Rcpp::DataFrame::create 被限制为 20 个参数？

Question

我们正在 Rcpp 函数中创建以下数据框：

  Rcpp::DataFrame res =
    Rcpp::DataFrame::create(
     Rcpp::Named("A")=a
    ,Rcpp::Named("B")=b
    ,Rcpp::Named("C")=c
    ,Rcpp::Named("D")=d
    ,Rcpp::Named("E")=e
    ,Rcpp::Named("F")=f
    ,Rcpp::Named("G")=g
    ,Rcpp::Named("H")=h
    ,Rcpp::Named("I")=i
    ,Rcpp::Named("J")=j
    ,Rcpp::Named("K")=k
    ,Rcpp::Named("L")=l
    ,Rcpp::Named("M")=m
    ,Rcpp::Named("N")=n
    ,Rcpp::Named("O")=o
    ,Rcpp::Named("P")=p
    ,Rcpp::Named("Q")=q
    ,Rcpp::Named("R")=r
    ,Rcpp::Named("S")=s
    ,Rcpp::Named("T")=t
    ,Rcpp::Named("U")=u
  );

此数据框旨在作为返回结果。但是由于以下错误而无法编译：

error: no matching function for call to Rcpp::DataFrame_Impl<Rcpp::PreserveStorage>::create
In file included from local/lib64/R/library/Rcpp/include/Rcpp/DataFrame.h:97:0,
                 from local/lib64/R/library/Rcpp/include/Rcpp.h:57,
                 from file54f121e6a937.cpp:1:
local/lib64/R/library/Rcpp/include/Rcpp/generated/DataFrame_generated.h:142:23: note: template<class T1, class T2, class T3, class T4, class T5, class T6, class T7, class T8, class T9, class T10, class T11, class T12, class T13, class T14, class T15, class T16, class T17, class T18, class T19, class T20> static Rcpp::DataFrame_Impl<StoragePolicy> Rcpp::DataFrame_Impl<StoragePolicy>::create(const T1&, const T2&, const T3&, const T4&, const T5&, const T6&, const T7&, const T8&, const T9&, const T10&, const T11&, const T12&, const T13&, const T14&, const T15&, const T16&, const T17&, const T18&, const T19&, const T20&) [with T1 = T1; T2 = T2; T3 = T3; T4 = T4; T5 = T5; T6 = T6; T7 = T7; T8 = T8; T9 = T9; T10 = T10; T11 = T11; T12 = T12; T13 = T13; T14 = T14; T15 = T15; T16 = T16; T17 = T17; T18 = T18; T19 = T19; T20 = T20; StoragePolicy = Rcpp::PreserveStorage]
 static DataFrame_Impl create( const T1& t1, const T2& t2, const T3& t3, const T4& t4, const T5& t5, const T6& t6, const T7& t7, const T8& t8, const T9& t9, const T10& t10, const T11& t11, const T12& t12, const T13& t13, const T14& t14, const T15& t15, const T16& t16, const T17& t17, const T18& t18, const T19& t19, const T20& t20 ) {
                       ^
local/lib64/R/library/Rcpp/include/Rcpp/generated/DataFrame_generated.h:142:23: note:   template argument deduction/substitution failed:
file54f121e6a937.cpp:771:7: note:   candidate expects 20 arguments, 21 provided

它可以很好地处理 20 个参数。我们如何克服这个问题？谢谢

Answer 1

是的。这在许多不同的地方都有涉及...

超出我的想象：

https://cran.r-project.org/web/packages/Rcpp/vignettes/Rcpp-FAQ.pdf#page=17

In essence, and in order to be able to compile it with the largest number of compilers, Rcpp is constrained by the older C++ standards which do not support variadic function arguments. So we actually use macros and code generator scripts to explicitly enumerate arguments, and that number has to stop at some limit. We chose 20.

用于创建超过 20 列的 data.frame 的方法是构建一个 list，然后强制转换为 data.frame。

示例代码：

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::List dynamic_df(Rcpp::DataFrame df) {

  // Number of variables in data.frame
  int num_vars = df.ncol();

  // Instantiate list with p variable entries
  Rcpp::List long_list(num_vars);

  // Make a variable to name columns
  Rcpp::CharacterVector namevec(num_vars);

  // Copy from data.frame into list.
  for (int i=0;i < num_vars; ++i) {
    long_list[i] = df(i); // Move vector from data frame to list
    namevec[i] = i;
  }

  // Add colnames
  long_list.attr("names") = namevec;

  // Coerce list to data.frame
  long_list.attr("row.names") = Rcpp::IntegerVector::create(NA_INTEGER, df.nrow());
  long_list.attr("class") = "data.frame";

  // Return result.. Will appear as data.frame
  return long_list;
}

/*** R

head(dynamic_df(mtcars))

*/

输出：

head(dynamic_df(mtcars))
#      0 1   2   3    4     5     6 7 8 9 10
# 1 21.0 6 160 110 3.90 2.620 16.46 0 1 4  4
# 2 21.0 6 160 110 3.90 2.875 17.02 0 1 4  4
# 3 22.8 4 108  93 3.85 2.320 18.61 1 1 4  1
# 4 21.4 6 258 110 3.08 3.215 19.44 1 0 3  1
# 5 18.7 8 360 175 3.15 3.440 17.02 0 0 3  2
# 6 18.1 6 225 105 2.76 3.460 20.22 1 0 3  1

不过，您真的应该考虑在重复条目中使用 Kevin 的 List_Builder class。

c.f。

how many vectors can be added in DataFrame::create( vec1, vec2 ... )?

Rcpp::DataFrame::create 被限制为 20 个参数？

Rcpp::DataFrame::create is limited by 20 arguments?

r

rcpp

rcpp11