std::ranges:: sample

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(C++17)

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All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
Set operations (on sorted ranges)
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Minimum/maximum operations
Permutation operations
Fold operations
Operations on uninitialized storage
Return types
Defined in header <algorithm>
Call signature
(1) (since C++20)
(2) (since C++20)
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 ) .
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 algorithm function objects (informally known as niebloids ), that is:

Parameters

first1, last1 - the range from which to make the sampling ( the population )
r - the range from which to make the sampling ( the population )
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 sample range.

Complexity

Linear : 𝓞 ( last - first ) .

Notes

This function may implement selection sampling or reservoir sampling .

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_reference_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 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<R> or std::random_access_iterator<O>) &&
              std::indirectly_copyable<ranges::iterator_t<R>, O> &&
              std::uniform_random_bit_generator<std::remove_reference_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

(C++20)
randomly re-orders elements in a range
(algorithm function object)
(C++17)
selects N random elements from a sequence
(function template)
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