(This article was first published on

**Rcpp Gallery**, and kindly contributed to R-bloggers)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.

`#include `
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;
}

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.

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