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

[Xiao He]

Awesome. Thank you Baptiste!

On Sat, May 18, 2013 at 4:32 PM, baptiste auguie
<baptiste.auguie at gmail.com>wrote:

> 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
>>>>
>>>>
>>>> _______________________________________________
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>>>> 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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