[Rcpp-devel] integer arrays as arguments for a module function

Sat, Aug 13, 2011 12:52 AM

I didn't find anything too relevant in the unit tests.  I got a bit stuck,
and asked a question on StackOverflow here:
http://stackoverflow.com/questions/7048888/stdvectorstdstring-to-char-array
.

One answer seemed particularly promising (as shown by the working demo:
http://ideone.com/U6QZ5 ).

However, when I put this code into a module, I get an error:
error: argument of type ?char* (Foo::)(const std::string&)? does not match
?char* (Foo::*)(const std::basic_string<char>&)?

Here's the example I'm working with.  Commenting out the std::transform and
it compiles fine.  Why does it work OK when not in a module, but fails when
in a module?

R code:
inc <- paste(readLines('tests/convertCharExample.txt.cpp'),collapse="\n")
fx <- cxxfunction( signature(), "" , include = inc, plugin = "Rcpp" )
a <- Module( "foo_mod", getDynLib(fx) )
b <- new(a$Foo,1:5)
b$convertExample()

convertCharExample.txt.cpp code:
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <iterator>
#include <cstring>

class Foo {
public:
  Foo(IntegerVector tail) {
    this->tail = tail;
  }

  char *convert(const std::string & s)
  {
    char *pc = new char[s.size()+1];
    std::strcpy(pc, s.c_str());
    return pc;
  }

  int convertExample() {
       std::vector<std::string>  vs;
       vs.push_back("std::string");
       vs.push_back("std::vector<std::string>");
       vs.push_back("char*");
       vs.push_back("std::vector<char*>");
       std::vector<char*>  vc;

       std::transform(vs.begin(), vs.end(), std::back_inserter(vc),
convert);

       for ( size_t i = 0 ; i < vc.size() ; i++ )
            std::cout << vc[i] << std::endl;

       for ( size_t i = 0 ; i < vc.size() ; i++ )
            delete [] vc[i];
       return 0;
  }
private:
  IntegerVector tail;
};

RCPP_MODULE(foo_mod){
using namespace Rcpp ;
 class_<Foo>( "Foo" )
          .constructor<IntegerVector>()
          .method( "convertExample", &Foo::convertExample ,"")
 ;
}

On Fri, Aug 12, 2011 at 5:31 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

On 12 August 2011 at 16:59, Chris DuBois wrote:
| Point taken: STL looks like the way to go in general.
|
| In my particular example, however, the arrays get immediately cast to
another
| structure foo (somebody else's code).  So I need to cast from
IntegerVector to
| int[] before they get cast to foo[].  What you suggested works perfectly
(and
| makes sense in hindsight).
|
| Is the story identical for char[]?  Where x is a CharacterVector, I
tried
|
| char* a = x.begin();
|
| and got
| error: cannot convert ?Rcpp::Vector<16>::iterator? to ?char*? in
| assignment

char can be a pain. It is something C and C++ didn't get quite right by
lacking a base type for string.  Doing it in plain C (as in *argv[] from
main()) is a pain but doable.

CharacterVector works as the equivalent of std::vector< std::string >. Out
of
each element (ie string) you can extract the underlying char* but you may
have to rely on strcmp etc.  Remember that you may be dealing with pointers
of pointers...

Have a peek at the unitTests/ directory to see if you find something.

Hope this helps,  Dirk

| Any help much appreciated.
| Chris
|
| On Fri, Aug 12, 2011 at 3:19 PM, Dirk Eddelbuettel <edd at debian.org>
wrote:
|
|
|     On 12 August 2011 at 14:50, Chris DuBois wrote:
|     | Hi all,
|     |
|     | I'm trying to figure out how to pass in an array of integers to a
|     function
|     | inside a module.  For example, adding the following function to
|     runit.Module.R
|     | works fine:
|     |
|     |         int bla3( IntegerVector x ) {
|     |               return sum(x);
|     |         }
|     |
|     | However, I need to pass an int array, rather than an IntegerVector.
|      Using int
|     | x[] in the arguments doesn't compile (though I'm unfamiliar with
C++ in
|     | general, so maybe this shouldn't work anyway).
|     |
|     | Alternatively, should I just cast x from an IntegerVector to an int
|     array?  I
|     | tried various permutations of as, vector, <int>, etc, and would
like to
|     learn
|     | the proper way of doing this.
|
|     You generally do not want old school x[] arrays in C++. Why?  Because
STL
|     vectors do _everything_ they do at (essentially) zero added cost,
free you
|     from malloc/free and still allow you to access the straight memory
should
|     you
|     need to (to talk to a C API, say).
|
|     So use IntegerVector for _the interface_. You can the, if you must,
do
|
|         IntegerVector x;
|
|         int a1[] = x.begin();     // STL-style iterator to beginning of
|     memory
|         int *a2  = x.begin();     // idem
|
|     Hope this helps,  Dirk
|
|     --
|     Two new Rcpp master classes for R and C++ integration scheduled for
|     New York (Sep 24) and San Francisco (Oct 8), more details are at
|     http://dirk.eddelbuettel.com/blog/2011/08/04#
|     rcpp_classes_2011-09_and_2011-10
|     http://www.revolutionanalytics.com/products/training/public/
|     rcpp-master-class.php
|
|

--
Two new Rcpp master classes for R and C++ integration scheduled for
New York (Sep 24) and San Francisco (Oct 8), more details are at

http://dirk.eddelbuettel.com/blog/2011/08/04#rcpp_classes_2011-09_and_2011-10

http://www.revolutionanalytics.com/products/training/public/rcpp-master-class.php

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[Rcpp-devel] integer arrays as arguments for a module function

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