I am trying to compute a function of branch “b1” from a root file with N elements say. The idea is to write the values of this function as branch in a new root tree so that I can use it as a friend.

For each element of branch I compute the function, and save it as element of a std::vector of size N.

When I write this vector into a new root file, it just gets written as one entry because its just one single std::vector object I guess. This creates a problem for adding as friend of original tree because the entry count don’t match with the branch “b” from original tree.

Is there a way to write the elements of std vector such that they are written as individual elements in the new tree? Basically I would like each element of the vector to be a leaf element.

How are you saving it to the new root file? If you are using a tree (since you say you want to use tree friends), just don’t declare the branch as a vector, but as a single value (int/double), and fill it once per each vector element (the vector should have the same size as entries in the original tree, if it’s going to be a friend) e.g.

double newval=0;
TTree *Tnew = new TTree("Tnew","new");
Tnew->Branch("v",&newval,"v/D");
// and then loop over the vector elements (e.g. vec[i]) and fill Tnew each time
for (int i=0; i<vec.size();++i) {
newval = vec[i];
Tnew->Fill();
}
//...

Thank you very much. This was exactly what I was looking for. I had tried something similar myself but I had the line equivalent to Tnew->Branch("v",&newval,"v/D") inside the loop which took extremely long time to save the tree and took up significantly more (1000x) space.