提高Rcpp浮点输出的精度

Question

我试图在Rcpp函数的双精度输出中打印更多数字,但无法弄清楚如何...我看过如何使用cout以完全精度打印双值？通用C++答案的其他地方,但我看不出怎么做Rcpp,除了使用printf,我认为是最后的手段......

require(inline)
code <- '
    double x=1.0;
    std::cout.precision(10); // compiles but does nothing
    Rcpp::Rcout.precision(10); // compiles but does nothing
    printf("(1) %1.10lf\\n",x);  // works but bad practice
    Rcpp::Rcout << "(2) " << x << std::endl;
    Rcpp::Rcout << "(3) " << std::setprecision(10) << x << std::endl;
    return Rcpp::wrap(0);
'
fun <- rcpp(sig=c(v=0),body=code,includes="#include <iomanip>")
fun(1)
## (1) 1.0000000000
## (2) 1
## (3) 1
## [1] 0

Answer 1

查看您的链接答案,您错过了以下呼叫std::fixed:

code2 <- '
     double x=1.0;
     Rcpp::Rcout.precision(10);
     Rcpp::Rcout << "(1) " << std::fixed << x << std::endl;
     return Rcpp::wrap(0);
'
fun2 <- rcpp(sig=c(v=0),body=code2,includes="#include <iomanip>")
fun2(1)
## (1) 1.0000000000
## [1] 0

Answer 2

你总是可以走另一条路:

# next line is really one line wrapped here
R> cppFunction('std::string ben(double val) { char buf[32]; \
                                              snprintf(buf, 31, "%15.15f", val);\
                                              return std::string(buf); }')
R> ben(1/3)
[1] "0.333333333333333"
R> ben(1e6/3)
[1] "333333.333333333313931"
R> ben(1e12/3)
[1] "333333333333.333312988281250"
R> 

在此期间,@ Manetheran还向您展示了标准的iomanip路线.

当然也有Rprintf().

## the double backslash is needed only for cppFunction
R> cppFunction('void ben2(double val) { Rprintf("%15.15f\\n", val); }')
R> ben2(1e12/3)
333333333333.333312988281250
R> ben2(1e6/3)
333333.333333333313931
R> 

哦,为了记录,这些也适用于你想要的输入:

R> ben(1)
[1] "1.000000000000000"
R> ben2(1)
1.000000000000000
R>