C++ STL: lower_bound 和 upper_bound
接口声明
以下有两个不同的版本
lower_bound
-
template <class ForwardIterator, class T> ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val); -
template <class ForwardIterator, class T, class Compare> ForwardIterator lower_bound (ForwardIterator first, ForwardIterator last, const T& val, Compare comp);
upper_bound
-
template <class ForwardIterator, class T> ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val); -
template <class ForwardIterator, class T, class Compare> ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val, Compare comp);
两个接口都是返回一个迭代器，且改迭代器指向一个非递减序列[first,last)内部 的一个元素地址，后续通过该迭代器继续访问剩余元素。
两个接口的主要区别是：
lower_bound 指向的是第一个大于等于val的位置
upper_bound 指向的是第一个大于val的位置
[外链图片转存失败,源站可能有防盗链机制,建议将图片保存下来直接上传(img-6B3E9GCI-1592484277761)(/Users/zhanghuigui/Library/Application Support/typora-user-images/image-20200618111744296.png)]
接口源码实现
Lower_bound
// function : lower_bound
/// @brief Returns an iterator pointing to the first element in the range
/// [first, last) that is not less than (i.e. greater or equal to) val.
/// @param [in] last : iterator to the last element of the range
/// @param [in] val : value to find
/// @param [in] comp : object for to compare two value_t objects
/// @return iterator to the element found
//-----------------------------------------------------------------------------
// 返回大于等于val的数据
template<class Iter_t, class Filter = filter_pass<value_iter<Iter_t> >,
class Compare = std::less<typename Filter::key> >
inline Iter_t lower_bound(Iter_t first, Iter_t last,
const typename Filter::key &val,
const Compare & comp = Compare(),
Filter flt = Filter())
{
assert((last - first) >= 0);
if (last == first) return last;
Iter_t itaux = internal_find_first(first, last, val, comp, flt);
return (itaux == (last - 1) and comp(flt(*itaux), val)) ? last : itaux;
};
//-----------------------------------------------------------------------------
// function : internal_find_first
/// @brief find if a value exist in the range [first, last).
/// Always return as valid iterator in the range [first, last-1]
/// If exist return the iterator to the first occurrence. If don't exist
/// return the first greater than val.
/// If val is greater than the *(last-1), return (last-1)
/// If val is lower than (*first), return first
//
/// @param [in] first : iterator to the first element of the range
/// @param [in] last : iterator to the last element of the range
/// @param [in] val : value to find
/// @param [in] comp : object for to compare two value_t objects
/// @return iterator to the element found,
//-----------------------------------------------------------------------------
// 使用internal function进行二分查找
template <class Iter_t, class Filter = filter_pass<value_iter<Iter_t> >,
class Compare = std::less<typename Filter::key> >
inline Iter_t internal_find_first(Iter_t first, Iter_t last,
const typename Filter::key &val,
const Compare & comp = Compare(),
Filter flt = Filter())
{
Iter_t LI = first, LS = last - 1, it_out = first;
while (LI != LS)
{
it_out = LI + ((LS - LI) >> 1);
if (comp(flt(*it_out), val))
LI = it_out + 1;
else LS = it_out;
};
return LS;
};
Upper bound
// function :upper_bound
/// @brief return the first element greather than val.If don't exist
/// return last
//
/// @param [in] first : iterator to the first element of the range
/// @param [in] last : iterator to the last element of the range
/// @param [in] val : value to find
/// @param [in] comp : object for to compare two value_t objects
/// @return iterator to the element found
/// @remarks
//-----------------------------------------------------------------------------
//返回大于val的数据
template<class Iter_t, class Filter = filter_pass<value_iter<Iter_t> >,
class Compare = std::less<typename Filter::key> >
inline Iter_t upper_bound(Iter_t first, Iter_t last,
const typename Filter::key &val,
const Compare & comp = Compare(),
Filter flt = Filter())
{
assert((last - first) >= 0);
if (last == first) return last;
Iter_t itaux = internal_find_last(first, last, val, comp, flt);
return (itaux == first and comp(val, flt(*itaux))) ? itaux : itaux + 1;
}
// function : internal_find_last
/// @brief find if a value exist in the range [first, last).
/// Always return as valid iterator in the range [first, last-1]
/// If exist return the iterator to the last occurrence.
/// If don't exist return the first lower than val.
/// If val is greater than *(last-1) return (last-1).
/// If is lower than the first, return first
//
/// @param [in] first : iterator to the first element of the range
/// @param [in] last : iterator to the last element of the range
/// @param [in] val : value to find
/// @param [in] comp : object for to compare two value_t objects
/// @return iterator to the element found, if not found return last
//-----------------------------------------------------------------------------
template<class Iter_t, class Filter = filter_pass<value_iter<Iter_t> >,
class Compare = std::less<typename Filter::key> >
inline Iter_t internal_find_last(Iter_t first, Iter_t last,
const typename Filter::key &val,
const Compare & comp = Compare(), Filter flt =
Filter())
{
Iter_t LI = first, LS = last - 1, it_out = first;
while (LI != LS)
{
it_out = LI + ((LS - LI + 1) >> 1);
if (comp(val, flt(*it_out))) LS = it_out - 1;
else LI = it_out;
};
return LS;
};
应用
// lower_bound/upper_bound example
#include 输出如下
lower_bound at position 3 # 第一个大于等于20的位置
upper_bound at position 6 # 第一个大于20的位置