std::random_shuffle, std::shuffle
(on partitioned ranges)
<algorithm>
Reorders the elements in the given range [
first,
last)
such that each possible permutation of those elements has equal probability of appearance.
[
0,
n)
.T
as std::remove_reference_t <URBG>, if any of the following conditions is satisfied, the behavior is undefined:
T
is not a UniformRandomBitGenerator.
T::result_type
is not convertible to std::iterator_traits <RandomIt>::difference_type.
If the type of *first is not Swappable (until C++11)RandomIt
is not ValueSwappable (since C++11), the behavior is undefined.
RandomIt
must meet the requirements of LegacyRandomAccessIterator.
Exactly std::distance (first, last) - 1 swaps.
See also the implementations in libstdc++ and libc++.
random_shuffle (1) |
---|
template<class RandomIt> void random_shuffle(RandomIt first, RandomIt last) { typedef typename std::iterator_traits <RandomIt>::difference_type diff_t; for (diff_t i = last - first - 1; i > 0; --i) { using std::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. } } |
random_shuffle (2) |
template<class RandomIt, class RandomFunc> void random_shuffle(RandomIt first, RandomIt last, RandomFunc&& r) { typedef typename std::iterator_traits <RandomIt>::difference_type diff_t; for (diff_t i = last - first - 1; i > 0; --i) { using std::swap ; swap(first[i], first[r(i + 1)]); } } |
shuffle (3) |
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; for (diff_t i = last - first - 1; i > 0; --i) { using std::swap ; swap(first[i], first[D(g, param_t(0, i))]); } } |
Note that the implementation is not dictated by the standard, so even if you use exactly the same RandomFunc
or URBG
(Uniform Random Number Generator) you may get different results with different standard library implementations.
The reason for removing std::random_shuffle
in C++17 is that the iterator-only version usually depends on std::rand , which is now also discussed for deprecation. (std::rand should be replaced with the classes of the <random> header, as std::rand is considered harmful.) In addition, the iterator-only std::random_shuffle
version usually depends on a global state. The std::shuffle
's shuffle algorithm is the preferred replacement, as it uses a URBG
as its 3rd parameter.
Randomly shuffles the sequence [
1,
10]
of integers:
#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'; }
Possible output:
8 6 10 4 2 3 7 1 9 5
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
DR | Applied to | Behavior as published | Correct behavior |
---|---|---|---|
LWG 395 | C++98 | the source of randomness of overload (1) was not specified, and std::rand could not be the source due to the C library requirement |
it is implementation-defined, and using std::rand is allowed |
LWG 552 (N2423) |
C++98 | r was not required to be the source of randomness of overload (2)[1] |
required |