我编写了一个将2提高到给定幂的函数。我想使用64位整数。在R中,bit64软件包的最大和最小限制如下:
来自R
:
> bit64::lim.integer64()
integer64
[1] -9223372036854775807 9223372036854775807
这是-(2^63)
和2^63
。
但是,由于某种原因,我的Rcpp
代码只能将2^62
传递回R。这是我的函数的代码,该代码将2提高到给定的幂(注意:我使用移位来实现这一点):
[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
中运行它时,我无法获得最大可能/极限值!
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
我错过了这里的东西吗?请提出建议,并感谢您的宝贵时间。
我的快速猜测(证明是正确的):最大值自身可能是标记为NA的那个。因此,计算该值的“一减”并尝试。
快速猜测:最大值可能是为NA标记的最大值。因此,计算该值的“一减”,然后尝试]
// [[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;
}
我将其添加到您的代码中并运行它会产生:
R> largeval()
C++ value: 9223372036854775806
integer64
[1] 9223372036854775806
R>
下面的完整代码,以防万一。
#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()
*/