[Rcpp-devel] Using formula

[Denis Haine]

Hi,

Sorry for a beginner's question. I'm trying to call an R function 
(glm()) inside my cpp code. The code compiles with no problem, but when 
I'm running it, it cannot find the second element of the formula, i.e. 
the x in y~x. The error message is: Error in eval (predvars; data, env) 
: object 'e' not found.

However, if I return 'e', it is correctly calculated. I guess the 
formula is not correctly evaluated, but I haven't found any examples 
that could point me in one direction or another.

#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector misclass(NumericMatrix obs_mat) {
   // Obtain environment containing function
   Rcpp::Environment base("package:stats");

   // Picking up glm() and summary() function from base stats
   Rcpp::Function glm_r = base["glm"];
   Rcpp::Function sum_r = base["summary.glm"];

   Rcpp::NumericVector d = obs_mat(_, 1);
   Rcpp::NumericVector e = no_init(n);
   Rcpp::NumericVector mod_coef = no_init(n);
   // e is calculated in other section of the code
   e = as<IntegerVector>(e);
   Rcpp::List mod_pois = glm_r(_["formula"] = "d ~ e",
                     _["family"] = "poisson");
   Rcpp::List mod_sum = sum_r(mod_pois);
   Rcpp::NumericMatrix M_coef = mod_sum[12];
   mod_coef = M_coef(2, 1);

   return mod_coef;
}

I also tried providing the formula in the call, i.e. NumericVector 
misclass(NumericMatrix obs_mat, Formula f) and using it in glm_r, i.e. 
glm_r(_["formula"] = f, etc. but with the same outcome.

Thanks,

Denis
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