使用 rcpp 将 R 函数传递给 C 例程

问题描述

我有一个来自下游库的 C 函数，我像这样在 C 中调用

I have a C function from a down-stream library that I call in C like this

result = cfunction(input_function)

input_function 是一个回调，需要有如下结构

input_function is a callback that needs to have the following structure

double input_function(const double &x)
{
  return(x*x);
}

其中 x*x 是用户定义的计算，通常要复杂得多.我想使用 Rcpp 包装 cfunction 以便 R 用户可以在任意 R 函数上调用它.

Where x*x is a user-defined computation that is usually much more complicated. I'd like to wrap cfunction using Rcpp so that the R user could call it on arbitrary R functions.

NumericVector rfunction(Function F){

  NumericVector result(1);

  // MAGIC THAT I DON'T KNOW HOW TO DO
  // SOMEHOW TURN F INTO COMPATIBLE input_funcion

  result[0] = cfunction(input_function);

  return(result);
  }

R 用户然后可能会执行 rfunction(function(x) {x*x}) 并得到正确的结果.

The R user then might do rfunction(function(x) {x*x}) and get the right result.

我知道在 cfunction 中调用 R 函数会降低速度，但我认为我可以在稍后弄清楚如何传递编译函数.我只想让这部分工作.

I am aware that calling R functions within cfunction will kill the speed but I figure that I can figure out how to pass compiled functions later on. I'd just like to get this part working.

我能找到的最接近我需要的是这个 https://sites.google.com/site/andrassali/computing/user-supplied-functions-in-rcppgsl 包装了一个函数，该函数使用回调函数，其中包含一个非常有用的第二个参数我可以填充 R 函数.

The closest thing I can find that does what I need is this https://sites.google.com/site/andrassali/computing/user-supplied-functions-in-rcppgsl which wraps a function that uses callback that has an oh-so-useful second parameter within which I could stuff the R function.

非常感谢您的建议.

推荐答案

一种可能的解决方案是将 R 函数保存到一个全局变量中并定义一个使用该全局变量的函数.我使用匿名命名空间使变量仅在编译单元内为人所知的示例实现:

One possible solution would be saving the R-function into a global variable and defining a function that uses that global variable. Example implementation where I use an anonymous namespace to make the variable known only within the compilation unit:

#include <Rcpp.h>

extern "C" {
    double cfunction(double (*input_function)(const double&)) {
        return input_function(42);
    }
}

namespace {
std::unique_ptr<Rcpp::Function> func;
}

double input_function(const double &x) {
    Rcpp::NumericVector result = (*func)(x);
    return result(0);
}


// [[Rcpp::export]]
double rfunction(Rcpp::Function F){
    func = std::make_unique<Rcpp::Function>(F);
    return cfunction(input_function);
}

/*** R
rfunction(sqrt)
rfunction(log)
*/

输出:

> Rcpp::sourceCpp('57137507/code.cpp')

> rfunction(sqrt)
[1] 6.480741

> rfunction(log)
[1] 3.73767

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