Reading Tree data error

[Abhinab_Mukhopadhyay]

Hello,

I am pretty new to trees and I am experiencing a problem of extracting vector data from branch of tree and storing it in variables. I have a branch that is vector which I have scanned for variables. I have attached an image below showing the scan of variables.

My code below is (modified from documentation):

TFile *input = new TFile("mc16_DrellYan_SpinHalf_HECOs_100_1000GeV.root","read");
TTree *MonAnaTree = (TTree*)input->Get("MonAnaTree");
std::vector<Float_t> *monPx = 0;
MonAnaTree->SetBranchAddress("monPx", &monPx);
for (Int_t i = 0; i<20; i++) {MonAnaTree->GetEntry(i);}

however, when I check the value for monPx i am only provided two values shown below .

root [21] (*monPx)
(std::vector<Float_t> &) { 268166.f, -280968.f }

Maybe it is due to my rustiness in C++ and ROOT but where am I going wrong ?

[Wile_E_Coyote]

monPx keeps the values (“Instances”) from the last GetEntry call (“Row”)

[Abhinab_Mukhopadhyay]

Hi , thank you for that. How would I extract the data , all values from the vector? Also just as curiosity what is an instance? is it something like an array index?

[Wile_E_Coyote]

“Row” is the entry number of the tree, and “Instance” is the vector’s element index (always for the current tree entry).

[Abhinab_Mukhopadhyay]

Thank you for the information. So for storing the values will this code work for storing all the vector values in an array:

Int_t array[20] = {0};

for (Int_t i = 0; i<20; i++) {MonAnaTree->GetEntry(i); array[i]=(*monPx)[i]  }

[Abhinab_Mukhopadhyay]

Hello sorry , this still stores just the last two values, How do you extract the original values of the data of monPx ? I am able to draw the histogram as shown below:

[Wile_E_Coyote]

(*monPx)[i] doesn’t really make sense to me.
from the screenshot in your first post, it seems to me that, for every tree entry, the size of the monPx vector is 2 (always?).