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std::random_shuffle, std::shuffle

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
random_shuffle
(until C++17)

Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Permutations
Numeric operations
C library
 
Defined in header <algorithm>
template< class RandomIt >
void random_shuffle( RandomIt first, RandomIt last );
(1) (deprecated in C++14)
(removed in C++17)
(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)
(deprecated in C++14)
(removed in C++17)
template< class RandomIt, class URBG >
void shuffle( RandomIt first, RandomIt last, URBG&& 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

[edit] 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 UniformRandomBitGenerator whose result type is convertible to std::iterator_traits<RandomIt>::difference_type
Type requirements
-
RandomIt must meet the requirements of ValueSwappable and LegacyRandomAccessIterator.
-
std::remove_reference_t<URBG> must meet the requirements of UniformRandomBitGenerator.

[edit] Return value

(none)

[edit] Complexity

Linear in the distance between first and last

[edit] Notes

Note that the implementation is not dictated by the standard, so even if you use exactly the same RandomFunc or URBG you may get different results with different standard library implementations.

[edit] 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)]);
        // rand() % (i+1) isn't actually correct, because the generated number
        // is not uniformly distributed for most values of i. A correct implementation
        // will need to essentially reimplement C++11 std::uniform_int_distribution,
        // which is beyond the scope of this example.
    }
}
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 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;
    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:

#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:

8 6 10 4 2 3 7 1 9 5

[edit] 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]