std::random_shuffle, std::shuffle

 

C++

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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)

 

Defined in header <algorithm>
template< class RandomIt > void random_shuffle( RandomIt first, RandomIt last );	(1)	(deprecated in C++14) (removed in C++17)
	(2)
template< class RandomIt, class RandomFunc > void random_shuffle( RandomIt first, RandomIt last, RandomFunc& r );			(until C++11)
template< class RandomIt, class RandomFunc > void random_shuffle( RandomIt first, RandomIt last, RandomFunc&& r );			(since C++11) (deprecated in C++14) (removed in C++17)
template< class RandomIt, class URBG > void shuffle( RandomIt first, RandomIt last, URBG&& g );		(3)	(since C++11)

Reorders the elements in the given range [first, last) such that each possible permutation of those elements has equal probability of appearance.

1) The source of randomness is implementation-defined, but the function std::rand is often used.

2) The source of randomness is the function object r.

If any of the following conditions is satisfied, the behavior is undefined:

• The return type of r is not convertible to std::iterator_traits<RandomIt>::difference_type.
• Given a positive value n of type std::iterator_traits<RandomIt>::difference_type, the result of r(n) is not a randomly chosen value in the interval [​0​, n).

3) The source of randomness is the object g.

Given the type T as std::remove_reference_t<URBG>, if any of the following conditions is satisfied, the behavior is undefined:

• T is not a UniformRandomBitGenerator.

T::result_type is not convertible to std::iterator_traits<RandomIt>::difference_type.

(until C++20)

If the type of *first is not Swappable(until C++11)RandomIt is not ValueSwappable(since C++11), the behavior is undefined.

Parameters

first, last	-	the range of elements to shuffle randomly
r	-	function object returning a randomly chosen value
g	-	generator object returning a randomly chosen value
Type requirements
- RandomIt must meet the requirements of LegacyRandomAccessIterator.

Complexity

Exactly std::distance(first, last) - 1 swaps.

Possible implementation

See also the implementations in libstdc++ and libc++.

random_shuffle (1)
template<class RandomIt> void random_shuffle(RandomIt first, RandomIt last) { typedef typename std::iterator_traits<RandomIt>::difference_type diff_t;   for (diff_t i = last - first - 1; i > 0; --i) { using std::swap; swap(first[i], first[std::rand() % (i + 1)]); // rand() % (i + 1) is not actually correct, because the generated number is // not uniformly distributed for most values of i. The correct code would be // a variation of the C++11 std::uniform_int_distribution implementation. } }
random_shuffle (2)
template<class RandomIt, class RandomFunc> void random_shuffle(RandomIt first, RandomIt last, RandomFunc&& r) { typedef typename std::iterator_traits<RandomIt>::difference_type diff_t;   for (diff_t i = last - first - 1; i > 0; --i) { using std::swap; swap(first[i], first[r(i + 1)]); } }
shuffle (3)
template<class RandomIt, class URBG> void shuffle(RandomIt first, RandomIt last, URBG&& g) { typedef typename std::iterator_traits<RandomIt>::difference_type diff_t; typedef std::uniform_int_distribution<diff_t> distr_t; typedef typename distr_t::param_type param_t;   distr_t D; for (diff_t i = last - first - 1; i > 0; --i) { using std::swap; swap(first[i], first[D(g, param_t(0, i))]); } }

Notes

Note that the implementation is not dictated by the standard, so even if you use exactly the same RandomFunc or URBG (Uniform Random Number Generator) you may get different results with different standard library implementations.

The reason for removing std::random_shuffle in C++17 is that the iterator-only version usually depends on std::rand, which is now also discussed for deprecation. (std::rand should be replaced with the classes of the <random> header, as std::rand is considered harmful.) In addition, the iterator-only std::random_shuffle version usually depends on a global state. The std::shuffle's shuffle algorithm is the preferred replacement, as it uses a URBG as its 3rd parameter.

Example

Randomly shuffles the sequence [1, 10] of integers:

Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
#include <vector>
 
int main()
{
    std::vector<int> v{1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
 
    std::random_device rd;
    std::mt19937 g(rd());
 
    std::shuffle(v.begin(), v.end(), g);
 
    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << '\n';
}

Possible output:

8 6 10 4 2 3 7 1 9 5

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 395	C++98	the source of randomness of overload (1) was not specified, and std::rand could not be the source due to the C library requirement	it is implementation-defined, and using std::rand is allowed
LWG 552 (N2423)	C++98	r was not required to be the source of randomness of overload (2)	required

See also

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