C# Binary Trees and Dictionaries

Question

I\'m struggling with the concept of when to use binary search trees and when to use dictionaries.

In my application I did a little experiment which used the C5 libra

Answer 1

It does make sense that a tree node would require more storage than a dictionary entry. A binary tree node needs to store the value and both the left and right subtrees. The generic Dictionary is implemented as a hash table which - I'm assuming - either uses a linked list for each bucket (value plus one pointer/reference) or some sort of remapping (just the value). I'd have to have a peek in Reflector to be sure, but for the purpose of this question I don't think it's that important.

The sparser the hash table, the less efficient in terms of storage/memory. If you create a hash table (dictionary) and initialize its capacity to 1 million, and only fill it with 10,000 elements, then I'm pretty sure it would eat up a lot more memory than a BST with 10,000 nodes.

Still, I wouldn't worry about any of this if the amount of nodes/keys is only in the thousands. That's going to be measured in the kilobytes, compared to gigabytes of physical RAM.

---

If the question is "why would you want to use a binary tree instead of a hash table?" Then the best answer IMO is that binary trees are ordered whereas hash tables are not. You can only search a hash table for keys that are exactly equal to something; with a tree, you can search for a range of values, nearest value, etc. This is a pretty important distinction if you're creating an index or something similar.