Pyroot loading a single value out of a vector in a tree

Newbie
,
1

Hello,
I wanted to create a tree with vectors in which I can save different values and the time I get these. To keep it simple i tried this code to create my file with the values.

file = ROOT.TFile.Open(name,“create”)
tree = ROOT.TTree(“tree”, “tree”)
pv1 = ROOT.vector(‘float’)(0)
tree.Branch(“pv1”, pv1)
…
tree.Fill()
file.Write()
I’m resizing my vectors while running in order to save the new values.

Now I’m trying to load those vectors again in order to print just a single random value but dunno how to do this. I tried :

file = ROOT.TFile(name,“READ”)
tree = file.Get(‘tree’)
pv1 = tree.Branch(“pv1”)
print(pv1[0])

but I’m getting the following error and I’ve the feeling there should be a better way to do this.

TypeError: ‘int’ object is not subscriptable

Would appreciate any kind of help or new input regarding my problem.
Henry

ROOT Version: 6.22.06
Platform: Ubuntu 20.04
Compiler: Not Provided

2

Maybe pv1 = tree.GetBranch("pv1") instead?

1 Like
3

This is working but it seems like my Values are not getting saved on my file.
I’m using this code to save them at the moment to try out everything I wanna use later.

 def load():
     file = ROOT.TFile.Open(name, "recreate")
     tree = ROOT.TTree("new","new")
     pv1 = np.empty(1000, "float64")
     pv2 = np.empty(1000,"float64")
     pv3 = np.empty(1000,"float64")
     time = np.empty(1000,"float64")
     tree.Branch("time",time,"time/F")
     tree.Branch("pv1",pv1,"pv1/F")
     tree.Branch("pv2",pv2,"pv2/F")
     tree.Branch("pv3",pv3,"pv3/F")
     return file,pv1,pv2,pv3,time,tree
 
 def save():
     tree.Fill()
     file.Write()
 
 
 while i < n:
     if i == 0:
         file,pv1,pv2,pv3,time, tree = load()
     pv1[i]=1
     pv2[i]=2
     pv3[i] = 3
     time[i] = 0
     i=i+1
     save()

But if I open my file again the values aren’t saved. Would you mind taking a quick look at this code? Maybe you are able to see my fault.

Thank you so far
Henry

4

I’m sure @etejedor will take a look once back from vacation

5

Hello,

You need to define your array branches as arrays of doubles, right now they are defined as single floats.

So if you define:

pv1 = np.empty(1000, "float64")

then you need:

tree.Branch("pv1",pv1,"pv1[1000]/D")

You can find examples of how to use the Branch pythonization here (look for the PyROOT box after the C++ docs):

Also, when you read the values back, you can do it with this syntax:

f = ROOT.TFile.Open(name)
t = f.new
for entry in t:
    for elem in entry.pv1:
         print(elem)
6

Thanks that helped a lot. Would you mind if I ask another question right away?

Thanks for your time so far.
Henry

7

Sure please ask away!

1 Like