Question

I have a program that needs to store data values and periodically get the last ‘x’ data values.

It initially thought a stack is the way to go but I need to be able to see more than just the top value – something like a PeekRange method where I can peek the last ‘x’ number of values.

At the moment I’m just using a list and get the last, say, 20 values like this:

var last20 = myList.Skip(myList.Count - 20).ToList();

The list grows all the time the program runs, but I only ever want the last 20 values. Could someone give some advice on a better data structure?

Answer 1

Well since you mentioned the stack, I guess you only need modifications at the end of the list?

In that case the list is actually a nice solution (cache efficient and with fast insertion/removal at the end). However your way of extracting the last few items is somewhat inefficient, because IEnumerable<T> won’t expose the random access provided by the List. So the Skip()-Implementation has to scan the whole List until it reaches the end (or do a runtime type check first to detect that the container implements IList<T>). It is more efficient, to either access the items directly by index, or (if you need a second array) to use List<T>.CopyTo().

If you need fast removal/insertion at the beginning, you may want to consider a ring buffer or (doubly) linked list (see LinkedList<T>). The linked list will be less cache-efficient, but it is easy and efficient to navigate and alter from both directions. The ring buffer is a bit harder to implement, but will be more cache- and space-efficient. So its probably better if only small value types or reference types are stored. Especially when the buffers size is fixed.