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std::ranges::shuffle

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
(C++11)
(C++11)
(C++17)

 
Constrained algorithms
All names in this menu belong to namespace std::ranges
       
       
    
     
         
       
       
(C++23)
(C++23)  
(C++23)
(C++23)  
(C++23)            
 
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)
1) Reorders the elements in the given range [firstlast) 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 algorithm function objects (informally known as niebloids), that is:

[edit] Parameters

first, last - the iterator-sentinel pair defining 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 {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 {};

[edit] 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

[edit] See also

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