Le 15/11/10 21:15, Romain Francois a écrit :
Hello,
Since people have whisperred about Rcpp, I'd like to play too.
On 11/15/2010 07:45 AM, Patrick Leyshock wrote:
Very helpful, thank you.
A couple other questions, please:
1. I've got a function written in C, named "my_c_function". In my R
code I call this function, passing to it an INTSXP and a STRSXP,
respectively:
result <- .Call("my_c_function", int_vector, str_vector)
The prototype of "my_c_function" is:
SEXP my_c_function(SEXP int_vec, SEXP str_vec);
Within my_c_function I am able to extract the values within the integer
vector, e.g. I can grab the first value with these lines of code:
int extracted_value;
extracted_value = *INTEGER(int_vec);
What I cannot figure out how to do is extract the value from the
STRSXP. I'm assuming that I can create a pointer to a character array,
then malloc enough memory to hold the value. Is there an analogous
operation on "INTEGER" for STRSXPs?
STRING_ELT(str_vec, 0)
gets the 0th component of str_vec, which is a CHARSXP, i.e., an SEXP for
a character string. The char* can be retrieved with CHAR, so the usual
paradigm is
const char *x = CHAR(STRING_ELT(str_vec, 0));
note the const-ness of the char* -- it's not mutable, because R is
managing char * memory.
The converse action, of assigning to an element, is
SET_STRING_ELT(str_vec, 0, mkChar("foo"));
mkChar() is creating a copy (if necessary) of "foo", managing it, and
returning a CHARSXP. Working through protection (which will likely be
your next obstacle ;) in this last example is a good exercise.
In Rcpp, you would wrap up the STRSXP into a CharacterVector and then
pull things in and out using indexing:
Rcpp::CharacterVector aladdin(str_vec) ;
std::string first = aladdin[0] ;
aladdin[0] = "foobar" ;
As it was pointed out to me offlist, the code above fails to compile.
This is what you get when you reply to emails while watching a Woody
Allen movie ...
Anyway, the reason for this is that the line:
std::string first = aladdin[0] ;
uses the __constructor__ of std::string and not the assignment operator.
The code below does work :
std::string first ;
Rcpp::CharacterVector aladdin(str_vec) ;
first = aladdin[0] ;
aladdin[0] = "foobar" ;
Romain
--
Romain Francois
Professional R Enthusiast
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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