Reorders the elements in the given range[first, last) such that each possible permutation of those elements has equal probability of appearance.

Ifthe type of*first is notSwappable(until C++11)RandomIt is notValueSwappable(since C++11), the behavior is undefined.

[edit]Parameters

[edit]Complexity

Exactlystd::distance(first, last)-1 swaps.

[edit]Possible implementation

See also the implementations inlibstdc++ andlibc++.

template<class RandomIt>void random_shuffle(RandomIt first, RandomIt last){typedeftypenamestd::iterator_traits<RandomIt>::difference_type diff_t; for(diff_t i= last- first-1; i>0;--i){usingstd::swap;        swap(first[i], first[std::rand()%(i+1)]);// rand() % (i + 1) is not actually correct, because the generated number is// not uniformly distributed for most values of i. The correct code would be// a variation of the C++11 std::uniform_int_distribution implementation.}}

template<class RandomIt,class RandomFunc>void random_shuffle(RandomIt first, RandomIt last, RandomFunc&& r){typedeftypenamestd::iterator_traits<RandomIt>::difference_type diff_t; for(diff_t i= last- first-1; i>0;--i){usingstd::swap;        swap(first[i], first[r(i+1)]);}}

template<class RandomIt,class URBG>void shuffle(RandomIt first, RandomIt last, URBG&& g){typedeftypenamestd::iterator_traits<RandomIt>::difference_type diff_t;typedefstd::uniform_int_distribution<diff_t> distr_t;typedeftypename distr_t::param_type param_t;     distr_t D;for(diff_t i= last- first-1; i>0;--i){usingstd::swap;        swap(first[i], first[D(g, param_t(0, i))]);}}

[edit]Notes

Note that the implementation is not dictated by the standard, so even if you use exactly the sameRandomFunc orURBG (Uniform Random Number Generator) you may get different results with different standard library implementations.

The reason for removingstd::random_shuffle in C++17 is that the iterator-only version usually depends onstd::rand, which is now also discussed for deprecation. (std::rand should be replaced with the classes of the<random> header, asstd::rand isconsidered harmful.) In addition, the iterator-onlystd::random_shuffle version usually depends on a global state. Thestd::shuffle's shuffle algorithm is the preferred replacement, as it uses aURBG as its 3rd parameter.

[edit]Example

#include <algorithm>#include <iostream>#include <iterator>#include <random>#include <vector> 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';}

8 6 10 4 2 3 7 1 9 5

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

1. ↑Overload(3) has the same defect, but that part of the resolution is not applicable to C++98.

[edit]See also

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