[Rd] .C and .Call: convolve example not working

[Thomas Lumley]

On Fri, Aug 12, 2011 at 1:06 AM, Lars Wißler <jahftw at googlemail.com> wrote:
> Dear R users,
>
> I want to call C code via the .C or .Call interface. This works fine
> with integer values but using doubles the array received in C will be
> set to zeros.
>
> I have tried the convolve examples (Writing R extensions, chapter 5.2)
> and still the resulting array consists of zeros.
>
> My code (shortened for my purposes. Original did not work either):
> ------------------------------------------------------------
> convolve.r
>
> a=array(1, c(4))
> b=array(3, c(4))
>
> print(a)
> print(b)
>
> .C("convolve",
>    as.double(a),
>    as.double(b))
>
> ----------------------------------------------------------------
> convolve.c
>
> void convolve(double* a, double* b){
>     int i;
>
>     for(i=0;i<4;i++) Rprintf("a: %d", a[i]);
>     for(i=0;i<4;i++) Rprintf("b: %d", b[i]);
> }


The C code here is wrong for two reasons.  Firstly, there's no
declaration for Rprintf(), because you don't include the header file.

Secondly, you're using %d to print, which means you are telling the
compiler you're passing ints to Rprintf(), but you are actually
passing doubles.

When I fix these problems the code works for me.

    -thomas

Thomas Lumley
Professor of Biostatistics
University of Auckland