merge with function aggregation with Rcpp

library(data.table)
library(plyr)
library(dplyr)
library(purrr)
library(magrittr)
library(Rcpp)

sourceCpp(code = '
#include <Rcpp.h>
#include <string>
#include <vector>
using namespace Rcpp;
using namespace std;

NumericVector add_narm(NumericVector x, NumericVector y) {
  NumericVector out(x.size());
  NumericVector::iterator x_it,y_it,out_it;
  for (x_it = x.begin(), y_it=y.begin(), out_it = out.begin();
       x_it != x.end(); ++x_it, ++y_it, ++out_it) {
    if (ISNA(*x_it)) {
      *out_it = *y_it;
    } else if (ISNA(*y_it)) {
      *out_it = *x_it;
    } else {
      *out_it = *x_it + *y_it;
    }
  }
  return out;
}

// [[Rcpp::export]]
DataFrame merge_all_cpp(List dfs){
  DataFrame out(dfs[0]);
  vector<string> df_names = out.names();
  for (int i = 1; i < dfs.length(); i++)
  {
    DataFrame tmp(dfs[i]);
    vector<string> tmp_names = tmp.names();
    for (int j = 0; j < tmp.length(); j++)
    {
      if (find(df_names.begin(), df_names.end(), tmp_names[j]) != df_names.end())
      {
        out[tmp_names[j]] = add_narm(
          as<NumericVector>(out[tmp_names[j]]),
          as<NumericVector>(tmp[tmp_names[j]]));
      }
      else
      {
        out.push_back(tmp[tmp_names[j]], tmp_names[j]);
        df_names.push_back(tmp_names[j]);
      }
    }
  }
  return out;
}');

t = proc.time()
dfs = list(w,x,y,z)
by = "SP"
overall_keys = list(w,x,y,z) %>% map(~select_(., .dots = by)) %>% rbindlist %>% distinct
dfs %<>% map(~merge(., overall_keys, by = by, all = TRUE))
## not use purrr
# overall_keys = dfs %>% lapply(function(x) select_(x, .dots = by)) %>% rbindlist %>% distinct
# dfs %<>% lapply(function(x) merge(x, overall_keys, by = by, all = TRUE))
f = function(x) x / length(dfs)
out7 = merge_all_cpp(dfs) %>% tbl_dt(FALSE) %>% mutate_each_(funs(f), by)
proc.time() - t
#   user  system elapsed
#   1.46    0.11    1.57
out7 %<>% select_(.dots = names(out4)) %>%
  arrange_(.dots = by)  %>% mutate_each(funs(as.integer))
all.equal(out4, out7) # TRUE