R Rcpp std::transform fail

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This code worked in 2015 but doesn't now. It reports that there is no matching function for call to transform. Is there some issue with finding the std code now?

#include <Rcpp.h>

using namespace Rcpp;  

// [[Rcpp::export]]
NumericVector vecpow(const NumericVector base, const NumericVector exp) {
  NumericVector out(base.size());
  std::transform(base.begin(), base.end(),
                 exp.begin(), out.begin(), ::pow);
  return out;
}

/*** R
vecpow(c(0:3), c(4:1))
***/

I found a solution here that involves replacing ::pow with static_cast<double(*)(double, double)>(::pow) At least, it works with the code above.

rcpp r
1 Answers

Yes, the world around us does change every now and then.

If you consult a C++ reference you see that std::transformed has changed. I am not enough of a language lawyer to see what it dislikes about std::pow now (though I can tell you that you called the wrong (C-level) ::pow) but here are two that work by using a lambda function as in the other older answer.

Code

#include <Rcpp.h>

// [[Rcpp::export]]
std::vector<double> vecpow(const std::vector<double> base,
                           const std::vector<double> exp) {

    std::vector<double> out(base.size());
    std::transform(base.begin(), base.end(), exp.begin(), out.begin(),
                   [&](double lhs, double rhs) -> double { 
                                        return std::pow(lhs, rhs); });
    return out;
}

// [[Rcpp::export]]
Rcpp::NumericVector vecpow2(Rcpp::NumericVector base, Rcpp::NumericVector exp) {

    Rcpp::NumericVector out(base.size());
    std::transform(base.begin(), base.end(), exp.begin(), out.begin(),
                   [&](double lhs, double rhs) -> double {
                                        return std::pow(lhs, rhs); });
    return out;
}

/*** R
vecpow(0:3, 4:1)
vecpow2(0:3, 4:1)
*/

Output

> Rcpp::sourceCpp("answer.cpp")

> vecpow(0:3, 4:1)
[1] 0 1 4 3

> vecpow2(0:3, 4:1)
[1] 0 1 4 3
>
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