# Using Boost’s foreach macro

January 30, 2013
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(This article was first published on Rcpp Gallery, and kindly contributed to R-bloggers)

Boost provides a macro, BOOST_FOREACH, that allows us to easily iterate over elements in a container, similar to what we might do in R with sapply. In particular, it frees us from having to deal with iterators as we do with std::for_each and std::transform. The macro is also compatible with the objects exposed by Rcpp.

Side note: C++11 has introduced a similar for-each looping construct of the form

for (T &elem : X) { /*do stuff*/ } 

However, CRAN does not (at the time of this posting) allow C++11 in uploads and hence this Boost solution might be preferred if you want to use a for-each construct in a package.

The BOOST_FOREACH macro is exposed when we use #include <boost/foreach.hpp>. Make sure the Boost libraries are in your includepath so that they can be found and included easily. Because it’s a header-only library we don’t have to worry about external dependencies or linking.

We’ll use a simple example where we square each element in a vector.

#include <Rcpp.h>
#include <boost/foreach.hpp>
using namespace Rcpp;

// the C-style upper-case macro name is a bit ugly; let's change it
// note: this could cause compiler errors if it conflicts with other includes
#define foreach BOOST_FOREACH

// [[Rcpp::export]]
NumericVector square( NumericVector x ) {

// elem is a reference to each element in x
// we can re-assign to these elements as well
foreach( double& elem, x ) {
elem = elem*elem;
}

return x;
}

square( 1:10 )

 [1]   1   4   9  16  25  36  49  64  81 100

square( matrix(1:16, nrow=4) )

     [,1] [,2] [,3] [,4]
[1,]    1   25   81  169
[2,]    4   36  100  196
[3,]    9   49  121  225
[4,]   16   64  144  256

## we check that the function handles various 'special' values
x <- c(1, 2, NA, 4, NaN, Inf, -Inf)
square(x)

[1]   1   4  NA  16 NaN Inf Inf


And a quick benchmark:

x <- rnorm(1E5)
library(microbenchmark)
microbenchmark(
square(x),
x^2
)

Unit: microseconds
expr    min     lq median     uq  max
1 square(x)  71.04  71.64  71.89  74.14 1518
2       x^2 346.36 350.70 359.18 433.33 1842

all.equal( square(x), x^2 )

[1] TRUE


If you are defining your own classes / containers and want them to be compatible with one of these for-each constructs, you will need to define some methods for iteration across these objects. See this post on SO for more details.

For more information on BOOST_FOREACH, check the documentation here.