Книга: Standard Template Library Programmer

random_shuffle

random_shuffle

Category: algorithms

Component type: function

Random_shuffle is an overloaded name; there are actually two random_shuffle functions.

template <class RandomAccessIterator>
void random_shuffle(RandomAccessIterator first, RandomAccessIterator last);
template <class RandomAccessIterator, class RandomNumberGenerator>
void random_shuffle(RandomAccessIterator first, RandomAccessIterator last, RandomNumberGenerator& rand)

Random_shuffle randomly rearranges the elements in the range [first, last): that is, it randomly picks one of the N! possible orderings, where N is last – first. [1] There are two different versions of random_shuffle. The first version uses an internal random number generator, and the second uses a Random Number Generator, a special kind of function object , that is explicitly passed as an argument.

Defined in the standard header algorithm, and in the nonstandard backward-compatibility header algo.h.

For the first version:

• RandomAccessIterator is a model of Random Access Iterator

For the second version:

• RandomAccessIterator is a model of Random Access Iterator

• RandomNumberGenerator is a model of Random Number Generator

• RandomAccessIterator's distance type is convertible to RandomNumberGenerator's argument type.

• [first, last) is a valid range.

• last – first is less than rand 's maximum value.

Linear in last – first . If last != first, exactly (last – first) – 1 swaps are performed.

const int N = 8;
int A[] = {1, 2, 3, 4, 5, 6, 7, 8};
random_shuffle(A, A + N);
copy(A, A + N, ostream_iterator<int>(cout, " "));
// The printed result might be 7 1 6 3 2 5 4 8,
// or any of 40,319 other possibilities.

[1] This algorithm is described in section 3.4.2 of Knuth (D. E. Knuth, The Art of Computer Programming. Volume 2: Seminumerical Algorithms, second edition. Addison-Wesley, 1981). Knuth credits Moses and Oakford (1963) and Durstenfeld (1964). Note that there are N! ways of arranging a sequence of N elements. Random_shuffle yields uniformly distributed results; that is, the probability of any particular ordering is 1/N!. The reason this comment is important is that there are a number of algorithms that seem at first sight to implement random shuffling of a sequence, but that do not in fact produce a uniform distribution over the N! possible orderings. That is, it's easy to get random shuffle wrong.

random_sample, random_sample_n, next_permutation, prev_permutation, Random Number Generator

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