Question

In C# I have a use case where I have a mapping from ints to collections.

• the ints are a dense (but not packed) set from 1 to n where n is not known.
• The cells will be loaded in random order.
• the marginal cost of each cell should be ideal (as good as a List<T> or T[])
• I’d like to have the cells default filled on demand

What is the best structure for this?

A List<T> would work well (better in space than a Dictionary<>) and by deriving from it I can get much of what I want, but is there something better? As in the best code is code you don’t write.

Answer 1

A Dictionary<int, Cell> sounds like a good match to me. Or you could use a List<Cell> quite easily, and just make sure that you expand it where necessary:

public static void EnsureCount<T>(List<T> list, int count) {     if (list.Count > count)     {         return;     }     if (list.Capacity < count)     {         // Always at least double the capacity, to reduce         // the number of expansions required         list.Capacity = Math.Max(list.Capacity*2, count);     }     list.AddRange(Enumerable.Repeat(default(T), list.Capacity-list.Count)); }