We introduced Boost in a first post doing some integer math. In this post we want to look at the very versatile Boost.Lexical_Cast library to convert text to numbers – see the Motivation for more.

As before, I should note that I write this post on a machine with Boost headers in a standard system location. So stuff just works. If you have to install Boost from source, and into a non-standard location, you may need to add a -I flag, not unlike how added the C++11 flag in this post .

#include <Rcpp.h>
#include <boost/lexical_cast.hpp>  	// one file, automatically found for me

using namespace Rcpp;

using boost::lexical_cast;
using boost::bad_lexical_cast;
 
// [[Rcpp::export]]
std::vector<double> lexicalCast(std::vector<std::string> v) {

    std::vector<double> res(v.size());

    for (int i=0; i<v.size(); i++) {
        try {
            res[i] = lexical_cast<double>(v[i]);
        } catch(bad_lexical_cast &) {
            res[i] = NA_REAL;
        }
    }

    return res;
}

This simple program uses the exceptions idiom we discussed to branch: when a value cannot be converted, a NA value is inserted.

We can test the example:

v <- c("1.23", ".4", "1000", "foo", "42", "pi/4")
lexicalCast(v)


[1]    1.23    0.40 1000.00      NA   42.00      NA