std::random_shuffle, std::shuffle

 

C++

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Symbols index

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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
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)	findfind_iffind_if_not (C++11) find_end find_first_of
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
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is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)
C library
qsort	bsearch

 

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.

3) The source of randomness is the object g.

Parameters

first, last	-	the range of elements to shuffle randomly
r	-	function object returning a randomly chosen value of type convertible to std::iterator_traits<RandomIt>::difference_type in the interval [​0​, n) if invoked as r(n)
g	-	a UniformRandomBitGenerator whose result type is convertible to std::iterator_traits<RandomIt>::difference_type
Type requirements
- RandomIt must meet the requirements of ValueSwappable and LegacyRandomAccessIterator.
- std::remove_reference_t<URBG> must meet the requirements of UniformRandomBitGenerator.

Return value

(none)

Complexity

Linear in the distance between first and last.

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.

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. A correct implementation // will need to essentially reimplement C++11 std::uniform_int_distribution, // which is beyond the scope of this example. } }
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
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))]); } }

Example

The following code randomly shuffles the integers 1~10:

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)