Rcpp Create DataFrame with Variable Number of Columns

If I understand your question correctly, it seems like it would be easiest to take advantage of the DataFrame constructor that takes a List as an argument (since the size of a List can be specified directly), and set the names of your columns via .attr("names") and a CharacterVector:

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::DataFrame myFunc(int n, Rcpp::List lst, 
                       Rcpp::CharacterVector Names = Rcpp::CharacterVector::create()) {

  Rcpp::List tmp(n + 2);
  tmp[0] = Rcpp::IntegerVector(3);
  tmp[1] = Rcpp::IntegerVector(3);

  Rcpp::CharacterVector lnames = Names.size() < lst.size() ?
    lst.attr("names") : Names;
  Rcpp::CharacterVector names(n + 2);
  names[0] = "Num";
  names[1] = "ID";

  for (std::size_t i = 0; i < n; i++) {
    // tmp[i + 2] = do_something(lst[i]);
    tmp[i + 2] = lst[i];
    if (std::string(lnames[i]).compare("") != 0) {
      names[i + 2] = lnames[i];
    } else {
      names[i + 2] = "V" + std::to_string(i);
    }
  }
  Rcpp::DataFrame result(tmp);
  result.attr("names") = names;
  return result;
}

There's a little extra going on there to allow the Names vector to be optional - e.g. if you just use a named list you can omit the third argument. 

lst1 <- list(1L:3L, 1:3 + .25, letters[1:3])
##
> myFunc(length(lst1), lst1, c("V1", "V2", "V3"))
#  Num ID V1   V2 V3
#1   0  0  1 1.25  a
#2   0  0  2 2.25  b
#3   0  0  3 3.25  c

lst2 <- list(
  Column1 = 1L:3L,
  Column2 = 1:3 + .25,
  Column3 = letters[1:3],
  Column4 = LETTERS[1:3])
##
> myFunc(length(lst2), lst2)
#  Num ID Column1 Column2 Column3 Column4
#1   0  0       1    1.25       a       A
#2   0  0       2    2.25       b       B
#3   0  0       3    3.25       c       C

Just be aware of the 20-length limit for this signature of the DataFrame constructor, as pointed out by @hrbrmstr.


It's an old question, but I think more people are struggling with this, like me. Starting from the other answers here, I arrived at a solution that isn't limited by the 20 column limit of the DataFrame constructor:

// [[Rcpp::plugins(cpp11)]]
#include <Rcpp.h>
#include <string>
#include <iostream>

using namespace Rcpp;

// [[Rcpp::export]]
List variableColumnList(int numColumns=30) {
    List retval;
    for (int i=0; i<numColumns; i++) {
        std::ostringstream colName;
        colName << "V" << i+1;
        retval.push_back( IntegerVector::create(100*i, 100*i + 1),colName.str());
    }
    return retval;
}

// [[Rcpp::export]]
DataFrame variableColumnListAsDF(int numColumns=30) {
    Function asDF("as.data.frame");

    return asDF(variableColumnList(numColumns));
}

// [[Rcpp::export]]
DataFrame variableColumnListAsTibble(int numColumns=30) {
    Function asTibble("tbl_df");

    return asTibble(variableColumnList(numColumns));
}

So build a C++ List first by pushing columns onto an empty List. (I generate the values and the column names on the fly here.) Then, either return that as an R list, or use one of two helper functions to convert them into a data.frame or tbl_df. One could do the latter from R, but I find this cleaner.

Tags:

C++

R

Rcpp

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