# STL transform + remove_copy for subsetting

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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.

#includeusing 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; }

We can illustrate this on a simple example or two:

a <- 1:5 subsetter(a, a %% 2 == 0) [1] 2 4 subsetter(a, a > 2) [1] 3 4 5

Casual benchmarking (not shown) shows this to be comparable to and even slightly faster than basic indexing in `R`

itself.

To

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