R devel - Aug 2011 - .C and .Call: convolve example not working

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]);
}

--------------------------------------------------------------

ouput:

[1] "starting C code..."
[1] 1 1 1 1
[1] 3 3 3 3
a: 30467528
a: 0
a: 0
a: 0
b: 0
b: 0
b: 0
b: 0

Any suggestions as to why this is happening and what I am doing wrong
would be much appreciated. I have tried .Call with conversion SEXP to
double with REAL(a)/REAL(b) with the same results (the entry first
entry of a has a different number, but is huge as well). I am quite
astonished with the results I am getting.

Thanks and regards,

Lars Wissler

On Fri, Aug 12, 2011 at 1:06 AM, Lars Wi?ler <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