std::random_shuffle, std::shuffle

Reorders the elements in the given range [ first ,  last )

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

Parameters

Complexity

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

Possible implementation

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

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.
    }
}

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)]);
    }
}

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

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

Example

#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';
}

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.

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

See also