Rcpp:与代理模型的行为不一致 [英] Rcpp: Inconsistent behavior with proxy model

问题描述

这篇帖子讨论了参数代理模型的一些问题通过 Rcpp.但是，当我实现此功能时:

This post discusses some issues with the proxy model for parameter passing in Rcpp. However, when I implemented this function:

// [[Rcpp::export]]
void test_size(NumericVector test){
  NumericVector test2(test);
  NumericVector test3 = NumericVector::create(1,1,1,1,1);
  test2 = test3;
  Rf_PrintValue(test);
}

我们得到:

> temp = c(2,2,2,2)
> test_size(temp)
[1] 2 2 2 2

所以问题是上一篇文章和这本book 在这种情况下说test2 应该是一个指向 R 底层 SEXP 对象的指针.但是，当我们分配 test2 = test3 时，这不适用于 test，因为 test NumericVector 保持不变.

So the problem is that the previous post and this book say that in this case test2 should be a pointer to the underlying SEXP object from R. However, when we assigned test2 = test3, this didn't apply to test because the test NumericVector remained unchanged.

更新

我正在添加一个示例，其中我认为分配没有像 Dirk 建议的那样工作，但当然我可能会误解这个问题.

I am adding an example where I think assignment isn't working as Dirk suggested, but of course I could be misunderstanding the problem.

所以假设我有以下功能:

So suppose I have the following function:

// [[Rcpp::export]]
NumericVector testing(){
  NumericMatrix mat(3,3);
  mat.row(0) = NumericVector::create(1,1,1);
  mat.row(1) = NumericVector::create(1,1,1);
  mat.row(2) = NumericVector::create(2,2,2);
  NumericVector test;
  NumericVector test2;
  for (int i = 0; i < mat.nrow(); i++){
    test = mat.row(i);
    if (test[0] == 1){
      test2 = test;
    }
  }
  return test2;
}

这个函数应该输出1,1,1，但它输出的是2,2,2.但是，当我用 test2 = clone(test) 替换 test2 = test 时，我得到了正确的输出.所以我想知道为什么我会得到这种行为，即使这只是 Dirk 建议的分配?

This function is supposed to output 1,1,1, but instead it outputs 2,2,2. However when I replace test2 = test with test2 = clone(test) then I get the correct output. So I was wondering why am I getting this behavior even though this is just assignment as Dirk suggested?

推荐答案

当您查看以下修改后的程序中的所有三个时，我会变得更容易:

I gets easier when you look at all three as in the modified program below:

R> testvecs(c(2,2,2,2))
$test
[1] 2 2 2 2

$test2
[1] 1 1 1 1 1

$test3
[1] 1 1 1 1 1

R> 

(现在完成的)代码在哪里

where the (now complete) code is

#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
List testvecs(NumericVector test){
  NumericVector test2(test);
  NumericVector test3 = NumericVector::create(1,1,1,1,1);
  test2 = test3;
  return List::create(Named("test") = test,
                      Named("test2") = test2,
                      Named("test3") = test3);
}

/*** R
testvecs(c(2,2,2,2))
*/

所以:

• test 传入且未更改，结果并不意外
• test2 被创建，然后覆盖
• test3 刚刚创建，并按预期出现
• test2 被指定为与 test3 相同，并且确实如此.

• test is incoming and unaltered, no surprise on the outcome
• test2 is created and then overwritten
• test3 is freshly created, and comes out as expected
• test2 is assigned to be the same as test3, and it is.

我发现这里没有任何不一致之处.

I see no inconsistency here.

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