[Rd] .Call and .C arguments

[Duncan Murdoch]

On 29/03/2010 7:56 AM, Roger Bergande wrote:
> ---------- 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.
>   

What makes you think those two numbers are different?  See FAQ 7.31.

Duncan Murdoch

>
>
> Is there a way to pass the arguments differently?
>
>
>
> I'm using Windows and Visual Studio C++ 2005 Express Edition and
> R-2.10.1.
>
>
>
>
>
> Please see the two simple examples to understand the issue.
>
>
>
> # C call from R
>
> .C("myroundC",as.double(1204.245))
>
>
>
>
>
> // C Code
>
>
>
> void myroundC(double *Amount){
>
>
>
>             *Amount = Rf_fround(*Amount,2);
>
>
>
> }
>
>
>
> #Return value in R
>
> [[1]]
>
> [1] 1204.24
>
>
>
>
>
>
>
> # C call from R
>
> .Call("myroundCall",as.double(1204.245))
>
>
>
> // 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);
>
>             UNPROTECT(1);
>
>             return(ans);
>
> }
>
>
>
> #Return value in R
>
> [1] 1204.24
>
>
>
> # expected value 1204.25
>
>
>
>
>
> Thanks a lot for your help.
>
> Best regards
>
> Roger Bergande
>
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