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# std::random_shuffle, std::shuffle

From cppreference.com
< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class RandomIt > void random_shuffle( RandomIt first, RandomIt last ); | (1) |
| (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)
template< class RandomIt, class URNG > void shuffle( RandomIt first, RandomIt last, URNG&& 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`.

### [edit] Parameters

first, last | - | the range of elements to shuffle randomly
r | - | function object returning a randomly chosen value of type convertible to `iterator_traits<RandomIt>::difference_type` in the interval [0,n) if invoked as `r(n)`
g | - | function object returning a randomly chosen value of type `URNG::result_type` in the interval [g.min(), g.max()] if invoked as `g()` (e.g. any of the uniform random number generators from `<random>`)
Type requirements
- `RandomIt` must meet the requirements of `ValueSwappable` and `RandomAccessIterator`.
- `URNG` must meet the requirements of `UniformRandomNumberGenerator`.

### [edit] Return value

(none)

### [edit] Complexity

Linear in the distance between `first` and `last`

### [edit] Possible implementation

First 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)]);
        }
    }
Second version
    template<class RandomIt, class UniformRandomNumberGenerator>
    void shuffle(RandomIt first, RandomIt last,
                 UniformRandomNumberGenerator&& g)
    {
        typedef typename std::iterator_traits<RandomIt>::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) {
            using std::swap;
            swap(first[i], first[D(g, param_t(0, i))]);
        }
    }

### [edit] 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

### [edit] See also

next_permutation | generates the next greater lexicographic permutation of a range of elements (function template) [edit]
prev_permutation | generates the next smaller lexicographic permutation of a range of elements (function template) [edit]

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