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

[JS Ubei]

thank you for your quick answer,

I'm far of the digits capacity and my values are not the result of a computation. I'm developping a R package to acces a specific data source. And I need precision a few better. How can I do ? 

When I try this In R console, this is correct and  what I need :

> my_value <- 29.958334
> my_value == 29.958334
[1] TRUE


But I need to do the first operation (my_value <- 29.958334) in C


Regards,

----- Message d'origine ----
De : jim holtman <jholtman at gmail.com>
À : JS Ubei <jsubei at yahoo.fr>
Cc : r-help at r-project.org
Envoyé le : Jeudi, 17 Juillet 2008, 14h56mn 01s
Objet : Re: [R] float and double precision with C code

FAQ 7.31

Also read

"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
>
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>



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

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