STL 源代码剖析 算法 stl_algo.h -- random_shuffle

random_shuffle

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描写叙述：将[first,last)的元素次序随机重排。

思路：

必须是 RandomAccessIterator 

1.遍历区间

2.产生[0,i)中的一个随机数rand。将*(first+rand)与i交换

??怎么证明随机性 

源代码：

template <class RandomAccessIterator>
inline void random_shuffle(RandomAccessIterator first,
                           RandomAccessIterator last) {
  __random_shuffle(first, last, distance_type(first));
}


template <class RandomAccessIterator, class RandomNumberGenerator>
void random_shuffle(RandomAccessIterator first, RandomAccessIterator last,
                    RandomNumberGenerator& rand) { //这里必须是传引用。由于随机数产生器有局部状态。每次被调用都会改变
  if (first == last) return;
  for (RandomAccessIterator i = first + 1; i != last; ++i)
    iter_swap(i, first + rand((i - first) + 1));
}


template <class RandomAccessIterator, class Distance>
void __random_shuffle(RandomAccessIterator first, RandomAccessIterator last,
                      Distance*) {
  if (first == last) return;
  for (RandomAccessIterator i = first + 1; i != last; ++i) 
#ifdef __STL_NO_DRAND48
    iter_swap(i, first + Distance(rand() % ((i - first) + 1)));
#else
  iter_swap(i, first + Distance(lrand48() % ((i - first) + 1)));
#endif
}

演示样例：

int main()
{
  vector<int> vec;
  for(int ix = 0; ix < 10; ix++)
    vec.push_back(ix);
  random_shuffle(vec.begin(), vec.end());
  copy(vec.begin(), vec.end(), ostream_iterator<int>(cout, " "));
}