我如何知道我的迭代器是否递减超过了向量的开头？

发布于 2024-12-03 07:05:54 字数 1101 浏览 1 评论 0原文

我正在通过向量前后移动迭代器。

我可以检查迭代器是否从末尾运行，如下所示：

++my_iterator;
if ( my_iterator == my_vector.end() )
{
    --my_iterator; // if I want to stop the iterator at the end.
    my_iterator = my_vector.begin(); // if I want the iterator to wraparound.
}

但是如何检查它是否从开头运行？

编辑：那我可以这样做吗？

--my_iterator;
if ( my_iterator == my_vector.rend() )
{
    my_iterator = my_vector.begin(); // if I want to stop the iterator at the beginning.
    my_iterator = --(my_vector.rbegin()); // if I want the iterator to wraparound.
}

或者我必须这样做吗？

std::vector< T >::iterator temp_reverse_iterator = reverse_iterator< T >( my_iterator );
++temp_reverse_iterator;
if ( temp_reverse_iterator == my_vector.rend() )
{
    my_iterator = my_vector.begin(); // if I want to stop the iterator at the beginning.
    my_iterator = --(my_vector.end()); // if I want the iterator to wraparound.
}
else
{
    my_iterator = temp_reverse_iterator.base(); // do I need to -- this?
}

这两个例子逻辑上合理吗？

原文

I am moving an iterator backward and forwards through a vector.

I can check if the iterator ran off the end like so:

++my_iterator;
if ( my_iterator == my_vector.end() )
{
    --my_iterator; // if I want to stop the iterator at the end.
    my_iterator = my_vector.begin(); // if I want the iterator to wraparound.
}

But how do I check if it ran off the beginning?

Edit: So can I do this?

--my_iterator;
if ( my_iterator == my_vector.rend() )
{
    my_iterator = my_vector.begin(); // if I want to stop the iterator at the beginning.
    my_iterator = --(my_vector.rbegin()); // if I want the iterator to wraparound.
}

Or do I have to do this?

std::vector< T >::iterator temp_reverse_iterator = reverse_iterator< T >( my_iterator );
++temp_reverse_iterator;
if ( temp_reverse_iterator == my_vector.rend() )
{
    my_iterator = my_vector.begin(); // if I want to stop the iterator at the beginning.
    my_iterator = --(my_vector.end()); // if I want the iterator to wraparound.
}
else
{
    my_iterator = temp_reverse_iterator.base(); // do I need to -- this?
}

And are both of these examples logically sound?

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怪我入戏太深 2024-12-10 07:05:54

我想知道如果您使用是否会更容易使用 Boost Circular Buffer 而不是 std::vector 作为底层数据结构。

但是，要回答您的实际问题：您可以通过检查迭代器是否等于 v.begin() 来检查是否绕过开头。

#include <vector>
#include <cassert>

template <class T> void
Increment( typename std::vector<T>::iterator& it, std::vector<T>& v )
{
  assert(v.size() > 0);

  ++it;
  if(it == v.end())
    it = v.begin();
}

template <class T> void
Decrement( typename std::vector<T>::iterator& it, std::vector<T>& v )
{
  assert(v.size() > 0);

  if(it == v.begin())
    it = v.end();
  --it;
}

int main() {
  std::vector<int> v;
  v.push_back(0);
  v.push_back(1);

  std::vector<int>::iterator it;

  it = v.begin();
  Decrement(it, v);
  assert(*it == 1);
  Increment(it, v);
  assert(*it == 0);
}

I wonder if it would be easier for you if you used a Boost Circular Buffer instead of a std::vector as your underlying data structure.

But, to answer your actual question: You check for wrapping past the beginning by checking to see if the iterator equals v.begin().

#include <vector>
#include <cassert>

template <class T> void
Increment( typename std::vector<T>::iterator& it, std::vector<T>& v )
{
  assert(v.size() > 0);

  ++it;
  if(it == v.end())
    it = v.begin();
}

template <class T> void
Decrement( typename std::vector<T>::iterator& it, std::vector<T>& v )
{
  assert(v.size() > 0);

  if(it == v.begin())
    it = v.end();
  --it;
}

int main() {
  std::vector<int> v;
  v.push_back(0);
  v.push_back(1);

  std::vector<int>::iterator it;

  it = v.begin();
  Decrement(it, v);
  assert(*it == 1);
  Increment(it, v);
  assert(*it == 0);
}

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