std::ranges::shuffle

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Execution support library (C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Constrained algorithms

All names in this menu belong to namespace std::ranges

Non-modifying sequence operations

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

Modifying sequence operations

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

Partitioning operations

is_partitioned    partition_point    partition    partition_copy    stable_partition

Sorting operations

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

Binary search operations (on sorted ranges)

lower_bound         upper_bound         binary_search      equal_range

Set operations (on sorted ranges)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

Heap operations

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

Minimum/maximum operations

max max_element         min min_element         minmax minmax_element     clamp

Permutation operations

is_permutation      next_permutation    prev_permutation

Fold operations

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

Numeric operations iota (C++23)             Random number generation generate_random (C++26)

Operations on uninitialized storage

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

Return types

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (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 [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 (C++20)	generates the next greater lexicographic permutation of a range of elements (niebloid)
ranges::prev_permutation (C++20)	generates the next smaller lexicographic permutation of a range of elements (niebloid)
random_shuffleshuffle (until C++17)(C++11)	randomly re-orders elements in a range (function template)