# STL random_shuffle for permutations

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The STL also contains random sampling and shuffling algorithms. We start by looking at `random_shuffle`

.

There are two forms. The first uses an internal RNG with its own seed; the second form allows for a function object conformant to the STL’s requirements (essentially, given `N`

produce a uniform draw greater or equal to zero and less than `N`

). This is useful for us as it lets us tie this to the same RNG which R uses.

#include <Rcpp.h> // wrapper around R's RNG such that we get a uniform distribution over // [0,n) as required by the STL algorithm inline int randWrapper(const int n) { return floor(unif_rand()*n); } // [[Rcpp::export]] Rcpp::NumericVector randomShuffle(Rcpp::NumericVector a) { // already added by sourceCpp(), but needed standalone Rcpp::RNGScope scope; // clone a into b to leave a alone Rcpp::NumericVector b = Rcpp::clone(a); std::random_shuffle(b.begin(), b.end(), randWrapper); return b; }

We can illustrate this on a simple example or two:

a <- 1:8 set.seed(42) randomShuffle(a) [1] 1 4 3 7 5 8 6 2 set.seed(42) randomShuffle(a) [1] 1 4 3 7 5 8 6 2

By tieing the STL implementation of the random permutation to the RNG from R, we are able to compute reproducible permutations, fast and from C++.

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