std::random_shuffle

Defined in header <algorithm>

template< class RandomIt >
void random_shuffle( RandomIt first, RandomIt last );

(1)

(until C++17)
(deprecated in C++14)

(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)
(until C++17)
(deprecated in C++14)

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.

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 `UniformRandomNumberGenerator` whose result type is convertible to `std::iterator_traits<RandomIt>::difference_type`
Type requirements
- `RandomIt` must meet the requirements of `ValueSwappable` and `RandomAccessIterator`.
- `URNG` must meet the requirements of `UniformRandomNumberGenerator`.

Return value

(none).

Complexity

Linear in the distance between first and last.

Possible implementation

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)]);
    }
}

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 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))]);
    }
}

Example

The following code randomly shuffles the integers 1..10:

#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);
  
    std::copy(v.begin(), v.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << "\n";
}

Possible output:

1	`8 6 10 4 2 3 7 1 9 5`

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)