[Rd] using C code to create data frame but always return as list

[Yu Gong]

there is my code,  expect return value  is a data frame but R say it is list:

SEXP Julia_R_MD_NA_DataFrame(jl_value_t* Var)
{
 SEXP ans,names,rownames;
 char evalcmd[4096];
 int i;
 const char* dfname="DataFrameName0tmp";
 jl_set_global(jl_main_module, jl_symbol(dfname), (jl_value_t*)Var);
 //Get Frame cols 
 sprintf(evalcmd,"size(%s,2)",dfname);
 jl_value_t* cols=jl_eval_string(evalcmd);
 int collen=jl_unbox_long(cols);
 jl_value_t* eachcolvector;
 //Create VECSXP

 //Create SEXP for Each Column and assign
 PROTECT(ans=allocVector(VECSXP,collen));
 for (i=0;i<collen;i++)
 {
  sprintf(evalcmd,"%s[%d]",dfname,i+1);
  eachcolvector=jl_eval_string(evalcmd);
  SET_VECTOR_ELT(ans,i,Julia_R_MD_NA(eachcolvector));
 }
 //set names attribute
 sprintf(evalcmd,"names(%s)",dfname);
 jl_value_t* ret=jl_eval_string(evalcmd);
 jl_value_t* onesymbol;
 if (jl_is_array(ret))
 {
  PROTECT(names=allocVector(STRSXP,collen));
  for (i=0;i<jl_array_len(ret);i++)
  { 
   onesymbol=jl_arrayref((jl_array_t*)ret,i);
   if (jl_is_symbol(onesymbol))
    SET_STRING_ELT(names,i,mkChar(((jl_sym_t*)onesymbol)->name));
  }
  setAttrib(ans,R_NamesSymbol,names);
  UNPROTECT(1);
 } 
 //set row names
 sprintf(evalcmd,"size(%s,1)",dfname);
 jl_value_t* rows=jl_eval_string(evalcmd);
 int rowlen=jl_unbox_long(rows);
 PROTECT(rownames=allocVector(INTSXP,rowlen));
 for (i=0;i<rowlen;i++)
  INTEGER(rownames)[i]=i+1;
 setAttrib(ans,R_RowNamesSymbol,rownames);
 UNPROTECT(1);
 //set class as data frame
 setAttrib(ans,R_ClassSymbol,mkString("data.frame"));
 UNPROTECT(1);
 return ans;
}

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