[R] More questions about R extension programming

[Thomas Lumley]

On Fri, 4 Oct 2003, Rajarshi Guha wrote:

> On Fri, 2003-10-03 at 17:01, Rajarshi Guha wrote:
> > Hi,
> >   I'm using a package that has a number of formats. I have C code to
> > parse these formats the results of which are generally integer arrays.
> >
> > I would like to utilize these modules in R rather than writing R code to
> > read in these files (and also to learn about R extensions).
>
> Thanks for the pointers to the above question. I have a few more!
>
> 1) I would like my C function to be passed a character string. Thus I
> define the function as
>
> SEXP _loadsets(SEXP filename)
> {
>     FILE *f;
>
>     PROTECT(filename = AS_CHARACTER(filename));
>     f = fopen(filename,"r");
>     UNPROTECT(1);
> .....
> .....
> }
>
> However compiling it gives me:
>
> loadset.c: In function `_loadsets':
> loadset.c:25: warning: passing arg 1 of `fopen' from incompatible
> pointer type
> gcc -shared -L/usr/local/lib -o loadset.so loadset.o
>
> How can I coerce/convert the SEXP type to a char*?

Your code can't possibly be right, because it defined filename as SEXP and
passes it to a function that accepts const char *.

You want

fopen(CHARACTER(STRING_ELT(filename,0)), "r")

Since filename is a vector of strings (with one element) you need
STRING_ELT to extract one of the strings. Now you have a SEXP and need
CHARACTER() to extract a pointer to the actual chars, just as I used
INTEGER() to extract a pointer to the actual ints in an integer vector
SEXP.

> 2) The function returns a list object whose elements themselves are
> lists.

No. It returns a list whose elements are integer vectors.

>		 Is there any way I can make those elements arrays rather than
> lists?

It's probably easiest to do this manipulation in R afterwards

>
> 3) I'm a little puzzled since I allocate a list (say length = 2) object
> by
>
> alist = allocVector(VECSXP,2);
>
> and use the same syntax for a vector object. From what I understand a
> vector is the same as a list. Is this true?

No (or yes, but not in the way you mean).  A list contains vectors or
lists as elements.  A vector contains numbers or strings.  The first
element to allocVector says what sort of thing you are allocating.

> 4) When writing C code does it make sense to differentiate between a
> list object and an array object? Or is it better to simply coerce the
> returned list objects (via as.array()) from within R.

It is often but not always sensible to do all the manipulation of
complicated attributes in R.  I think you mean array(), not as.array().

	-thomas


Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle