std::ranges::sample - cppreference.com (original) (raw)

1. Selects M = min(n, last - first) elements from the sequence [first, last) (without replacement) such that each possible sample has equal probability of appearance, and writes those selected elements into the range beginning at out.

The algorithm is stable (preserves the relative order of the selected elements) only if I models std::forward_iterator.

The behavior is undefined if out is in [first, last).

1. 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 algorithm function objects (informally known as niebloids), that is:

• Explicit template argument lists cannot be specified when calling 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 inhibited.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

An iterator equal to out + M, that is the end of the resulting sample range.

[edit] Complexity

Linear: 𝓞(last - first).

[edit] Notes

This function may implement selection sampling or reservoir sampling.

[edit] Possible implementation

struct sample_fn { template<std::input_iterator I, std::sentinel_for S, std::weakly_incrementable O, class Gen> requires (std::forward_iterator or std::random_access_iterator) && std::indirectly_copyable<I, O> && std::uniform_random_bit_generator<std::remove_reference_t> O operator()(I first, S last, O out, std::iter_difference_t n, Gen&& gen) const { using diff_t = std::iter_difference_t; using distrib_t = std::uniform_int_distribution; using param_t = typename distrib_t::param_type; distrib_t D{};   if constexpr (std::forward_iterator) { // this branch preserves "stability" of the sample elements 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 or std::random_access_iterator) && std::indirectly_copyable<ranges::iterator_t, O> && std::uniform_random_bit_generator<std::remove_reference_t> O operator()(R&& r, O out, ranges::range_difference_t n, Gen&& gen) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(out), n, std::forward(gen)); } };   inline constexpr sample_fn sample {};

[edit] Example

#include #include #include #include #include #include   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 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 }

[edit] See also

| | randomly re-orders elements in a range(algorithm function object)[edit] | | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | selects N random elements from a sequence (function template) [edit] |