[Rcpp-devel] Pass a function name as an argument.

Hi Xiao,

The problem is that sourceCpp can only accept arguments of types that can
be converted to R using Rcpp::as (this is detailed in the help topic for
sourceCpp and in the Rcpp vignettes). Plain C function pointers
aren't convertible in this fashion so the compilation fails.

If your intention is to pass an R function to C++ you can however use the
Rcpp::Function type in your signature. This is explained in more detail in
this Rcpp Gallery article:

http://gallery.rcpp.org/articles/r-function-from-c++/

J.J.

On Sat, May 18, 2013 at 1:31 PM, Xiao He <praguewatermelon at gmail.com>wrote:

Hi everyone, I have two questions regarding passing a function name as an
argument.

(1). Suppose that I have a function foo() shown below. The function takes
a NumericVector and a function pointer that points to a function that takes
a NumericVector and returns a double. Note that the function pointer will
only point to functions not exposed to R.

double foo(NumericVector x, double (*f)(NumericVector x) ){
double output;
 output=(*f)(x);
return output;
}

When I try to compile it as is using sourceCpp(), it's fine, but when I
add "// [[Rcpp::export]]" above the function definition, I get an error
message:

Error in sourceCpp("foo.cpp") :
  Error 1 occurred building shared library.
foo.cpp:229: error: expected initializer before ?SEXP?

So I wonder how I can fix this mistake.


(2). Imagine a more complex scenario: suppose there are two functions
available to be passed to foo(). But the two functions differ in the number
of arguments each has (see fun1() and fun2() below). I wonder if there is
any way to deal with this.

double fun1(NumericVector x){
double total=0;
 for(int i=0;i<x.size();i++)
total+=x(i);
 return total/10;
}


double fun2(NumericVector x, int n){
double total=0;
 for(int i=0;i<x.size();i++)
total+=x(i)+add;
 return total/n;
}
 Thank you in advance!


Best,
-Xiao