How to get a certain element in a list, given the position?

Question

So I've got a list:

list<Object> myList; myList.push_back(Object myObject); 

I'm not sure but I'm confident that this would be the "0th" element in the array. Is there any function I can use that will return "myObject"?

Object copy = myList.find_element(0); 

?

Answer 1

If you frequently need to access the Nth element of a sequence, std::list, which is implemented as a doubly linked list, is probably not the right choice. std::vector or std::deque would likely be better.

That said, you can get an iterator to the Nth element using std::advance:

std::list<Object> l; // add elements to list 'l'...  unsigned N = /* index of the element you want to retrieve */; if (l.size() > N) {     std::list<Object>::iterator it = l.begin();     std::advance(it, N);     // 'it' points to the element at index 'N' } 

For a container that doesn't provide random access, like std::list, std::advance calls operator++ on the iterator N times. Alternatively, if your Standard Library implementation provides it, you may call std::next:

if (l.size() > N) {     std::list<Object>::iterator it = std::next(l.begin(), N); } 

std::next is effectively wraps a call to std::advance, making it easier to advance an iterator N times with fewer lines of code and fewer mutable variables. std::next was added in C++11.

Answer 2

std::list doesn't provide any function to get element given an index. You may try to get it by writing some code, which I wouldn't recommend, because that would be inefficient if you frequently need to do so.

What you need is : std::vector. Use it as:

std::vector<Object> objects; objects.push_back(myObject);  Object const & x = objects[0];    //index isn't checked Object const & y = objects.at(0); //index is checked