Using Boost's foreach macro

Kevin Ushey — written Jan 30, 2013 — source

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 this post was initially written) 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;
// We can now use the BH package
// [[Rcpp::depends(BH)]]

// 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)
[1]   1   4  NA  16 NaN Inf Inf

And a quick benchmark:

x <- rnorm(1E5)
Unit: microseconds
      expr   min     lq median    uq  max neval
 square(x)  70.0  70.78     71  72.0 1498   100
       x^2 151.5 152.11    153 561.9 2022   100
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.

tags: basics  boost 

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