How Do I Choose Between a Hash Table and a Trie (Prefix Tree)?

Question

So if I have to choose between a hash table or a prefix tree what are the discriminating factors that would lead me to choose one over the other. From my own naive point of

Answer 1

Everyone knows hash table and its uses but it is not exactly constant look up time , it depends on how big the hash table is , the computational complexity of the hash function.

Creating huge hash tables for efficient lookup is not an elegant solution in most of the industrial scenarios where even small latency/scalability matters (e.g.: high frequency trading). You have to care about the data structures to be optimized for space it takes up in memory too to reduce cache miss.

A very good example where trie better suits the requirements is messaging middleware . You have a million subscribers and publishers of messages to various categories (in JMS terms - Topics or exchanges) , in such cases if you want to filter out messages based on topics (which are actually strings) , you definitely do not want create hash table for the million subscriptions with million topics . A better approach is store the topics in trie , so when filtering is done based on topic match , its complexity is independent of number of topics/subscriptions/publishers (only depends on the length of string). I like it because you can be creative with this data structure to optimize space requirements and hence have lower cache miss.