std::random_shuffle, std::shuffle

 

C++

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Algorithm library

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Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
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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)
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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 random number generator is implementation-defined, but 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 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++.

First version
template< class RandomIt > void random_shuffle( RandomIt first, RandomIt last ) { typename std::iterator_traits<RandomIt>::difference_type i, n; n = last - first; for (i = n-1; i > 0; --i) { using std::swap; swap(first[i], first[std::rand() % (i+1)]); // rand() % (i+1) isn't 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. } }
Second version
template<class RandomIt, class RandomFunc> void random_shuffle(RandomIt first, RandomIt last, RandomFunc&& r) { typename std::iterator_traits<RandomIt>::difference_type i, n; n = last - first; for (i = n-1; i > 0; --i) { using std::swap; swap(first[i], first[r(i+1)]); } }
Third version
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; diff_t n = last - first; for (diff_t i = n-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 <random>
#include <algorithm>
#include <iterator>
#include <iostream>
#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

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)