反向迭代器算法

Reverse Iterator Arithmetic

本文关键字：算法 迭代器      更新时间：2023-10-16

所有，我试图在列表中的元素之间反向进行O(n^2)比较，所以我使用反向迭代器。

代码如下所示

#include <list>
struct Element {
 double a;
 double b;
};
typedef std::list<Element> ElementList;
class DoStuff {
public:
  DoStuff();
  void removeDuplicates(ElementList & incList) const {
     for(ElementList::reverse_iterator stackIter = incList.rbegin(); stackIter != incList.rend(); ++stackIter) {
        bool uniqueElement = true;
        for(ElementList::reverse_iterator searchIter = stackIter+1; searchIter != incList.rend() && uniqueElement; ++searchIter) {
            //Check stuff and make uniqueElement = true;
         } 
     }
  }
};
int main() {
  std::list<Element> fullList;
  DoStuff foo;
  foo.removeDuplicates(fullList);
}

我得到一个编译错误的searchIter创建…为什么…

这是有效的，但它是愚蠢的阅读:

ElementList::reverse_iterator searchIter = stackIter;
searchIter++;
for( ; searchIter != incList.rend() && uniqueElement; ++searchIter) {
}

错误如下:

In file included from /usr/local/include/c++/6.1.0/bits/stl_algobase.h:67:0,
                 from /usr/local/include/c++/6.1.0/list:60,
                 from main.cpp:1:
/usr/local/include/c++/6.1.0/bits/stl_iterator.h: In instantiation of 'std::reverse_iterator<_Iterator> std::reverse_iterator<_Iterator>::operator+(std::reverse_iterator<_Iterator>::difference_type) const [with _Iterator = std::_List_iterator<Element>; std::reverse_iterator<_Iterator>::difference_type = long int]':
main.cpp:16:66:   required from here
/usr/local/include/c++/6.1.0/bits/stl_iterator.h:233:41: error: no match for 'operator-' (operand types are 'const std::_List_iterator<Element>' and 'std::reverse_iterator<std::_List_iterator<Element> >::difference_type {aka long int}')
       { return reverse_iterator(current - __n); }

对于某些迭代器it和整数n，语法it + n要求该迭代器为"随机访问迭代器"。列表迭代器不满足这个要求。

要解决"stupid To read"的问题，您可以使用std::next:

for(ElementList::reverse_iterator searchIter = std::next(stackIter); ...

或者，输入更少:

for(auto searchIter = std::next(stackIter); ...
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