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[Rcpp-devel] How to set a default value when call a R function in Rcpp

6 messages · Jingyu He, Dirk Eddelbuettel, Qiang Kou

Original
Jingyu He
# 
Hi,


I'm writing a R package by Rcpp and I have a problem when setting default
value when call a R function in Rcpp. Here is an example to pass a R
function to C++, provided by http://adv-r.had.co.nz/Rcpp.html

#include <Rcpp.h>using namespace Rcpp;// [[Rcpp::export]]RObject
callWithOne(Function f) {
  return f(1);}

The question is, how to set the default value for f? I would like to have
the choice to pass a function or not, like

callWithOne(Function f = NULL) {
  // if pass f, run f(1)
  // else, run something else}


Thanks!

Best,
Jingyu He
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Dirk Eddelbuettel
#
On 11 November 2016 at 05:16, Jingyu He wrote:
| I'm writing a R package by Rcpp and I have a problem when setting default value
| when call a R function in Rcpp. Here is an example to pass a R function to C++,
| provided by?http://adv-r.had.co.nz/Rcpp.html
| 
| #include <Rcpp.h>
| using namespace Rcpp;
| // [[Rcpp::export]]
| RObject callWithOne(Function f) {
|   return f(1);
| }
| 
| The question is, how to set the default value for f? I would like to have the
| choice to pass a function or not, like
| 
| callWithOne(Function f = NULL) {
|   // if pass f, run f(1)
|   // else, run something else
| }


1) Don't crosspost. If you decided to post on StackOverflow, stay on
StackOverflow. Now that you posted here, maybe delete the question on
StackOverflow.

2) I don't understand the question.  Wouldn't the following do:


#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
RObject callWithOne(Function f) {
  const int arg = 1;
  return f(arg);
}


If it is a constant, as you seem to imply, why not make it one?

But if it is a variable, just pass it down.

Dirk
Jingyu He
# 
Hi,

I've deleted the post on StackOverflow. Sorry for the ambiguity, the
problem is not arg, but the Function f.


#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
RObject cppfunction(Function rfunction = NULL) {
      if( rfunction == NULL){
      cout << 1 + 1 << endl;  // if we don't pass rfunction to cppfunction,
run something not related to rfunction()
      }else{
     rfunction()  // if we pass rfunction to cppfunction, run it
      }
}

rfunction is an argument of cppfunction. Sometimes I need to pass rfunction
to cppfunction sometimes not. So I need to set a default rfunction
(something like NULL indicating not passing it).

But
   RObject cppfunction(Function rfunction = NULL)
doesn't work. How can I make it?

Thanks!

Best,
Jingyu
On Thu, Nov 10, 2016 at 11:41 PM Dirk Eddelbuettel <edd at debian.org> wrote:

        
On 11 November 2016 at 05:16, Jingyu He wrote:

        
| I'm writing a R package by Rcpp and I have a problem when setting default
value
| when call a R function in Rcpp. Here is an example to pass a R function
to C++,
| provided by http://adv-r.had.co.nz/Rcpp.html
|
| #include <Rcpp.h>
| using namespace Rcpp;
| // [[Rcpp::export]]
| RObject callWithOne(Function f) {
|   return f(1);
| }
|
| The question is, how to set the default value for f? I would like to have
the
| choice to pass a function or not, like
|
| callWithOne(Function f = NULL) {
|   // if pass f, run f(1)
|   // else, run something else
| }


1) Don't crosspost. If you decided to post on StackOverflow, stay on
StackOverflow. Now that you posted here, maybe delete the question on
StackOverflow.

2) I don't understand the question.  Wouldn't the following do:


#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
RObject callWithOne(Function f) {
  const int arg = 1;
  return f(arg);
}


If it is a constant, as you seem to imply, why not make it one?

But if it is a variable, just pass it down.

Dirk

--
http://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
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      Dirk Eddelbuettel
    

    

      
      #
    

  


  

      
On 11 November 2016 at 05:53, Jingyu He wrote:

        
| I've deleted the post on StackOverflow. Sorry for the ambiguity, the problem is
| not arg, but the Function f.?
| 
| 
| #include <Rcpp.h>
| using namespace Rcpp;
| // [[Rcpp::export]]
| RObject cppfunction(Function rfunction = NULL) {
| ? ? ? if( rfunction == NULL){
| ? ? ? cout << 1 + 1 << endl; ?// if we don't pass rfunction to cppfunction, run
| something not related to rfunction()?
| ? ? ? }else{
| ? ? ?rfunction() ?// if we pass rfunction to cppfunction, run it
| ? ? ? }
| }
| 
| rfunction is an argument of cppfunction. Sometimes I need to pass rfunction to
| cppfunction sometimes not. So I need to set a default rfunction (something like
| NULL indicating not passing it).?
| 
| But?
| ? ?RObject cppfunction(Function rfunction = NULL)?
| doesn't work. How can I make it?

Well that is a different question :) But it has a nice answer: use

   Rcpp::Nullable<Rcpp::Function> rfunction

and test via

   if (rfunction.isNull())

(Untested, but should work)

Dirk

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Qiang Kou
    

    

      
      #
    

  


  

      
Hi, Jingyu,

I post a small example below. I think this is what you want.

Best,

KK

#include <Rcpp.h>

using namespace Rcpp;

//[[Rcpp::export]]
void test(Nullable<Function> f = R_NilValue) {
  if (f.isNotNull()) {
    Rcpp::Rcout << "Not NULL fun" << std::endl;
    Function f2(f);
    f2();
  } else {
    Rcout << "NULL fun" << std::endl;
  }
}

/* R code

            

      
      
      
    

        
+ print("test fun")
+ }

            

      
      
      
    

        
Not NULL fun
[1] "test fun"

            

      
      
      
    

        
NULL fun
*/

        
On Fri, Nov 11, 2016 at 7:18 AM, Dirk Eddelbuettel <edd at debian.org> wrote:

        

            

      
      
      
    

        

      
      
    

        

  
    

      
      
    

  


  


      

    

    

      
      

        


  

        

      Jingyu He
    

    

      
      #
    

  


  

      
It works, thanks a lot!
Qiang Kou <qkou at umail.iu.edu>?2016?11?11? ??09:07???

            

      
      
      
    

        

      
      
    

        
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