NAME¶

std::random_shuffle,std::shuffle - std::random_shuffle,std::shuffle

Synopsis¶

Defined in header <algorithm>
template< class RandomIt > (deprecated in C++14)
void random_shuffle( RandomIt first, (1) (removed in C++17)
RandomIt last );
template< class RandomIt, class
RandomFunc > (until C++11)
void random_shuffle( RandomIt first,
RandomIt last, RandomFunc& r );
template< class RandomIt, class (since C++11)
RandomFunc > (2) (deprecated in C++14)
void random_shuffle( RandomIt first, (removed in C++17)
RandomIt last, RandomFunc&& r );
template< class RandomIt, class URBG
> (3) (since C++11)
void shuffle( RandomIt first,
RandomIt last, URBG&& g );

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

1) The source of randomness is implementation-defined, but the function std::rand is
often used.
2) The source of randomness is the function object r.
If any of the following conditions is satisfied, the behavior is undefined:
* The return type of r is not convertible to
std::iterator_traits<RandomIt>::difference_type.
* Given a positive value n of type
std::iterator_traits<RandomIt>::difference_type, the result of r(n) is not a
randomly chosen value in the interval [0, n).
3) The source of randomness is the object g.
Given the type 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 (until C++20)
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.

Parameters¶

first, last - the range of elements to shuffle randomly
r - function object returning a randomly chosen value
g - generator object returning a randomly chosen value

Type requirements¶

-
RandomIt must meet the requirements of LegacyRandomAccessIterator.

Complexity¶

Exactly std::distance(first, last) - 1 swaps.

Possible implementation¶

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))]);
}
}

Notes¶

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.

Example¶

Randomly shuffles the sequence [1, 10] of integers:

// Run this code

#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

Defect reports

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

DR Applied to Behavior as published Correct behavior
the source of randomness of
overload (1) was not specified, it is implementation-defined,
LWG 395 C++98 and and using std::rand is allowed
std::rand could not be the source
due to the C library requirement
LWG 552 r was not required to be the
(N2423) C++98 source required
of randomness of overload (2)^[1]

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

See also¶

generates the next greater lexicographic permutation of a range of
next_permutation elements
(function template)
generates the next smaller lexicographic permutation of a range of
prev_permutation elements
(function template)
ranges::shuffle randomly re-orders elements in a range
(C++20) (niebloid)