R help - Apr 2007 - Problem with length of array while calling C from R

Hello,

my problem is that I don't know how long must be an array of double while
calling C from R.

R-Code:
> array <- c(1,1,1)
> save <- .C ( "Ctest" , a = array )
C-Code:
void Ctest ( double *array )
{ ...
  array = (double*) realloc ( array , new_number * sizeof(double) ) ;
  ...
}

The length of "array" will be compute in C.

At the end save$a has a length of 3 and not the length of the allocated array in
C.

What can I do?

Thank you in advance.

Sven Kn?ppel

>>>>> "Sven" == Sven Kn?ppel <sven.knueppel at
reilich.net>
>>>>>     on Tue, 24 Apr 2007 13:53:09 +0200 writes:
    Sven> Hello,
    Sven> my problem is that I don't know how long must be an array of
double while calling C from R.

    Sven> R-Code:
    >> array <- c(1,1,1)
    >> save <- .C ( "Ctest" , a = array )

    Sven> C-Code: void Ctest ( double *array ) { ...  array     Sven>
(double*) realloc ( array , new_number *
    Sven> sizeof(double) ) ; ...  }

    Sven> The length of "array" will be compute in C.

    Sven> At the end save$a has a length of 3 and not the length of the
allocated array in C.

    Sven> What can I do?

Either you learn to use  .Call()  where you pass whole R
objects, and can use  length(.) on them in your C code,

or, simpler in this case, but much less powerful in general,
you change your R code to

   ss <- .C("Ctest", a = myarray, n = length(myarray))

and the C code to

    void Ctest (double *array, int *n) { 
         .........
    }

and then make use of  *n  inside the C code.

Martin Maechler, ETH Zurich