2013-01-15-a-second-boost-example.md

title	author	license	tags	summary	layout	src
A second example of using Boost	Dirk Eddelbuettel	GPL (>= 2)	basics boost	This post shows how to use some string functionality from Boost	post	2013-01-15-a-second-boost-example.cpp

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 initially wrote this post on a machine with Boost in a standard system location. So stuff just works. Others may have had to install Boost from source, and into a non-standard location, which may have required an -I flag, not unlike how we initially added the C++11 flag in this post before the corresponding plugin was added.

This is now automated thanks to the BH package which, if installed, provides Boost headers for use by R in compilations just like this one.

{% highlight cpp %} // We can now use the BH package // [[Rcpp::depends(BH)]]

#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 lexicalCast(std::vectorstd::string v) {

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

for (unsigned 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;

} {% endhighlight %}

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:

{% highlight r %} v <- c("1.23", ".4", "1000", "foo", "42", "pi/4") lexicalCast(v) {% endhighlight %}

[1]    1.23    0.40 1000.00      NA   42.00      NA