What is the fastest way to sum all the values of one branch? What I am doing know is:
TH1F h("h", "h", 1, 1, 2);
chain->Draw("branchname>>h", "", "goff");
h.GetMean() * h.GetEntries()This works, but it is doing much more than what I need.
Second question: Is there a general reduce function to apply recursively?
f(f(f(f(x0, x1), x2), x3), ..., xn)in my case f(a, b) -> a + b
I would imagine that the fastest way would be to disable all the branches except the ones you need, then write a small loop that does chain->GetEntry() and adds the branch value to a variable. That also doesn’t put the values into bins like your method does.
Jean-François
[quote=“jfcaron”]I would imagine that the fastest way would be to disable all the branches except the ones you need, then write a small loop that does chain->GetEntry() and adds the branch value to a variable. That also doesn’t put the values into bins like your method does.
Jean-François[/quote]
Yes, thank you. By the way I don’t need only a fast computation, but also a fast code to be written. Writing an explicit loop, linking variables with SetBranchAddress, manually accumulate, … is very verbose for this simple problem.
[quote=“wiso”]Second question: Is there a general reduce function to apply recursively?
f(f(f(f(x0, x1), x2), x3), ..., xn)in my case f(a, b) -> a + b[/quote]
Is it somehow a ROOT-related question? May be something like std::accumulate/std::inner_product can help (though they work with sequences)?
C++ is not a functional programming language, so you have to clarify what do you understand as ‘reduce function to apply recursively’. Oh, and accumulate/inner_product both contains iteration under the hood instead of recursion you want.
P.S. Ah, and if you were asking about something TTree-related, please ignore my reply, since I understood your question as a general C++ question 
(though why not write your own TTreeIterator which works as an input iterator (C++ iterator category)) )) .
And about ‘too much code’ - with ROOT 6 we have new classes in $ROOTSYS/tree/treeplayer - TTreeReader and others who intend to significantly improve/simplify work with trees. I’m not sure if they are documented already in a ROOT user’s guide, but probably, you can have a look
[quote=“tpochep”][quote=“wiso”]Second question: Is there a general reduce function to apply recursively?
f(f(f(f(x0, x1), x2), x3), ..., xn)in my case f(a, b) -> a + b[/quote]
Is it somehow a ROOT-related question? May be something like std::accumulate/std::inner_product can help (though they work with sequences)?
C++ is not a functional programming language, so you have to clarify what do you understand as ‘reduce function to apply recursively’. Oh, and accumulate/inner_product both contains iteration under the hood instead of recursion you want.
[/quote]
C++ is not a functional programming language, but you can use functional programming pattern in C++, in fact std::accumulate does what I need, but I would like to have it directly in the TTree, for example:
tree->reduce("my_branch ** 2", function)where function is what you prefer: TF1, functor, *function, lambda, … for example
float function(float a, float b) {return a + b; }I can dump the content of a branch in a std::vector and then use std::transform, std::accumulate, but I need to dump everything in memory and it is not necessary.
[quote]
P.S. Ah, and if you were asking about something TTree-related, please ignore my reply, since I understood your question as a general C++ question (though why not write your own TTreeIterator which works as an input iterator (C++ iterator category)) )) .
And about ‘too much code’ - with ROOT 6 we have new classes in $ROOTSYS/tree/treeplayer - TTreeReader and others who intend to significantly improve/simplify work with trees. I’m not sure if they are documented already in a ROOT user’s guide, but probably, you can have a look[/quote]
thank you. I am curious to see it.
Well, I guess you can easily implement such a general reduce yourself, right? All you need is - somehow wrap the code for quite low-level SetBranchAddress/GetBranch/etc. into something type-safe and more robust (and here we have TTreeReader I believe 
) + functors or whatever (some people also like C++11 lambdas )
I have the same question and my solution is a variant of yours. Instead of filling the histogram with branch values, fill the histogram with a fixed value (within the bin boundaries) and use the branch values as weights:
chain->Draw(" 1.5 >>h", “branchname”, “goff”);
double sum = h->GetBinContent(1)
Isn’t
RDataFrame DF
DF.Sum(“column”)
What you look for ?
In 2021: yes