NAME¶

std::ranges::shuffle - std::ranges::shuffle

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::random_access_iterator I, std::sentinel_for<I> S, class
Gen >
(since
requires std::permutable<I> && (1) C++20)
std::uniform_random_bit_generator<std::remove_reference_t<Gen>>

I shuffle( I first, S last, Gen&& gen );
template< ranges::random_access_range R, class Gen >

requires std::permutable<ranges::iterator_t<R>> && (since
std::uniform_random_bit_generator<std::remove_reference_t<Gen>> (2) C++20)
ranges::borrowed_iterator_t<R>

shuffle( R&& r, Gen&& gen );

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¶

// 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¶

ranges::next_permutation generates the next greater lexicographic permutation of a
(C++20) range of elements
(niebloid)
ranges::prev_permutation generates the next smaller lexicographic permutation of a
(C++20) range of elements
(niebloid)
random_shuffle
shuffle randomly re-orders elements in a range
(until C++17) (function template)
(C++11)