TTree:Draw() - size of second component of 2D vector?

Dear experts,

I have a TTree filled with 2D vectors (ie. std::vector<std::vector>) for which a dictionary was generated using genreflex. The first component of the 2D vector is fixed-sized, the second ones are variable.

With TTree:Draw(), it is easy to plot the size of the first component of the 2D vector: Draw("@v.size()","","").
However, I do not manage to get the size of the second component-vector, for a particular entry in the first component (which in my case is always defined):

  • Draw("@v[0].size()","","") gives the same as above (ie not what one would expect)
  • Draw("@(v[0]).size()","","") doesn’t work
  • Draw(“v[0].size()”,"","") doesn’t work either
  • Draw(“Length$(v[0][])”,"","") always returns 1. Trying Draw(“Iteration$:v[0][]”,"",“colz”) confirms that the second index is not looped over… Is that expected?

Is there a way around this? Otherwise I’ll resort to storing a vector containing the sizes of these second components, but this is obviously inefficient…


PS: I’m using ROOT 6.02/05


[quote]Draw(“Iteration$:v[0][]”,"",“colz”) confirms that the second index is not looped over… Is that expected?[/quote]No … and I can not reproduce this problem.

[quote]Draw(“Length$(v[0][])”,"","") always returns 1. [/quote]I can not reproduce this problem either and actually in my example, it seems to be the answer to your original question. With:{ vector<vector<double>> v; v.push_back({1,2,3,4}); v.push_back({10,11}); TTree *t = new TTree("t","for vec"); t->Branch("vec",&v); t->Fill(); v.clear(); v.push_back({1,2,3,4,5,6}); v.push_back({10,11,12,13,14}); t->Fill(); t->Scan("Length$(vec[0][])"); t->Scan("Length$(vec[1][])"); auto c = new TCanvas("c1"); c->Divide(2); c->cd(1); t->Draw("Length$(vec[0][])"); c->cd(2); t->Draw("Iteration$:vec[0][]","","colz"); }
I get [code]************************

  • Row * Length$(v *

  •    0 *         4 *
  •    1 *         6 *

  • Row * Length$(v *

  •    0 *         2 *
  •    1 *         5 *

and the attached picture.


1 Like

Dear Philippe,

After taking a closer look I found out there was an issue with the vectors written to my tree… So it’s good to see confirmed that the Length$() syntax works for this!

Many thanks,