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std::random_shuffle, std::shuffle

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:

#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)^[1]	required

Overload (3) has the same defect, but that part of the resolution is not applicable to C++98.

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)