如何始终保留集合的n个元素？

Question

What is the best way to keep n elements of a collection in c# ?

Remove old elements when adding a new one.

This is what I did:

List<int> listOf5Elements = new List<int>();
for(var i = 0; i<200; i++)
{
    listOf5Elements.Add(i);
    if (listOf5Elements.Count() == 6)
        listOf5Elements.RemoveAt(0);
}

LinkedList<int> linkedOf5elements = new LinkedList<int>();
for (var i = 0; i < 200; i++)
{
    linkedOf5elements.AddLast(i);
    if (linkedOf5elements.Count() == 6)
        linkedOf5elements.RemoveFirst();
}

Queue<int> queueOf5Elements = new Queue<int>();
for (var i = 0; i < 200; i++)
{
    queueOf5Elements.Enqueue(i);
    if (queueOf5Elements.Count() == 6)
        queueOf5Elements.Dequeue();
}

Is there any other way to do it?

Answer 1

What you want is a circular buffer . There are many implementations out there. Here is one .

It's efficient in both time and space; in time because an insert is O(1) and in space because the new element overwrites the oldest in-place.

Your first attempt is not efficient because RemoveAt(0) is O(n) because the elements have to shift.

The second isn't too bad, but has some overhead in allocating the new slot, and deleting the old one.

The third attempt is the best because the .NET queue is impemented as a circular buffer, but there's some overhead dealing with resizing. Since you don't care about resizing, a true circular buffer with a fixed size would be a bit more efficient.