[英]C++ “vector iterator not decrementable”?
I am attempting to solve this UVa problem . 我试图解决这个UVa问题 。
And I am trying to use Vector to solve the problem. 我正在尝试使用Vector来解决问题。 I need to simulate something like circular linked list, so I use an iterator to access the elements.
我需要模拟像循环链表这样的东西,所以我使用迭代器来访问元素。 But after trying, I found Vector iterator having some problem about increment and decrement, and I cannot erase the element by using a reverse_iterator as argument.
但是在尝试之后,我发现Vector迭代器有一些关于增量和减量的问题,并且我不能通过使用reverse_iterator作为参数来擦除元素。 I am confused now.
我现在很困惑。 Is there any wrong with my code because I missed some important details or I should solve this problem in another way??
我的代码有什么问题,因为我错过了一些重要的细节,或者我应该以另一种方式解决这个问题?
Thanks in advance. 提前致谢。
Here is my code 这是我的代码
#include <iostream>
#include <vector>
#include <iomanip>
using namespace std;
vector<int> people;
int main()
{
int n, k, m; // k -> counter clockwise, m -> cloclwise
while (cin >> n >> k >> m)
{
if (n == 0 && k == 0 && m == 0)
return 0;
for (int i = 1; i <= n; i++)
people.push_back(i);
vector<int>::iterator k_pos = people.begin();
vector<int>::reverse_iterator m_pos = people.rbegin();
//cout << n << " " << k << " " << m << endl;
while (!people.empty())
{
int k_choose, m_choose;
for (int i = 1; i < k; i++)
{
k_pos++;
if (k_pos == people.end()) // if reach the end, go to begin
k_pos = people.begin();
}
k_choose = *k_pos;
cout << k_choose << endl;
for (int i = 1; i < m; i++)
{
m_pos++;
if (m_pos == people.rend())
m_pos = people.rbegin();
}
m_choose = *m_pos;
if (k_choose == m_choose)
{
cout << setw(3) << k_choose << ",";
people.erase(k_pos); // erase the element
}
else
{
cout << setw(3) << k_choose << setw(3) << m_choose << ",";
k_pos = people.erase(k_pos); // erase the element
//vector<int>::iterator temp;
//for (temp = people.begin(); *temp != *m_pos; temp++)
//{
//}
//cout << "ok" << endl;
people.erase(--m_pos.base());*****problem
}
vector<int>::iterator temp;
for (temp = people.begin(); temp != people.end(); temp++)
cout << *temp << endl;
k_pos++; *****problem
if (k_pos == people.end()) // point to next
k_pos = people.begin();
m_pos++; *****problem
if (m_pos == people.rend()) // point to next
m_pos = people.rbegin();
}
}
return 0;
}
After erasing or pushing in a vector all the iterators to it may become invalid(if the vector gets reallocated). 擦除或推入向量后,所有迭代器都可能变为无效(如果向量被重新分配)。 That is why after erase is performed in the else m_pos may become invalid.
这就是为什么在else中执行擦除后m_pos可能变得无效。 My advise is to use indices(at least that is what I do for competetive programming).
我的建议是使用指数(至少这是我为竞争性编程所做的)。
What if m_pos
is equal to people.rbegin()
? 如果
m_pos
等于people.rbegin()
怎么办? Therefore --m_pos
is not a valid iterator and that's can be the source of problem. 因此
--m_pos
不是有效的迭代器,而且可能是问题的根源。
Also it is possible, that you erased an element pointed by --m_pos
in the line before: 还有可能,您删除了之前行中的
--m_pos
指向的元素:
k_pos = people.erase(k_pos); // erase the element
The other thing you can improve in your code is setting k_pos
and m_pos
iterators in a more efficient way. 您可以在代码中改进的另一件事是以更有效的方式设置
k_pos
和m_pos
迭代器。 Instead: 代替:
for (int i = 1; i < k; i++)
{
k_pos++;
if (k_pos == people.end()) // if reach the end, go to begin
k_pos = people.begin();
}
You can write: 你可以写:
#include <iterator>
std::advance(people.begin(), k % people.size());
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