Question

---

This is a reproducible C++ program:

#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’s here:

## -*- 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

---

Output of this program:

In C++ element 0 is 40.7021

---

The problem is that the value 40.702147 is getting “truncated” to 40.7021.
I want it full.
What is the way out?

Answer 1

The value itself is not being truncated ^(a), the cout output stream is using the defaults for output.

You can look into the iomanip header to find ways to force a specific output format, such as:

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

This complete program here:

#include <iostream>
#include <iomanip>

int main (void) {
    double d = 40.702147;
    std::cout << d << '\n';
    std::cout << std::setprecision (9) << d << '\n';
    return 0;
}

outputs:

40.7021
40.702147

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^(a): Keep in mind it may be truncated if it cannot be represented exactly as an IEEE754 double precision value but that’s not the case here – doubles have about fifteen decimal digits of precision and you’re only seeing six.