std::ranges::shuffle
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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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 );
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(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 );
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(2) | (since C++20) |
Reorders the elements in the target range [first, last) or r such that each possible permutation of those elements has equal probability of appearance. The source of randomness is gen.
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.
Parameters
| first, last | - | the iterator-sentinel pair defining the target range |
| r | - | the target range |
| gen | - | the random number generator |
Return value
The past the end iterator of the target range.
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::next(ranges::begin(r), ranges::end(r)),
std::forward<Gen>(gen));
}
};
inline constexpr shuffle_fn shuffle{};
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Example
Run this code
#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
(until C++17)(C++11) |
randomly re-orders elements in a range (function template) |
(C++20) |
generates the next greater lexicographic permutation of a range of elements (algorithm function object) |
(C++20) |
generates the next smaller lexicographic permutation of a range of elements (algorithm function object) |