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

[Xiao He]

Hi JJ,

Thank you for the reply. I would actually like to pass C++ functions to the
C++ function foo(). To be more specific, the user will indicate in the R
environment which of the two functions (fun1 or fun2) to pass to foo().
foo() will somehow use the chosen one accordingly. I suppose one option is
to wrap foo() inside another function. The wrapper function can take input
from the user from R, and then based on what the input is choose the
appropriate function (fun1 or fun2). But the question of how to pass
functions with different numbers of arguments still remains.  I wonder if
you have any thoughts. Thank you again!

Best,
Xiao




On Sat, May 18, 2013 at 12:45 PM, JJ Allaire <jj.allaire at gmail.com> wrote:

> 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
>>
>>
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>> Rcpp-devel at lists.r-forge.r-project.org
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>>
>
>
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