如何为指定的列表编写迭代器 [英] How can I write a iterator for list as specified

问题描述

我对list的迭代器有些困惑.我写了一段代码.

 #include   ><   iostream  > 
 #include   <  列表 > 
 #include   <  向量 > 
 #include   <  算法 > 

使用 命名空间 std；

 int  main()
{
   vector< int> vector1；
    for (未签名  int 索引=  0 ;索引<  10 ; ++ index)
   {
      vector1.push_back(index);
   }
   // 我可以在vector的迭代器上添加一个常量
   find(vector1.begin()，vector1.begin()+  5 ， 2 ));
   
   list< int> list1;
    for (未签名  int 索引=  0 ;索引<  10 ; ++ index)
   {
      list1.push_back(index);
   }

   // ，但是当我尝试在列表中进行操作时，我发现它没有用.
   find(list1.begin()，list1.begin()+  5 ， 2 );
} 

如何为指定的列表编写迭代器?请帮助我.

解决方案

简短答案:您不能说listl.begin() + 5，因为列表迭代器不属于随机访问迭代器类别 [双向迭代器类别 [listl.begin() + 5即可获得第5个项目的迭代器.在向量中，迭代器实现很容易找出当前项目中五分之五的项目的迭代器，因为向量基本上是一个数组，因此步进迭代器只是一个加法.如果是链表，则要步进迭代器(效率低下)，则必须执行N次链接查找.仍然可以允许列表迭代器使用恒定数量的项目来执行它，但stl容器不包含算法性能较慢的方法.他们的迭代器也是如此.要查看关于迭代器类别的含义，请阅读以下站点之一:
http://www.cplusplus.com/reference/std/iterator/RandomAccessIterator/ [ ^ ]
http://anaturb.net/C/iterators.htm [ list< int> ::: const iterartor it = listl.begin(); for ( int i = 0 ; i< ; 5; ++ i) ++ it;

如果< const> std :: list的迭代器包含随机访问运算符，则它们还应该使用for循环执行N的增量/减量，该循环非常无效并且看起来很奇怪.某些算法在处理列表时需要前进几步，但是如果您的代码包含巨大(可能是随机大小)的前进/后退步骤，那么您可能选择了错误的stl容器类型.

I am a little bit confused about iterator for list. I wrote a piece of code.

#include <iostream>
#include <list>
#include <vector>
#include <algorithm>

using namespace std;

int main()
{
   vector<int>vector1;
   for(unsigned int index = 0; index < 10; ++index)
   {
      vector1.push_back(index);
   }
   // I can add a constant on the iterator of vector
   find(vector1.begin(), vector1.begin() + 5, 2); 
   
   list<int>list1;
   for(unsigned int index = 0; index < 10; ++index)
   {
      list1.push_back(index);
   }

   // but when I try to do it on list, I found it didn`t work.
   find(list1.begin(), list1.begin() + 5, 2); 
}

How can I write a iterator for list as specified? Please help me.

解决方案

Short answer: You cant say listl.begin() + 5 because the list iterator doesn''t fall into the random access iterator category[^]. The iterator of the stl list is in the bidirectional iterator category[^].
Long answer: There are iterator categories each of them giving an iterator some ''special abilities''. For example a random access iterator provides that you can access items by performing ''indexing'' with it like you did - you are basically getting the iterator for the 5th item by saying listl.begin() + 5. In a vector its easy for an iterator implementation to find out the iterator of an item that is 5 past five the current item because the vector is basically an array so stepping the iterator is just an addition. In case of a linked list you have to perform N number of link lookups if you want to step your iterator, its inefficient. It would still be possible to allow for a list iterator to step it with a constant number of items but stl containers doesn''t contain methods whose performance is algorithmically slow. The same is true for their iterators. To check out what I mean on iterator categories read one of the following sites:
http://www.cplusplus.com/reference/std/iterator/RandomAccessIterator/[^]
http://anaturb.net/C/iterators.htm[^]

EDIT: If you still insist on writing listl.begin() + 5 then you have to do that by incrementing the list iterator in a for loop with single steps by using one of the operations (++) available for bidirectional iterators:

list<int>::const iterartor it = listl.begin();
for (int i=0; i<5; ++i)
    ++it;

If the iterator of <const>std::list contained random access operators they should also do the increment/decrement by N with a for loop that is pretty ineffective and looks weird. Some algorithms require to step forward a few steps when processing a list but if your code contains huge (maybe randomly sized) forward/backward steps then you have probably chosen the wrong type of stl container.

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