如何从 C++ List 中获取指定索引处的元素
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How to get an element at specified index from c++ List
提问by user2398815
I have a list:
我有一个list：
list<Student>* l;
and I would like to get an element at a specified index. Example:
我想在指定的索引处获取一个元素。例子：
l->get(4)//getting 4th element
Is there a function or method in listwhich enables it to do so?
是否有一个函数或方法list可以让它这样做？
回答by juanchopanza
std::listdoes not have a random access iterator, so you have to step 4 times from the front iterator. You can do this manually or with std::advance, or std::nextin C++11, but bear in mind that both O(N) operations for a list.
std::list没有随机访问迭代器，所以你必须从前面的迭代器开始 4 次。您可以手动或使用std::advance或C++11 中的std::next执行此操作，但请记住，列表的两个 O(N) 操作。
#include <iterator>
#include <list>
....
std::list<Student> l; // look, no pointers!
auto l_front = l.begin();
std::advance(l_front, 4);
std::cout << *l_front << '\n';
Edit: The original question asked about vector too. This is now irrelevant, but may be informative nonetheless:
编辑：原始问题也询问了向量。这现在无关紧要，但仍然可以提供信息：
std::vectordoes have random access iterators, so you can perform the equivalent operation in O(1) via the std::advance, std::nextif you have C++11 support, the []operator, or the at()member function:
std::vector确实具有随机访问迭代器，因此如果您有 C++11 支持、运算符或成员函数，则可以通过std::advance, std::next在 O(1) 中执行等效操作：[]at()
std::vector<Student> v = ...;
std::cout << v[4] << '\n'; // UB if v has less than 4 elements
std::cout << v.at(4) << '\n'; // throws if v has less than 4 elements
回答by Patch92
Here's a get()function that returns the _ith Studentin _list.
这里有一个get()返回函数_i日Student在_list。
Student get(list<Student> _list, int _i){
list<Student>::iterator it = _list.begin();
for(int i=0; i<_i; i++){
++it;
}
return *it;
}
回答by srikanta
If you want random access to elements, you should use a vectorand then you can use []operator to get the 4th element.
如果你想随机访问元素，你应该使用 avector然后你可以使用[]运算符来获取第 4 个元素。
vector<Student> myvector (5); // initializes the vector with 5 elements`
myvector[3]; // gets the 4th element in the vector
回答by simpleuser
For std::vectoryou can use
因为std::vector你可以使用
myVector.at(i)//retrieve ith element
myVector.at(i)//检索第i个元素
