Namespaces
Variants
Views
Actions

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

From cppreference.com
< cpp‎ | algorithm
(Undo revision 99207 by 92.223.67.192 (talk))
m (Text replace - "{{concept" to "{{named req")
Line 35: Line 35:
 
{{par | first, last | the range of elements to shuffle randomly}}
 
{{par | first, last | the range of elements to shuffle randomly}}
 
{{par | r | function object returning a randomly chosen value of type convertible to  {{c|std::iterator_traits<RandomIt>::difference_type}} in the interval [0,n) if invoked as {{tt|r(n)}} }}
 
{{par | r | function object returning a randomly chosen value of type convertible to  {{c|std::iterator_traits<RandomIt>::difference_type}} in the interval [0,n) if invoked as {{tt|r(n)}} }}
−
{{par | g | a {{concept|UniformRandomBitGenerator}} whose result type is convertible to {{c|std::iterator_traits<RandomIt>::difference_type}} }}
+
{{par | g | a {{named req|UniformRandomBitGenerator}} whose result type is convertible to {{c|std::iterator_traits<RandomIt>::difference_type}} }}
 
{{par hreq}}
 
{{par hreq}}
 
{{par req concept | RandomIt | RandomAccessIterator | ValueSwappable}}
 
{{par req concept | RandomIt | RandomAccessIterator | ValueSwappable}}

Revision as of 14:22, 15 June 2018

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
random_shuffleshuffle
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
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

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

Template:par req concept Template:par req concept

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)]);
        // 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))]);
    }
}

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);
 
    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

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]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/random_shuffle&oldid=103190"