Is it possible to write a C++ function that gets an R dataFrame as input, then modifies the dataFrame (in our case taking a subset) and returns the new data frame (in this question, returning a sub-dataframe) ?
My code below may make my question more clear:

**code**:

```
# Suppose I have the data frame below created in R:
myDF = data.frame(id = rep(c(1,2), each = 5), alph = letters[1:10], mess = rnorm(10))

# Suppose I want to write a C++ function that gets id as inout and returns 
# a sub-dataframe corresponding to that id (**If it's possible to return 
# DataFrame in C++**)

# Auxiliary function --> helps get a sub vector:
arma::vec myVecSubset(arma::vec vecMain, arma::vec IDVec, int ID){
  arma::uvec AuxVec = find(IDVec == ID);
  arma::vec rslt = arma::vec(AuxVec.size());
  for (int i = 0; i < AuxVec.size(); i++){
    rslt[i] = vecMain[AuxVec[i]];
  }
  return rslt;
}

# Here is my C++ function:
Rcpp::DataFrame myVecSubset(Rcpp::DataFrame myDF, int ID){
  arma::vec id = Rcpp::as<arma::vec>(myDF["id"]);
  arma::vec alph = Rcpp::as<arma::vec>(myDF["alpha"]);
  arma::vec mess = Rcpp::as<arma::vec>(myDF["mess"]);

  // here I take a sub-vector:
  arma::vec id_sub = myVecSubset(id, id, int ID);
  arma::vec alph_sub = myVecSubset(alph, id, int ID);
  arma::vec mess_sub = myVecSubset(mess, id, int ID);

  // here is the CHALLENGE: How to combine these vectors into a new data frame???
  ???
}

```

In summary, there are actually two main question:
1) Is there any better way to take the sub-dataframe above in C++? (wish I could simple say myDF[myDF$id == ID,]!!!)

2) Is there anyway that I can combine id\_sub, alpha\_sub, and mess\_sub into an R data frame and return it?

I really appreciate your help.