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std::ranges::shuffle

Defined in header <algorithm>

Call signature

Defined in header `<algorithm>`
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)

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 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.

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 {};

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