Difference between revisions of "cpp/algorithm/random shuffle"

Latest revision as of 23:49, 27 March 2024

Reorders the elements in the given range [first, last) such that each possible permutation of those elements has equal probability of appearance.

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

[edit] Parameters

[edit] Complexity

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

[edit] 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))]);
    }
}

[edit] 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.

[edit] 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

[edit] 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.

[edit] See also