std::ranges::sample
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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template< std::input_iterator I, std::sentinel_for<I> S,
std::weakly_incrementable O, class Gen >
requires (std::forward_iterator<I> || std::random_access_iterator<O>) &&
std::indirectly_copyable<I, O> &&
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
O sample( I first, S last, O d_first,
std::iter_difference_t<I> count, Gen&& gen );
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(1) | (since C++20) |
template< ranges::input_range R, std::weakly_incrementable O, class Gen >
requires (ranges::forward_range<R> || std::random_access_iterator<O>) &&
std::indirectly_copyable<ranges::iterator_t<R>, O> &&
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
O sample( R&& r, O d_first, ranges::range_difference_t<R> count, Gen&& gen );
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(2) | (since C++20) |
Randomly copies count different elements from the source range [first, last) or r to the destination range beginning at d_first, such that each possible combination has equal probability of appearance. The source of randomness is gen.
If count is greater than ranges::distance(first, last) or ranges::distance(r), all elements in the source range will be copied.
The algorithm is stable (preserves the relative order of the selected elements) only if I or ranges::iterator_t<R> models forward_iterator.
If out is in the source range, the behavior is undefined.
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
Parameters
| first, last | - | the iterator-sentinel pair defining the source range |
| r | - | the source range |
| d_first | - | the beginning of the destination range |
| count | - | the sample size |
| gen | - | the random number generator |
Return value
The past-the-end iterator of the destination range.
Complexity
1) Linear in
ranges::distance(first, last).2) Linear in
ranges::distance(r).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 d_first,
std::iter_difference_t<I> count, 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 (count = ranges::min(count, rest); count != 0; ++first)
if (D(gen, param_t(0, --rest)) < count)
{
*out++ = *first;
--count;
}
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 != count; ++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 < count)
out[i] = *first;
}
return out + sample_size;
}
}
template<ranges::input_range R>
auto get_end(R&& r)
{
return ranges::end(r);
}
template<ranges::forward_range R>
auto get_end(R&& r)
{
return ranges::next(ranges::begin(r), ranges::end(r));
}
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 d_first,
ranges::range_difference_t<R> count, Gen&& gen) const
{
return (*this)(ranges::begin(r), get_end(r),
std::move(out), count, std::forward<Gen>(gen));
}
};
inline constexpr sample_fn sample{};
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Example
Run this code
#include <algorithm>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <random>
#include <vector>
void print(const auto& rem, const auto& v)
{
std::cout << rem << " = [" << std::size(v) << "] { ";
for const auto& 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++17) |
selects N random elements from a sequence (function template) |
(C++20) |
randomly re-orders elements in a range (algorithm function object) |