标准库里的lower_bound()和upper_bound()函数

使用一个东西，不明白它的道理，不高明
——侯捷老师

1. lower_bound()函数

功能：返回第一个不小于val的元素位置的迭代器

1.1 函数声明

// default (1)
template 
  ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last,
                               const T& val);

// custom (2)   

template 
  ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last,
                               const T& val, Compare comp);

1.2 等价操作实现

template 
  ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val)
{
  ForwardIterator it;
  iterator_traits::difference_type count, step;
  count = distance(first,last);
  while (count>0)
  {
    it = first; step=count/2; advance (it,step);
    if (*it

1.3 示例程式

void test_lower_bound() {
        vector vec {10, 30, 20, 40 ,20, 20, 50};
        std::sort(vec.begin(), vec.end());

        auto it = std::lower_bound(vec.begin(), vec.end(), 20);
        cout << std::distance(vec.begin(), it) << " " << *it << endl;
    }

输出结果：1 20

1.4 参考链接

http://www.cplusplus.com/reference/algorithm/lower_bound/

2. upper_bound()函数

功能:返回大于第一个比比较值大的元素位置的迭代器

2.1 函数声明

// default (1)
template 
  ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last,
                               const T& val);

// custom (2)   

template 
  ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last,
                               const T& val, Compare comp);

2.2 等价操作实现

template 
  ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val)
{
  ForwardIterator it;
  iterator_traits::difference_type count, step;
  count = std::distance(first,last);
  while (count>0)
  {
    it = first; step=count/2; std::advance (it,step);
    if (!(val<*it))                 // or: if (!comp(val,*it)), for version (2)
      { first=++it; count-=step+1;  }
    else count=step;
  }
  return first;
}

2.3 示例程式

void test_upper_bound() {
        int myints[] = {10,20,30,30,20,10,10,20};
        std::vector v(myints,myints+8);           // 10 20 30 30 20 10 10 20

        std::sort (v.begin(), v.end());                // 10 10 10 20 20 20 30 30

        std::vector::iterator low,up;
        low=std::lower_bound (v.begin(), v.end(), 20); //      ^
        up= std::upper_bound (v.begin(), v.end(), 20); //                ^

        std::cout << "lower_bound at position " << (low- v.begin()) << '\n';
        std::cout << "upper_bound at position " << (up - v.begin()) << '\n';
    }

输出结果：
lower_bound at position 3
upper_bound at position 6

2.4 参考链接

http://www.cplusplus.com/reference/algorithm/upper_bound/