c++ - Handling List in Rcpp -

i trying take rcpp::charactermatrix , convert each row own element within rcpp::list.

however, function have written has odd behavior every entry of list corresponds last row of matrix. why so? pointer related concept? please explain.

function:

#include <rcpp.h> using namespace rcpp; // [[rcpp::export]] list char_expand_list(charactermatrix a) {   charactervector b(a.ncol());    list output;    for(int i=0;i<a.nrow();i++) {     for(int j=0;j<a.ncol();j++) {       b[j] = a(i,j);     }      output.push_back(b);   }    return output; } 

test matrix:

this matrix a passed above function.

mat = structure(c("a", "b", "c", "a", "b", "c", "a", "b", "c"), .dim = c(3l, 3l))

 > mat          [,1] [,2] [,3]     [1,] "a"  "a"  "a"      [2,] "b"  "b"  "b"      [3,] "c"  "c"  "c" 

output:

above function should take matrix input , return list of rows of matrix so:

> char_expand_list(mat) [[1]] [1] "a" "a" "a"  [[2]] [1] "b" "b" "b"  [[3]] [1] "c" "c" "c" 

but instead getting different:

> char_expand_list(mat) [[1]] [1] "c" "c" "c"  [[2]] [1] "c" "c" "c"  [[3]] [1] "c" "c" "c" 

now there reproducible example...

it very, very, bad use .push_back() on rcpp data types end copying , fro ever expanding object rcpp data types hide r object must recreated. result, should use std::list std::vector<t> type t (i.e. std::string). @ end, use rcpp::wrap return correct object.

e.g.

// [[rcpp::export]] rcpp::list char_expand_list(rcpp::charactermatrix a) {   std::vector<std::string> b(a.ncol());    std::list<std::vector<std::string> > o;    for(int i=0;i<a.nrow();i++) {     for(int j=0;j<a.ncol();j++) {       b[j] = a(i,j);     }      o.push_back(b);   }    return rcpp::wrap(o); } 

alternatively, aim keep rcpp::list, declare size expecting head of time , still use std::vector<t> element.

e.g.

// [[rcpp::export]] rcpp::list char_expand_list(rcpp::charactermatrix a) {   std::vector<std::string> b(a.ncol());    rcpp::list o(a.nrow());    for(int i=0;i<a.nrow();i++) {     for(int j=0;j<a.ncol();j++) {       b[j] = a(i,j);     }      o[i] = b;   }    return o; } 

regarding main issue though, list attribute able store last pointer location. hence, repeated lines of "c" throughout list.