std::ranges::shuffle
Defined in header <algorithm> |
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| Call signature | ||
template< std::random_access_iterator I, std::sentinel_for<I> S, class Gen >
requires std::permutable<I> &&
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
I shuffle( I first, S last, Gen&& gen ); |
(1) | (since C++20) |
template< ranges::random_access_range R, class Gen >
requires std::permutable<ranges::iterator_t<R>> &&
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
ranges::borrowed_iterator_t<R> shuffle( R&& r, Gen&& gen ); |
(2) | (since C++20) |
1) Reorders the elements in the given range
[first, last) such that each possible permutation of those elements has equal probability of appearance.
2) Same as
(1), but uses
r as the 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
| first, last | - | the range of elements to shuffle randomly |
| r | - | the range of elements to shuffle randomly |
| gen | - | the random number generator |
Return value
An iterator equal to last.
Complexity
Exactly (last - first) - 1 swaps.
Possible implementation
struct shuffle_fn {
template<std::random_access_iterator I, std::sentinel_for<I> S, class Gen>
requires std::permutable<I> &&
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
I operator()( I first, S last, Gen&& gen ) const {
using diff_t = std::iter_difference_t<I>;
using distr_t = std::uniform_int_distribution<diff_t>;
using param_t = typename distr_t::param_type;
distr_t D;
const auto n{ last - first };
for (diff_t i{1}; i < n; ++i) {
ranges::iter_swap(first + i, first + D(gen, param_t(0, i)));
}
return ranges::next(first, last);
}
template<ranges::random_access_range R, class Gen>
requires std::permutable<ranges::iterator_t<R>> &&
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>
ranges::borrowed_iterator_t<R> operator()( R&& r, Gen&& gen ) const {
return (*this)(ranges::begin(r), ranges::end(r), std::forward<Gen>(gen));
}
};
inline constexpr shuffle_fn shuffle{};
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Example
#include <algorithm>
#include <array>
#include <iostream>
#include <random>
void print(const auto& a) {
for (const auto e : a) { std::cout << e << ' '; }
std::cout << "\n";
}
int main()
{
std::array a{'A', 'B', 'C', 'D', 'E', 'F'};
print(a);
std::random_device rd;
std::mt19937 gen{rd()};
for (int i{}; i != 3; ++i) {
std::ranges::shuffle(a, gen);
print(a);
}
}
Possible output:
A B C D E F F E A C D B E C B F A D B A E C F D
See also
|
(C++20)
|
generates the next greater lexicographic permutation of a range of elements (niebloid) |
|
(C++20)
|
generates the next smaller lexicographic permutation of a range of elements (niebloid) |
|
(until C++17)(C++11)
|
randomly re-orders elements in a range (function template) |
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https://en.cppreference.com/w/cpp/algorithm/ranges/shuffle
