Problem when filling tree from Pythia

[imanol097]

I am trying to fill a ntuple from Pythia data. The problem is that the generated data does not correspond with the data saved in the ntuple. When I read the data in the tree, some strange numbers appears close to zeros like 10E-320. Pythia data types are <class ‘float’>, and I am filling the tree with this code:

# Creating file
file = TFile.Open("FileName, "recreate")

# Tree
tree = TTree("tree", "tree")
i      = 0

for i in range(0, len(pythia.event.size())):

    # Data from Pythia
    phi_i = np.array(pythia.event[i].phi())

    # Creating branch in the first iteration
    if i ==0: tree.Branch("phi", phi_i, "phi/F")

    # Filling
    tree.Fill()
    
    i+=1


# Writing tree
tree.Write()

# Writing file
file.Write()

file.Close()

I have checked the phi values in the loop and they are fine, the problem arises when filling the tree.

Someone know which is the problem?

Cheers,
Imanol

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[dastudillo]

If phi_i is an array of fixed size N, you need to save it also as array in the tree, e.g.

tree.Branch("phi", phi_i, "phi[N]/F")

[imanol097]

As far as I know, it is a float, as I pointed in the descripcion of the error. When I print the type of the variable it shows this: <class ‘float’>

[dastudillo]

you have

@imanol097

phi_i = np.array(...

If it is a simple number, why not do simply phi_i = pythia.event[i].phi()?

[imanol097]

Oh, yeah. If I don’t do that it gives an error:

TypeError: Template method resolution failed:
  none of the 8 overloaded methods succeeded. Full details:
  int TTree::Branch(TList* list, int bufsize = 32000, int splitlevel = 99) =>
    TypeError: could not convert argument 1
  TBranch* TTree::Branch(const char* name, char* address, const char* leaflist, int bufsize = 32000) =>
    TypeError: could not convert argument 2 (could not convert argument to buffer or nullptr)
  TBranch* TTree::Branch(const char* name, int address, const char* leaflist, int bufsize = 32000) =>
    TypeError: could not convert argument 2 (int/long conversion expects an integer object)
  int TTree::Branch(TCollection* list, int bufsize = 32000, int splitlevel = 99, const char* name = "") =>
    TypeError: could not convert argument 1
  int TTree::Branch(const char* folder, int bufsize = 32000, int splitlevel = 99) =>
    TypeError: could not convert argument 2 (int/long conversion expects an integer object)
  TBranch* TTree::Branch(const char* name, long address, const char* leaflist, int bufsize = 32000) =>
    TypeError: could not convert argument 2 (int/long conversion expects an integer object)
  TBranch* TTree::Branch(const char* name, void* address, const char* leaflist, int bufsize = 32000) =>
    TypeError: could not convert argument 2
  TBranch* TTree::Branch(const char* name, const char* classname, void* addobj, int bufsize = 32000, int splitlevel = 99) =>
    TypeError: could not convert argument 2 (bad argument type for built-in operation)
  Failed to instantiate "Branch(std::string,double,std::string)"
´´´

Maybe I am not using well the method ? In fact in the https://root.cern.ch/doc/master/classTTree.html#addcolumnoffundamentaltypes, the array syntax is the recomended one

[dastudillo]

Indeed, the second argument in Branch has to point to the variable; in C++, you’d do &Phi_i or simply the name of an array, which is the pointer, but in Python you use an array of 1 element; the example you link to says

# Basic type branch (float) - use array of length 1
n = array('f', [ 1.5 ])
t.Branch('floatb', n, 'floatb/F')

So, if pythia.event[i].phi() is a float, try

phi_i = np.array( 'f', [pythia.event[i].phi()] )

keeping tree.Branch("phi", phi_i, "phi/F")

[imanol097]

I have tried

phi_i = array('f', [1.5])
tree.Branch('phi', phi_i, 'phi/F')

And the result is this:

Some good values but a lot of non-sense ones

[dastudillo]

import ROOT
from array import array

# Creating file
file = ROOT.TFile.Open("aa.root", "recreate")

# Tree
tree = ROOT.TTree("tree", "tree")
phi_i = array('f', [0])
tree.Branch("phi", phi_i, "phi/F")

for i in range(0, len(pythia.event.size())):
    phi_i[0] = pythia.event[i].phi()
    # Filling
    tree.Fill()    

# Writing file
file.Write()

file.Close()

[imanol097]

Yeah, now It seems to work.
So the trick is to predefine the variable with a random value at the beggining and redefine it later? I don’t understand why.

[dastudillo]

You were creating a new array at every iteration of the loop, but only the address of the first one was associated with the branch.

[imanol097]

ok, i see. So to avoid this you redefine the value of the array linked to the tree at every iteration instead of creating a new array right?