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std::random_shuffle, std::shuffle

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< cpp‎ | algorithm
Revision as of 08:54, 9 June 2013 by P12 (Talk | contribs)

 
 
 
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.

Contents

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.

Return value

(none)

Complexity

Linear in the distance between first and last

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))]);
    }
}

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);
 
    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

See also

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