通过Rcpp和int64_t R包将最大的C++变量值传递给R

Question

我已经写了一个函数来将2提升到给定的能力。我想使用64位整数。在R中，bit64包具有以下最大和最小限制：

来自R

> bit64::lim.integer64()
integer64
[1] -9223372036854775807 9223372036854775807 

这是-(2^63)和2^63。

但是，由于某些原因，我的Rcpp代码只能将2^62传回R。下面是将2提升到给定功率的函数代码(注意:我使用位移位来实现这一点)：

C++代码：

// [[Rcpp::export]]
Rcpp::NumericVector i2_to_the_power_j ( int64_t j )
{

  int64_t base = 1;
  int64_t value = base << j;

  // cout << "C++ value: " << value << "\n";

  // Create a vector of length 1 with `value` as the sole contents
  const   std::vector<int64_t> v(1, value);
  const size_t len = v.size();

  Rcpp::NumericVector nn(len);         // storage vehicle we return them in

  // transfers values 'keeping bits' but changing type
  // using reinterpret_cast would get us a warning
  std::memcpy(&(nn[0]), &(v[0]), len * sizeof(double));

  nn.attr("class") = "integer64";
  return nn;

  return value;
}

但是，当我在R中运行这个程序时，我无法获得最大的可能/限制值！

来自R

>library(Rcpp)
>library(bit64)

> sourceCpp("./hilbert_curve_copy.cpp")

> # I can get 2^62
> i2_to_the_power_j(62)
integer64
[1] 4611686018427387904

> # ...but I cannot get 2^63
> i2_to_the_power_j(63)
integer64
[1] <NA>

> # I cannot get 2^63, despite bit64 package claiming it can
> # handle integers of this size
> bit64::lim.integer64()
integer64
[1] -9223372036854775807 9223372036854775807 

我漏掉了什么东西吗？请指点，谢谢你抽出时间。

Answer 1

对我的快速猜测(被证明是正确的)：最大值本身可能是标记为NA的值。因此，计算‘1减去’这个值，并尝试它。

快速猜测我的:最大值可能是标记为NA的值。因此，计算‘1-’这个值并尝试它。

// [[Rcpp::export]]
Rcpp::NumericVector largeval ( ) {
  int64_t val = 9223372036854775807LL - 1;
  Rcpp::Rcout << "C++ value: " << val << "\n";
  Rcpp::NumericVector dbl(1);
  std::memcpy(&(dbl[0]), &val, sizeof(double));
  dbl.attr("class") = "integer64";
  return dbl;
}

我将它添加到您的代码中并运行它，结果是：

R> largeval()
C++ value: 9223372036854775806
integer64
[1] 9223372036854775806
R> 

下面是完整的代码以防万一。

代码

#include <Rcpp.h>

// [[Rcpp::export]]
Rcpp::NumericVector i2_to_the_power_j ( int64_t j )
{

  int64_t base = 1;
  int64_t value = base << j;

  // cout << "C++ value: " << value << "\n";

  // Create a vector of length 1 with `value` as the sole contents
  const   std::vector<int64_t> v(1, value);
  const size_t len = v.size();

  Rcpp::NumericVector nn(len);         // storage vehicle we return them in

  // transfers values 'keeping bits' but changing type
  // using reinterpret_cast would get us a warning
  std::memcpy(&(nn[0]), &(v[0]), len * sizeof(double));

  nn.attr("class") = "integer64";
  return nn;

  return value;
}

// [[Rcpp::export]]
Rcpp::NumericVector largeval ( ) {
  int64_t val = 9223372036854775807LL - 1;
  Rcpp::Rcout << "C++ value: " << val << "\n";
  Rcpp::NumericVector dbl(1);
  std::memcpy(&(dbl[0]), &val, sizeof(double));
  dbl.attr("class") = "integer64";
  return dbl;
}


/*** R
library(bit64)
# I can get 2^62
i2_to_the_power_j(62)

# ...but I cannot get 2^63
i2_to_the_power_j(63)

# I cannot get 2^63, despite bit64 package claiming it can
# handle integers of this size
bit64::lim.integer64()

largeval()
*/