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std::ranges::shuffle

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
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 ranges::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>

      ranges::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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] Parameters

first, last - the range of elements to shuffle randomly
r - the range of elements to shuffle randomly
gen - the random number generator.

[edit] Return value

An iterator equal to last.

[edit] Complexity

Exactly (last - first) - 1 swaps.

[edit] 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{}; 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::move(gen));
      }
};
 
inline constexpr shuffle_fn shuffle{};

[edit] 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

[edit] See also

generates the next greater lexicographic permutation of a range of elements
(niebloid) [edit]
generates the next smaller lexicographic permutation of a range of elements
(niebloid) [edit]
(until C++17)(C++11)
randomly re-orders elements in a range
(function template) [edit]