如何防止双值被截断

How to prevent double values from getting truncated?

本文关键字：何防止      更新时间：2023-10-16

这是一个可复制的C++程序：

#include <RInside.h>
#include <Rcpp.h>
int main (int argc, char *argv[])
{
    RInside R (argc, argv);
    SEXP ans = R.parseEval ("m <- 40.702147");
    Rcpp::NumericVector v (ans);           
    for (int i=0; i< v.size(); i++) 
    {           
        std::cout << "In C++ element " << i << " is " << v[i] << std::endl;
    }
    return 0;
}

Makefile在这里：

## -*- mode: make; tab-width: 8; -*-
##
## Simple Makefile
##
## TODO: 
##  proper configure for non-Debian file locations,   [ Done ]
##  allow RHOME to be set for non-default R etc
## comment this out if you need a different version of R, 
## and set set R_HOME accordingly as an environment variable
R_HOME :=       $(shell R RHOME)
sources :=      $(wildcard *.cpp)
programs :=         $(sources:.cpp=)

## include headers and libraries for R 
RCPPFLAGS :=        $(shell $(R_HOME)/bin/R CMD config --cppflags)
RLDFLAGS :=         $(shell $(R_HOME)/bin/R CMD config --ldflags)
RBLAS :=        $(shell $(R_HOME)/bin/R CMD config BLAS_LIBS)
RLAPACK :=      $(shell $(R_HOME)/bin/R CMD config LAPACK_LIBS)
## if you need to set an rpath to R itself, also uncomment
#RRPATH :=      -Wl,-rpath,$(R_HOME)/lib
## include headers and libraries for Rcpp interface classes
RCPPINCL :=         $(shell echo 'Rcpp:::CxxFlags()' | $(R_HOME)/bin/R --vanilla --slave)
RCPPLIBS :=         $(shell echo 'Rcpp:::LdFlags()'  | $(R_HOME)/bin/R --vanilla --slave)

## include headers and libraries for RInside embedding classes
RINSIDEINCL :=      $(shell echo 'RInside:::CxxFlags()' | $(R_HOME)/bin/R --vanilla --slave)
RINSIDELIBS :=      $(shell echo 'RInside:::LdFlags()'  | $(R_HOME)/bin/R --vanilla --slave)
## compiler etc settings used in default make rules
CXX :=          $(shell $(R_HOME)/bin/R CMD config CXX)
CPPFLAGS :=         -Wall $(shell $(R_HOME)/bin/R CMD config CPPFLAGS)
CXXFLAGS :=         $(RCPPFLAGS) $(RCPPINCL) $(RINSIDEINCL) $(shell $(R_HOME)/bin/R CMD config CXXFLAGS)
LDLIBS :=       $(RLDFLAGS) $(RRPATH) $(RBLAS) $(RLAPACK) $(RCPPLIBS) $(RINSIDELIBS)
all:            $(programs)
            @test -x /usr/bin/strip && strip $^
run:            $(programs)
            @for p in $(programs); do echo; echo "Running $$p:"; ./$$p; done
clean:
            rm -vf $(programs)
            rm -vrf *.dSYM
runAll:
            for p in $(programs); do echo "Running $$p"; ./$$p; done

该程序的输出：

In C++ element 0 is 40.7021

问题是值40.702147被"截断"为40.7021
我要满的
出路是什么？

值本身没有被截断（a），cout输出流使用默认值进行输出。

您可以查看iomanip标头以找到强制使用特定输出格式的方法，例如：

std::cout << std::setprecision (9) << v[i] << 'n';

这个完整的程序在这里：

#include <iostream>
#include <iomanip>
int main (void) {
    double d = 40.702147;
    std::cout << d << 'n';
    std::cout << std::setprecision (9) << d << 'n';
    return 0;
}

输出：

40.7021
40.702147

（a）：请记住，如果不能准确地表示为IEEE754双精度值，那么它可能会被截断，但这里的情况并非如此-双精度大约有十五位小数，而你只看到六位。

您可以通过以下方式指定std::cout精度：

std::cout.precision(16);

或使用

std::cout << std::setprecision (9) << val;

正如paxdiablo和Joachim建议的

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