implementing apply function in Rcpp

c++, rcpp, syntactic-sugar

Solution

There is no sugar function for `apply`. The easiest way of doing what you want is to call `as<double>`, i.e.:

output[i]=as<double>(f(x(i,_)));

You could also embed this in a type that would call `as` for you, something like:

template <typename T>
class F {
public: 
  F( SEXP f_) : f(f_){}

  inline T operator()(NumericVector x){
    return as<T>(f(x)) ;  
  }

private:
  Function f ;
} ;

so that you could do:

// [[Rcpp::export]]
NumericVector apply_cpp(NumericMatrix x,int dim,F<double> f){
  if(dim==1){
    NumericVector output(x.nrow());
    for(int i=0;i<x.nrow();i++){
      output[i]=f(x(i,_));
    } 
    return output ;
  }
  else {
    NumericVector output(x.ncol());

    for(int i=0;i<x.ncol();i++){
      output[i]=f(x(_,i));
    }  
    return output ;
  }
} 

The `F` template from above assumes that the function takes a `NumericVector` and returns something that can be converted to a `double`. You could also embed type information about both inputs and outputs. Something like this (expressed in C++11):

template <typename T, typename... Args>
class F {
public: 
  F( SEXP f_) : f(f_){}

  inline T operator()(Args... args){
    return as<T>(f(args...)) ;  
  }

private:
  Function f ;
} ;

Then the signature would become:

// [[Rcpp::export]]
NumericVector apply_cpp(NumericMatrix x,int dim,F<double,NumericVector> f){

Problem

Original source