FreeBSD Manual Pages
std::ranges::sample(3) C++ Standard Libary std::ranges::sample(3) NAME std::ranges::sample - std::ranges::sample Synopsis Defined in header <algorithm> Call signature template< std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O, class Gen > requires (std::forward_iterator<I> || std::random_access_itera- tor<O>) && std::indirectly_copyable<I, O> && (1) (since C++20) std::uniform_random_bit_generator<std::remove_reference_t<Gen>> O sample( I first, S last, O out, std::iter_difference_t<I> n, Gen&& gen ); template< ranges::input_range R, std::weakly_incrementable O, class Gen > requires (ranges::forward_range<R> || std::random_access_itera- tor<O>) && (since std::indirectly_copyable<ranges::iterator_t<R>, O> && (2) C++20) std::uniform_random_bit_generator<std::remove_reference_t<Gen>> O sample( R&& r, O out, ranges::range_difference_t<R> n, Gen&& gen ); 1) Selects M = min(n, last - first) elements from the sequence [first, last) (without replacement) such that each possible sample has equal prob- ability of appearance, and writes those selected elements into the range begin- ning at out. The algorithm is stable (preserves the relative order of the se- lected elements) only if I models std::forward_iterator. The behavior is undefined if out is in [first, last). 2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as first, and ranges::end(r) as last. The function-like entities described on this page are niebloids, that is: * Explicit template argument lists cannot be specified when call- ing any of them. * None of them are visible to argument-dependent lookup. * When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is in- hibited. In practice, they may be implemented as function objects, or with special compiler extensions. Parameters first1, last1 - the range from which to make the sampling (the popu- lation) r - the range from which to make the sampling (the popu- lation) out - the output iterator where the samples are written n - number of samples to take gen - the random number generator used as the source of randomness Return value An iterator equal to out + M, that is the end of the resulting sam- ple range. Complexity Linear: (last - first). Notes This function may implement selection sampling or reservoir sam- pling. Possible implementation struct sample_fn { template<std::input_iterator I, std::sentinel_for<I> S, std::weakly_incrementable O, class Gen> requires (std::forward_iterator<I> or std::random_access_iterator<O>) && std::indirectly_copyable<I, O> && std::uniform_random_bit_generator<std::remove_refer- ence_t<Gen>> O operator()(I first, S last, O out, std::iter_difference_t<I> n, Gen&& gen) const { using diff_t = std::iter_difference_t<I>; using distrib_t = std::uniform_int_distribution<diff_t>; using param_t = typename distrib_t::param_type; distrib_t D{}; if constexpr (std::forward_iterator<I>) { // this branch preserves "stability" of the sample ele- ments auto rest{ranges::distance(first, last)}; for (n = ranges::min(n, rest); n != 0; ++first) if (D(gen, param_t(0, --rest)) < n) { *out++ = *first; --n; } return out; } else { // O is a random_access_iterator diff_t sample_size{}; // copy [first, first + M) elements to "random access" output for (; first != last && sample_size != n; ++first) out[sample_size++] = *first; // overwrite some of the copied elements with randomly selected ones for (auto pop_size{sample_size}; first != last; ++first, ++pop_size) { const auto i{D(gen, param_t{0, pop_size})}; if (i < n) out[i] = *first; } return out + sample_size; } } template<ranges::input_range R, std::weakly_incrementable O, class Gen> requires (ranges::forward_range<R> or std::random_access_itera- tor<O>) && std::indirectly_copyable<ranges::iterator_t<R>, O> && std::uniform_random_bit_generator<std::remove_refer- ence_t<Gen>> O operator()(R&& r, O out, ranges::range_difference_t<R> n, Gen&& gen) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(out), n, std::forward<Gen>(gen)); } }; inline constexpr sample_fn sample {}; Example // Run this code #include <algorithm> #include <iomanip> #include <iostream> #include <iterator> #include <random> #include <vector> void print(auto const& rem, auto const& v) { std::cout << rem << " = [" << std::size(v) << "] { "; for (auto const& e : v) std::cout << e << ' '; std::cout << "}\n"; } int main() { const auto in = {1, 2, 3, 4, 5, 6}; print("in", in); std::vector<int> out; const int max = in.size() + 2; auto gen = std::mt19937{std::random_device{}()}; for (int n{}; n != max; ++n) { out.clear(); std::ranges::sample(in, std::back_inserter(out), n, gen); std::cout << "n = " << n; print(", out", out); } } Possible output: in = [6] { 1 2 3 4 5 6 } n = 0, out = [0] { } n = 1, out = [1] { 5 } n = 2, out = [2] { 4 5 } n = 3, out = [3] { 2 3 5 } n = 4, out = [4] { 2 4 5 6 } n = 5, out = [5] { 1 2 3 5 6 } n = 6, out = [6] { 1 2 3 4 5 6 } n = 7, out = [6] { 1 2 3 4 5 6 } See also ranges::shuffle randomly re-orders elements in a range (C++20) (niebloid) sample selects N random elements from a sequence (C++17) (function template) http://cppreference.com 2024.06.10 std::ranges::sample(3)
NAME | Synopsis | Parameters | Return value | Complexity | Notes | Possible implementation | Example | Possible output: | See also
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=std::ranges::sample&sektion=3&manpath=FreeBSD+Ports+15.1.quarterly>