| title | author | license | tags | summary | layout | src |
|---|---|---|---|---|---|---|
STL transform + remove_copy for subsetting |
Dirk Eddelbuettel |
GPL (>= 2) |
stl featured |
The STL's transform and remove_copy functions can subset |
post |
2012-12-29-stl-transform-for-subsetting.cpp |
We have seen the use of the STL transform functions in the posts STL transform and Transforming a matrix. We use the same logic in conjuction with a logical (ie boolean) vector in order subset an initial vector.
{% highlight cpp %} #include <Rcpp.h>
using namespace Rcpp; using namespace std;
const double flagval = DBL_MIN; // works //const double flagval = NA_REAL; // does not
// simple double value 'flagging' function inline double flag(double a, bool b) { return b ? a : flagval; }
// [[Rcpp::export]] NumericVector subsetter(NumericVector a, LogicalVector b) { // We use the flag() function to mark values of 'a' // for which 'b' is false with the 'flagval' transform(a.begin(), a.end(), b.begin(), a.begin(), flag);
// We use sugar's sum to compute how many true values to expect
NumericVector res = NumericVector(sum(b));
// And then copy the ones different from flagval from a into
// res using the remove_copy function from the STL
remove_copy(a.begin(), a.end(), res.begin(), flagval);
return res;
} {% endhighlight %}
We can illustrate this on a simple example or two:
{% highlight r %} a <- 1:5 subsetter(a, a %% 2 == 0) {% endhighlight %}
[1] 2 4
{% highlight r %} subsetter(a, a > 2) {% endhighlight %}
[1] 3 4 5
Casual benchmarking (not shown) shows this to be comparable to and
even slightly faster than basic indexing in R itself.