TTree::Draw THnSparse

Dear ROOT (pyROOT) experts,

I’ve been trying to figure out what the best way for filling a THnSparse from a TTree is.
As far as I could find, there is only one example using a THnSparse which does the filling manually.
What I was hoping for is a feature like chain.Draw(“var1:var2:var3:var4…>>MyTHn”, “cut_string”) in order to create a THnSparse on the fly.
but when I try this with 4 variables, it creates a TH3F. When I try this with more than 4 variables, I get the following error message:

Too many variables. Use the option “para”, “gl5d” or “candle” to display more than 4 variables.

And none of these options seem to help in creating THnSparse as far as I could try.

Is there any feature available like this which would allow for creating ThnSparse directly from the TTree?

Many thanks in advance,

Hi Navid,

I don’t think TTree.Draw() can fill THnSparse, @pcanal please correct me if I’m wrong.

I would have thought that RDataFrame can do it, like this

import ROOT
from array import array

h = ROOT.THnSparseD("", "", dim, array('i', (nbin1, nbin2, ...)), array('d', (min1, min2, ...)), array('d', (max1, max2, ...)))
h = ROOT.RDataFrame(treeName, fileName).Fill(h, ["var1", "var2", ...])

But somehow I get an error

ActionHelpers.hxx:382:21: error: member reference base type 'THnSparseT<TArrayD> *' is not a structure or union
  fObjects[slot]->Fill(x0, x1, ...)

@eguiraud Do you have an idea?

I think PyROOT is somehow hiding the actual problem: THnSparse is not copy-constructible and as such cannot be used with Fill.

I see, so is this still the latest recommendation on using THnSparse with RDataFrame?

Yes that’s the Python workaround, thanks for digging that up! It’s fairly ugly – if there is enough request we could add a proper method to RDF to fill a THnSparse.

Thanks a lot for the quick responses! Unfortunately, in our current working environment RDFs are not included. I will try it once our environment is updated.
But on the other hand could the Fill THnSparse feature also be implemented with TTree::Draw or are there technical show stoppers?

After seeing this feature request from '08, I was hoping the feature would already be in place.

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