标准库里is_sorted()、is_sorted_until()函数

使用一个东西，不明白它的道理，不高明
——侯捷老师

1. is_sorted()函数

功能：判断序列[first, last)是否是排序的

1.1 函数声明

// default(1)
template 
  bool is_sorted (ForwardIterator first, ForwardIterator last);

// custom(2)
template 
  bool is_sorted (ForwardIterator first, ForwardIterator last, Compare comp);

1.2 等价操作实现

template 
  bool is_sorted (ForwardIterator first, ForwardIterator last)
{
  if (first==last) return true;
  ForwardIterator next = first;
  while (++next!=last) {
    if (*next<*first)     // or, if (comp(*next,*first)) for version (2)
      return false;
    ++first;
  }
  return true;
}

1.3 示例程式

void test_is_sorted() {
        std::array foo {2, 4, 1, 3};
        do {
            std::prev_permutation(foo.begin(), foo.end());
            
            cout << "foo: ";
            for (auto& x : foo) {
                cout << " " << x;
            }
            cout << endl;
        } while (!std::is_sorted(foo.begin(), foo.end()));
        cout << "the range is sorted." << endl;
    }

输出结果：

image.png

1.4 源码探究

发现内部调用的是is_sorted_until()函数

template
    inline bool
    is_sorted(_ForwardIterator __first, _ForwardIterator __last,
          _Compare __comp)
    { return std::is_sorted_until(__first, __last, __comp) == __last; }

1.5 参考链接

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

2. is_sorted_until()函数

功能：返回第一个不满足排序的元素迭代器

2.1 函数声明

// default (1)
template 
  ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last);
// custom (2)   
template 
  ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last,
                                   Compare comp);

2.2 等价操作实现

template 
  ForwardIterator is_sorted_until (ForwardIterator first, ForwardIterator last)
{
  if (first==last) return first;
  ForwardIterator next = first;
  while (++next!=last) {
    if (*next<*first) return next;
    ++first;
  }
  return last;
}

2.3 示例程式

void test_is_sorted_until() {
        std::array foo = {2,4,1,3};
        std::array::iterator it;
        
        do {
            std::prev_permutation(foo.begin(), foo.end());
            
            cout << "foo: ";
            for (auto& x: foo) {
                cout << " " << x;
            }
            
            it = std::is_sorted_until(foo.begin(), foo.end());
            cout << "( " << (it - foo.begin()) << " elements sorted" << endl;
        } while (it != foo.end());
        cout << "the range is sorted" << endl;
    }

输出结果：

image.png

2.4 源码探究

is_sorted_until函数

template
    inline _ForwardIterator
    is_sorted_until(_ForwardIterator __first, _ForwardIterator __last,
            _Compare __comp)
    {
      // concept requirements
      __glibcxx_function_requires(_ForwardIteratorConcept<_ForwardIterator>)
      __glibcxx_function_requires(_BinaryPredicateConcept<_Compare,
        typename iterator_traits<_ForwardIterator>::value_type,
        typename iterator_traits<_ForwardIterator>::value_type>)
      __glibcxx_requires_valid_range(__first, __last);

      return std::__is_sorted_until(__first, __last,
                    __gnu_cxx::__ops::__iter_comp_iter(__comp));
    }

__is_sorted_until()函数:

template
    _ForwardIterator
    __is_sorted_until(_ForwardIterator __first, _ForwardIterator __last,
              _Compare __comp)
    {
      if (__first == __last)
    return __last;

      _ForwardIterator __next = __first;
      for (++__next; __next != __last; __first = __next, ++__next)
    if (__comp(__next, __first))
      return __next;
      return __next;
    }

2.5 参考链接

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