How to create and use a branch with a C char array from PyROOT

[harelamnon]

I have a brand new TTree, and I want to create in it a branch on it that will hold a C char array (of a known length) so that I can access this char array from python, modify it, and “Fill” the tree.

I actually managed to create such a branch with the monstrosity below, but I can’t modify the resulting python object. I find lots of old relevant posts, but they all refer to http://cern.ch/wlav/pyroot/tpytree.html which no longer exists (and may well be outdated due to the new pyroot).

R.gInterpreter.Declare("struct a5 { char c[5]; };") 
Ra5 = getattr( R, 'a5' )
a5 = Ra5() 
tr.Branch( 'dummy', R.AddressOf(a5), 'dummy/C' ) 

ROOT Version: 6.18/04
Platform: Ubuntu
Compiler: Not Provided

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[etejedor]

Hi,

You can use a character array in Python, with the array.array type. Here you can find an explanation:

Branching array of char in ttree

Gabriel, not exactly: CINT saves you by allocating a larger buffer for the string variable that you allocate, and probably initializes it to \0, or what you did in the C++ macro would not have worked (try compiling it, and see it crash at runtime in several places: the assignments beyond the empty string bounds as well as in the print/scan). In python, you get exactly what you asked for: a character array. That is not a string. You can get the same behavior by creating a larger buffer, and th…

So something like:
s=array('c', 'ab\0')

which you can pass to tr.Branch and you can modify before each fill with the value you want for the next tree entry.