std::random_shuffle, std::shuffle

 

C++

Language

Standard library headers

Concepts

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

 

Algorithm library

Functions
Non-modifying sequence operations
all_of any_of none_of (C++11) (C++11) (C++11) for_each count count_if mismatch equal	find find_if find_if_not (C++11) find_end find_first_of adjacent_find search search_n
Modifying sequence operations
copy copy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) fill fill_n transform generate generate_n remove remove_if remove_copy remove_copy_if	replace replace_if replace_copy replace_copy_if swap swap_ranges iter_swap reverse reverse_copy rotate rotate_copy random_shuffle shuffle   (C++11) unique unique_copy
Partitioning operations
is_partitioned (C++11) partition partition_copy (C++11)	stable_partition partition_point (C++11)
Sorting operations (on sorted ranges)
is_sorted (C++11) is_sorted_until (C++11) sort	partial_sort partial_sort_copy stable_sort nth_element
Binary search operations (on sorted ranges)
lower_bound upper_bound	binary_search equal_range
Set operations (on sorted ranges)
merge inplace_merge includes	set_difference set_intersection set_symmetric_difference set_union
Heap operations
is_heap is_heap_until (C++11) make_heap	push_heap pop_heap sort_heap
Minimum/maximum operations
max max_element min min_element minmax (C++11) minmax_element (C++11)	lexicographical_compare is_permutation (C++11) next_permutation prev_permutation
Numeric operations
iota (C++11) accumulate inner_product	adjacent_difference partial_sum
C library
qsort	bsearch

 

Defined in header <algorithm>
template< class RandomAccessIterator > void random_shuffle( RandomAccessIterator first, RandomAccessIterator last );	(1)
template< class RandomAccessIterator, class RandomNumberGenerator > void random_shuffle( RandomAccessIterator first, RandomAccessIterator last,                      RandomNumberGenerator& r ); template< class RandomAccessIterator, class RandomNumberGenerator > void random_shuffle( RandomAccessIterator first, RandomAccessIterator last,                      RandomNumberGenerator&& r );	(2)	(until C++11) (since C++11)
template< class RandomAccessIterator, class UniformRandomNumberGenerator > void shuffle( RandomAccessIterator first, RandomAccessIterator last,               UniformRandomNumberGenerator&& 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 random number generator is implementation-defined, the function std::rand is often used.

2) The random number generator is the function object r.

3) The random number generator is the function object g.

Parameters

first, last	-	the range of elements to shuffle randomly
r	-	function object returning a randomly chosen value of type iterator_traits<RandomAccessIterator>::difference_type in the interval [0,n) if invoked as r(n)
g	-	function object returning a randomly chosen value of type UniformRandomNumberGenerator::result_type in the interval [g.min(), g.max()] if invoked as g() (intended to be used with uniform random number generators from the standard library)

Return value

(none)

Complexity

linear in the distance between first and last

Possible implementation

First version
template<class RandomAccessIterator, class RandomNumberGenerator> void random_shuffle(RandomAccessIterator first, RandomAccessIterator last, RandomNumberGenerator&& r) { typename std::iterator_traits<RandomAccessIterator>::difference_type i, n; n = last - first; for (i = n-1; i > 0; --i) { std::swap(first[i], first[r(i+1)]); } }
Second version
template<class RandomAccessIterator, class UniformRandomNumberGenerator> void shuffle(RandomAccessIterator first, RandomAccessIterator last, UniformRandomNumberGenerator&& g) { typedef typename std::iterator_traits<RandomAccessIterator>::difference_type diff_t; typedef typename std::make_unsigned<diff_t>::type udiff_t; typedef typename std::uniform_int_distribution<udiff_t> distr_t; typedef typename distr_t::param_type param_t;   distr_t D; diff_t n = last - first; for (diff_t i = n-1; i > 0; --i) { std::swap(first[i], first[D(g, param_t(0, i))]); } }

Example

The following code randomly shuffles the integers 1..10

run this code

#include <random>
#include <algorithm>
#include <iterator>
#include <iostream>
 
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);
 
    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

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)