B: Hash table - Parker Core Knowledge

Understanding the Context

What Is a Hash Table?

A hash table, also known as a hash map or hash map, is a data structure that implements an associative array—a collection where values are stored and retrieved using keys. It allows for fast insertion, deletion, and lookup operations, typically in average-case constant time, O(1). What makes hash tables revolutionary is their ability to map keys to values efficiently using a hash function—a mathematical function that converts a key (string, number, object, etc.) into an index for an array.

How Does a Hash Table Work?

Key Insights

The core process of a hash table involves three main steps:

1. Hashing
   The key is passed into a hash function, which outputs a numeric index. This index determines the array location where the corresponding value is stored.

2. Collision Handling
   Since hash functions map a potentially large set of keys into a fixed array size, collisions—where two different keys produce the same index—are inevitable. Common techniques to resolve collisions include chaining (using linked lists) and open addressing (probing for next available slots).

3. Retrieving Values
   When retrieving a value, the same hash function is applied to the key, matching the index, and the associated value is quickly accessed—often in constant time.

This streamlined approach enables hash tables to outperform other data structures like arrays or linked lists in key-based access scenarios.

Final Thoughts

Key Advantages of Hash Tables

• Fast Access Time: Average-case O(1) for insert, delete, and lookup operations.
  
• Efficient Memory Usage: Dynamic arrays reduce wasted space.
  
• Simple Interface: Typically supports intuitive APIs like put(key, value), get(key), and remove(key).

Common Challenges and Trade-Offs

Despite their performance benefits, hash tables come with certain limitations: