如何在 Rcpp 的函数参数中将默认值设置为 Rcpp::Function? [英] How to set default value to a Rcpp::Function in argument of function in Rcpp?

问题描述

我想将函数参数中的默认值设置为 Rcpp::Function 参数.

I want to set default value in argument of function to Rcpp::Function argument.

仅仅简单的赋值，Rcpp::Function func = mean，是不可能的.它返回错误:<代码>没有来自<重载的函数类型>"的可行转换到'Rcpp::Function'(又名'Function_Impl')

Just simple assignment, Rcpp::Function func = mean, is not possible. It returns error: no viable conversion from '<overloaded function type>' to 'Rcpp::Function' (aka 'Function_Impl<PreserveStorage>')

或者，我尝试过这样的操作:Rcpp::Function func = Function("mean")，但同样，它不起作用.它返回警告消息:无法解析 C++ 默认值 'Function("mean")' for argument func of function.

Or, I tried something like this: Rcpp::Function func = Function("mean"), but again, it is not working. It returns warning message: Unable to parse C++ default value 'Function("mean")' for argument func of function.

例如，我有自己的最大函数叫做 maxC:

For example, I have my own function to maximum called maxC:

// [[Rcpp::export]]
double maxC(NumericVector x) {
  double max;
  max = *std::max_element(x.begin(), x.end());

  return max;
}

现在，我想使用它 (maxC) 作为另一个函数的默认参数，例如:

Now, I want to use it (maxC) as default argument to another function, for example like this:

// [[Rcpp::export]]
double aggregate(NumericVector x, Rcpp::Function func = maxC) {
  double agg;
  agg = Rcpp::as<double>(func(x));

  return agg;   
}

但它不起作用.有什么建议么?谢谢你.

But it doesn't work. Any suggestions? Thank you.

推荐答案

我不相信你可以用这种方式设置默认函数... 最好的办法是将函数设置为 NULL value 然后让代码稍后执行适当的默认值.例如...

I do not believe you can set a default function in this way... The best that can be achieved is setting function to a NULL value and then having the code execute the appropriate default later on. For example...

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericVector func_defaults(Rcpp::NumericVector x,
                                  Rcpp::Nullable<Rcpp::Function> f = R_NilValue) {

  if (f.isNotNull()) {
    Rcpp::NumericVector res = Rcpp::as<Rcpp::Function>(f)(x);
    return res;
  }

  Rcpp::Environment global_funcs = Rcpp::Environment::global_env();
  Rcpp::Function mean_r = global_funcs["mean"];
  return mean_r(x);
}

测试:

func_defaults(c(2.5,3,1))
# [1] 2.166667
func_defaults(c(2.5,3,1), mean)
# [1] 2.166667
func_defaults(c(2.5,3,1), median)
# [1] 2.5

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