如何防止双值被截断？

Question

---

这是一个可重现的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

---

该计划的输出:

In C++ element 0 is 40.7021

---

问题是该值40.702147被"截断"为40.7021.
我想要它充实.
出路是什么？

Answer 1

值本身未被截断^(a),cout输出流使用输出的默认值.

您可以查看iomanip标题以查找强制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:

#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,如果它不能完全表示为IEEE754双精度值,它可能会被截断但在这里不是这种情况 - 双精度具有大约15个十进制数字的精度,而你只看到6个.