# Re: [R] float and double precision with C code

From: jim holtman <jholtman_at_gmail.com>
Date: Thu, 17 Jul 2008 08:56:01 -0400

"What Every Computer Scientist Should Know About Floating-Point Arithmetic", ACM Computing Surveys, 23/1, 5–48, also available via http://docs.sun.com/source/806-3568/ncg_goldberg.html.

On Thu, Jul 17, 2008 at 8:47 AM, JS Ubei <jsubei_at_yahoo.fr> wrote:
> Hi all,
>
> There is a mistake for wich I need your ligths :
>
> ///////////////////////////////////////
>
> I have a small C code :
>
> SEXP testData()
> {
> SEXP result;
> void * rans;
> float * my_data;
> int my_data_length;
>
> my_data_length = 2;
> my_data = new float[my_data_length];
>
> my_data[0] = 29.958334;
> my_data[1] = 29.875;
>
> PROTECT(result = allocVector(REALSXP, my_data_length));
> rans = (void *)REAL(result);
>
> for(int i=0; i < my_data_length; i++) ((double *)rans)[i] = (double)(my_data[i]);
>
> Rprintf("C value #1 : %f\n", ((double *)rans)[0]);
> Rprintf("C value #2 : %f\n", ((double *)rans)[1]);
>
> delete(my_data);
>
> UNPROTECT(1);
> return(result);
> }
> ///////////////////////////////////////
>
> And the R corresponding function :
>
> testData <- function()
> {
> result <- .Call("testData", PACKAGE="my_package")
> print(paste("R value #1 :", result[1]))
> print(paste("R value #2 :", result[2]))
> return(result)
> }
>
> ///////////////////////////////////////
> And in R console, after compilation :
>
>> my_result <- testData()
> C value #1 : 29.958334
> C value #2 : 29.875000
> [1] "R value #1 : 29.9583339691162"
> [1] "R value #2 : 29.875"
>
>> my_result[1] == 29.958334
> [1] FALSE
>
> ???
>
> How can I do to conserve my values ?
>
> Regards,
>
>
>
> _______________________________________________________________
> e http://mail.yahoo.fr
>
> ______________________________________________
> R-help_at_r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> and provide commented, minimal, self-contained, reproducible code.
>

```--
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?

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