Understanding Hash Tables: The Backbone of Efficient Data Storage
Tags: algorithms, computerscience, javascript, hashtable
Published: 2024-05-29 12:31:34

In the realm of computer science and programming, hash tables are indispensable tools that provide efficient data storage and retrieval capabilities. Whether you're a seasoned developer or a beginner, understanding hash tables can significantly enhance your problem-solving toolkit. In this post, we will explore what exactly hash tables are, how they work, the mechanics of hashing, and their application. Let's get started

<img src="https://media.giphy.com/media/v1.Y2lkPTc5MGI3NjExc2Zwb3MwdW1pMjI3bHo2YWhtOTBqYmViZjMxaHdua2NhbGx1MXB1diZlcD12MV9pbnRlcm5hbF9naWZfYnlfaWQmY3Q9Zw/l0amJzVHIAfl7jMDos/giphy.gif" width="100%" height="100%" />

### What is a Hash Table? 

Well, that's a good question, and as the long story short, a hash table is a data structure that maps keys to values for highly efficient lookup. Think of it as a digital version of a dictionary, where each word (key) maps to a definition (value).

The image below describes what a hash table looks like:

<img src="https://dev-to-uploads.s3.amazonaws.com/uploads/articles/qakn4m8aj7nywjytsozx.png" alt="Understanding Hash Tables: The Backbone of Efficient Data Storage - A general view of a HashTable" />
    <figcaption >A general view of a HashTable</figcaption>
</figure>

From the image above, there are three important things to mention:

- The "hash" function: The "hash" function transforms keys, such as "ab" into 1234 and "cd" into 5678. This transformation maps the keys to specific indices in an array, enabling quick data retrieval.
- Mapping to indexes: The hash values are mapped to indices in the array using a modulo operation to ensure they fit within the array's bounds. For example, 1234 maps the key "ab" to index 4 because 1234 % 5 (the length of the hash table) equals 4.
- Dealing with collisions: To handle cases like the keys "ef" and "gh", or "cd", "ij", and "kl" being stored at the same index, linked lists are used to manage these situations, also called collisions. By storing multiple keys at the same index in a chain, the hash table allows efficient data retrieval even in the presence of collisions.

Let's take a look at these items separately.

#### Hash Function

A hash function is a crucial component of a hash table. It takes an input (the "key") and returns an integer, which is used as the index where the value associated with the key is stored. For instance, the key "ab" might be transformed into the index 1234, and "cd' into 5678. 
An example of a simple hash function for strings is shown below:

```js
function hash(key, tableSize) {
  let hash = 23;

  for (let i = 0; i < key.length; i++) {
    hash = (hash * key.charCodeAt(i)) % tableSize;
  }

  return hash;
}
```

This function iterates over each character in the input string, multiplying its Unicode value with the hash variable, and uses the modulo operation to ensure the resulting hash value fits within the bounds of the hash table's length. The use of a prime number (23) as the initial value helps create a more uniform distribution of hash values, reducing the likelihood of collisions in the hash table.

#### Collision Handling

A collision occurs when two keys hash to the same index, as in the case of the keys "ef" and "gh" in the image above. In situations like this, two techniques could be used to solve this problem: **separate chaining**, or **open addressing**.

- Separate Chaining: One of the most common methods for handling collisions. When a collision occurs, the key-value pairs that hash to the same index are stored in a linked list at that index.
- Open Addressing: This method involves finding another slot within the hash table for the colliding key. Techniques like `linear probing`, `quadratic probing`, and `double hashing` fall under this category.

### Advantages of Hash Tables

- Fast Data Access: Hash tables provide average-case constant time complexity, O(1), for both insertions and lookups, making them highly efficient. 
- Flexibility: They can store a wide variety of data types and can dynamically resize to accommodate more entries.

### Applications of Hash Tables

There are a myriad of ways you could use Hash Table, the most common applications are:

- Databases: Hash tables are used in database indexing to quickly retrieve records.
- Caches: Acting as a fundamental piece of the implementation of caches for fast data retrieval. 
- Sets and Maps in Programming Languages: Many programming languages use hash tables to implement sets and maps, such as Python's `dict` and `set`, and Java's `HashMap`.

### Conclusion

Hash tables are an example of efficient data storage and retrieval, due to their average-case constant time complexity for basic operations. By understanding the mechanisms of hashing and collision resolution, you can harness the power of hash tables to optimize your programs and solve complex problems with ease. Whether you are preparing for a coding interview or applying it to a project, hash tables are a fundamental tool that can make your data management tasks more efficient and effective.

To recap, remember that: 
- A hash table is a data structure that maps keys to values.
- The average case of a hash table is constant time complexity, O(1), for both insertions and lookups.
- A hash function is a crucial component of a hash table.
- The use of a prime number helps create a more uniform distribution of hash values, reducing the likelihood of collisions in the hash table.
- The most common technique to deal with collisions is Separate Chaining.
- Hash tables are great for database indexing and cache. 

### Resources

#### Tutorials 

[How to Implement a Hash Table in JavaScript](https://www.youtube.com/watch?v=UOxTMOCTEZk): A YouTube video tutorial where Ben Awad explains in detail how to implement a Hash Table in Javascript.

#### Books

[Cracking the Coding Interview](https://www.crackingthecodinginterview.com/): 150 Programming Interview Questions and Solutions.