[STL]lower_bound&upper_bound

源码

lower_bound

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

upper_bound

template <class ForwardIterator, class T>
  ForwardIterator upper_bound (ForwardIterator first, ForwardIterator last, const T& val)
{
  ForwardIterator it;
  iterator_traits<ForwardIterator>::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;
}

\ 看起来就是个二分，不过有点奇怪

用途

\ lower_bound

\ "returns an iterator pointing to the first element in the range [first,last) which does not compare less than val."

\ 返回一个序列中第一个不小于val值的参数。注意该序列必须已经排序

\ upper_bound

\ "returns an iterator pointing to the first element in the range [first,last) which compares greater than val."

\ 返回一个序列中第一个严格大于val值的参数。注意该序列必须已经排序

\ "Unlike lower_bound, the value pointed by the iterator returned by this function cannot be equivalent to val, only greater."

\ 不想lower_bound,upper_bound只返回严格大于val的值

参数

first, last

\ Forward iterators to the initial and final positions of a sorted (or properly partitioned) sequence. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.

\ 在一个已经排序的序列中传入起始和结束的迭代器。程序的范围为\([first,last)\)，也就是说包含第一个迭代器指向的值，但不包含最后一个迭代器指向的值。

val

\ Value of the lower bound to search for in the range.

\ lowerbound要在范围内找的那个数

\ For (1), T shall be a type supporting being compared with elements of the range [first,last) as the right-hand side operand of operator<.

\ val的类型必须能和序列中的元素用\(<\)比较

comp

\ Binary function that accepts two arguments (the first of the type pointed by ForwardIterator, and the second, always val), and returns a value convertible to bool. The value returned indicates whether the first argument is considered to go before the second.

\ 返回值为bool的程序并且有两个参数（第一个值，第二个值）。返回第一个参数是否应该优先于第二个参数处理。其实就是判断小于

\ The function shall not modify any of its arguments.

\ 这个函数不能修改仍和他的传入参数。显而易见

\ This can either be a function pointer or a function object.

\ 珂以是一个函数的指针，也珂以是一个函数的实体

时间复杂度

Performs approximately \(log_2(N)+1\)