Question

The case

I have compiled shared library for dyn.loading into R. Then when I call the wrapper function in R I get result rounded with truncated part after decimal point. I am probably missing something well known as it is my first try to use .C function, but I can’t find what is the cause of rounding. There is my code below.

My C code

#include <freesteam/steam_pT.h>
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <R.h>

// gcc -Wall -fPIC -c Rsteam_h_pT_2.c -o  Rsteam_h_pT_2.o -I/usr/local/include -I/opt/R-2.14.0/lib/R/include -L/usr/local/lib -lfreesteam
// gcc -Wall -shared -o Rsteam_h_pT_2.so Rsteam_h_pT_2.o -I/usr/local/include -I/opt/R-2.14.0/lib/R/include -L/usr/local/lib -lfreesteam

// function
double steam_h_pT(double p, double T){
SteamState S = freesteam_set_pT(p, T);
double s = freesteam_h(S);
return s;
}


// wrapper
double steam_h_pT(double, double);
void steam_h_pT_R_wrapper(double *p, double *T, double *result) { *result = steam_h_pT(*p, *T); }

My R function

steam_h_pT <- function(p_Pa,T_K)
 {
 dyn.load('Rsteam_h_pT.so')
 # Call the C function
 returned_data = .C('steam_h_pT_R_wrapper', p=as.numeric(p_Pa), T= as.numeric(T_K), result=numeric(1))
 # Return the value of the result parameter
 return(returned_data$result)
 }

My R session

> steam_h_pT(100000,400)
[1] 2730398
> str(steam_h_pT(100000,400))
num 2730398
> 

Expected result from original code

bash>./steam_h_pT 1e5 400
2730397.845968

Now, why it gets rounded in R?

Answer 1

It the representation due to the digits argument to the print function. The number is still stored correctly.

2730397 + 0.845968
[1] 2730398
print(2730397 + 0.845968, digits=15)
[1] 2730397.845968

You can set the global default with options(digits=15).