STLalgorithm算法shuffle,random_shuffle(46)

2016-02-24

字数统计: 540字 | 阅读时长≈ 2分

shuffle原型：

template <class RandomAccessIterator, class URNG>
void shuffle (RandomAccessIterator first, RandomAccessIterator last, URNG&& g);

该函数是用来打乱范围内元素的顺序的。

其行为类似：

template <class RandomAccessIterator, class URNG>
void shuffle (RandomAccessIterator first, RandomAccessIterator last, URNG&& g)
{
for (auto i=(last-first)-1; i>0; –i) {
std::uniform_int_distribution<decltype(i)> d(0,i);
swap (first[i], first[d(g)]);
}
}

一个简单的例子：

// shuffle algorithm example
#include // std::cout
#include // std::shuffle
#include // std::array
#include // std::default_random_engine
#include // std::chrono::system_clock

int main () {
std::array<int,9> foo {1,2,3,4,5,6,7,8,9};

// obtain a time-based seed:
unsigned seed = std::chrono::system_clock::now().time_since_epoch().count();

shuffle (foo.begin(), foo.end(), std::default_random_engine(seed));

std::cout << “shuffled elements:”;
for (int& x: foo) std::cout << ‘ ‘ << x;
std::cout << ‘\n’;

return 0;
}

运行截图：

random_shuffle原型：

std::random_shuffle

C++98
C++11

generator by default (1)

template
void random_shuffle (RandomAccessIterator first, RandomAccessIterator last);

specific generator (2)

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

该函数也是用来打乱序列的顺序的。简单来说就是使序列里元素的位置变得随机。

行为类似于：

template <class RandomAccessIterator, class RandomNumberGenerator>
void random_shuffle (RandomAccessIterator first, RandomAccessIterator last,
RandomNumberGenerator& gen)
{
iterator_traits::difference_type i, n;
n = (last-first);
for (i=n-1; i>0; –i) {
swap (first[i],first[gen(i+1)]);
}
}

一个简单的例子：

// random_shuffle example
#include // std::cout
#include // std::random_shuffle
#include // std::vector
#include // std::time
#include // std::rand, std::srand

// random generator function:
int myrandom (int i) { return std::rand()%i;}

int main () {
std::srand ( unsigned ( std::time(0) ) );
std::vector myvector;

// set some values:
for (int i=1; i<10; ++i) myvector.push_back(i); // 1 2 3 4 5 6 7 8 9

// using built-in random generator:
std::random_shuffle ( myvector.begin(), myvector.end() );

// using myrandom:
std::random_shuffle ( myvector.begin(), myvector.end(), myrandom);

// print out content:
std::cout << “myvector contains:”;
for (std::vector::iterator it=myvector.begin(); it!=myvector.end(); ++it)
std::cout << ‘ ‘ << *it;

std::cout << ‘\n’;

return 0;
}

运行截图：