std::ranges::sample
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> or
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 out, std::iter_difference_t<I> n, Gen&& gen ); |
(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 out, ranges::range_difference_t<R> n, Gen&& gen ); |
(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 niebloids, that is:
- Explicit template argument lists may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
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 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
#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 (niebloid) |
|
(C++17)
|
selects n random elements from a sequence (function template) |
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