Code to demonstrate a feature of the RDataFrame::Alias function I do not understand.

alias_demo.cc

#include <iostream>

#include<TTree.h>
#include <TROOT.h>
#include <ROOT/RDataFrame.hxx>

using namespace std;

// Helper function adapted from 
// https://root.cern.ch/doc/master/df001__introduction_8C.html
// Creates an input example tree
void fill_tree(const char *treeName, const char *fileName)
{
   ROOT::RDataFrame d(10);
   int i(0);
   d.Define("oldVar", [&i]() { return (double)i++; })
      .Snapshot(treeName, fileName);
}

int main() {
    // Preparation
    auto fileName = "demo.root";
    auto treeName = "demotree";
    fill_tree(treeName, fileName);

    // Read input
    ROOT::RDataFrame rdf(treeName, fileName,{"oldVar"});
    cout << "The mean value of oldVar is: " << *rdf.Mean("oldVar") << endl;

    // On one subnode of the original data frame, we simply alias
    // "newVar" to be identical to "oldVar"
    auto r1 = rdf.Alias("newVar","oldVar");
    auto mean_r1 = *r1.Mean("newVar");
    cout << "The mean value in r1 is: " << mean_r1 << ", expected: 4.5" << endl;

    // On a second subnode, we define newVar to be twice oldVar
    // This is where the problem appears
    auto r2 = rdf.Define("newVar","2*oldVar");
    auto mean_r2 = *r2.Mean("newVar");
    cout << "The mean value in r2 is: " << mean_r2 << ", expected: 9.0"<< endl;

    // The problem goes away if we use another name for our variable
    auto r3 = rdf.Define("otherVar","2*oldVar");
    auto mean_r3 = *r3.Mean("otherVar");
    cout << "The mean value in r3 is: " << mean_r3 << ", expected: 9.0"<< endl;

}

Compiled with

g++ alias_demo.cc `root-config --cflags --glibs` -std=c++11 -o alias_demo.o

and ROOT version 6.16/00

Output:

$ ./alias_demo.o
The mean value of oldVar is: 4.5			 
The mean value in r1 is: 4.5, expected: 4.5  # Good
The mean value in r2 is: 4.5, expected: 9.0  # Bad
The mean value in r3 is: 9, expected: 9.0	 # Good

The problem can be solved by using replacing Alias -> Define on Line 32.