[Rcpp-devel] Trailing Index issues, handled as pointer

[Mario Deng]

Hello everyone,

I am trying to implement a data structure using Rcpp. But I am already struggling with my first method.
How things (should) work: I want to my function ("createVectorList") a data frame or matrix. Each column of the matrix gets sorted. This also holds for the rownames - storing the sorted rownames for each sort column gives me big memory footprint, but log(n) access while reading later.
Basically everything is fine, but when accessing some data through a for loop I get "address 0x0, cause 'memory not mapped'".

Here is my Code:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
List createVectorList(NumericMatrix df, CharacterVector rownames) {
  List sorted_rownames(df.ncol());
  for(int i = 0; i < df.ncol(); i++){
    NumericVector sorted_vec = df(_,i);
    std::sort(sorted_vec.begin(), sorted_vec.end());
    NumericVector one_col = df(_,i);
    df(_,i)=sorted_vec;
    IntegerVector matched = match( sorted_vec, one_col);
    CharacterVector one_sorted_row(df.nrow());    
    for(int j = 0; j < one_sorted_row.length(); j++){
      int match_id=matched[j];
      String r_name=rownames[match_id];
      one_sorted_row[j]=r_name;
    }
    sorted_rownames[i]=one_sorted_row;
  }
  return Rcpp::List::create(Rcpp::Named("Matrix") = df, Rcpp::Named("rowIDX")=sorted_rownames);
}

There trouble is within these lines:

    for(int j = 0; j < one_sorted_row.length(); j++){
      int match_id=matched[j];
      String r_name=rownames[match_id];
      one_sorted_row[j]=r_name;
    }

When executing this, I get the error named above. 
What I thought about: rownames should be of the type "Proxy" or so, maybe thats the point?

Maybe one of you has a hint.

With beste regards,

Mario 
__
MD
mariodeng at googlemail.com