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

[baptiste auguie]

Hi,

I asked the same thing once, and Dirk came up with some helpful ideas and a
whipped up a demo for the gallery

http://stackoverflow.com/questions/14428687/rcpparmadillo-pass-user-defined-function

http://gallery.rcpp.org/articles/passing-cpp-function-pointers/

HTH,

baptiste

On 18 May 2013 19:00, Xiao He <praguewatermelon at gmail.com> wrote:

> 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
>>>
>>>
>>> _______________________________________________
>>> Rcpp-devel mailing list
>>> Rcpp-devel at lists.r-forge.r-project.org
>>> https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/rcpp-devel
>>>
>>
>>
>
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