[Rd] .Call and .C arguments

Roger Bergande bergarog at gmail.com
Mon Mar 29 13:56:14 CEST 2010

---------- Forwarded message ----------
From: Roger.Bergande at swisslife.ch
Date: Mon, 29 Mar 2010 13:51:12 +0200
Subject: .Call and .C arguments
To: bergarog at gmail.com

Dear List

My question is about .C and .Call

I was experimenting with the .C and .Call interface as I came across the
following behaviour.

The passed values are not the same in C.  I 'm calling a function in C
with the argument as.double(1204.245) but in the debug mode in C the
value has changed to 1204.2449999999999999.

Is there a way to pass the arguments differently?

I'm using Windows and Visual Studio C++ 2005 Express Edition and

Please see the two simple examples to understand the issue.

# C call from R


// C Code

void myroundC(double *Amount){

            *Amount = Rf_fround(*Amount,2);


#Return value in R


[1] 1204.24

# C call from R


// C Code

SEXP myroundCall(SEXP a){

            double *ap = REAL(a), *ansp;

            SEXP ans;

            PROTECT(ans = allocVector(REALSXP, 1));

            ansp = REAL(ans);

            *ansp = Rf_fround(*ap,2);




#Return value in R

[1] 1204.24

# expected value 1204.25

Thanks a lot for your help.

Best regards

Roger Bergande

More information about the R-devel mailing list