hog? Is there a way to avoid having to carry around a TTree which doesn’t actually do anything directly? E.g., is there a reasonably straightforward way to do something like overload TFormula so that it knows about my Event class rather than using TTrees and branches for that effect?

Many thanks,

Hi Ben,

Going through the TTreeFormula has several advantages both in term of functionality (support for the internal arrays) and speed (the string is parsed just once), the alternative is to generate a proper function (that would need to handle the internal loop, etc.) and either interpret it or compile it (via ACLiC) [and access it via a TMethodCall).