In the realm of computer science, data structures are essential to efficiently manage and manipulate large volumes of data. One such data structure is the Hash Table, which is primarily used to store and retrieve data in constant time.

In this article, we will explore the Hash Table data structure, its implementation in Python, and the importance of its hash function. Hash Table vs.

Dictionary: An Abstract Data Type

A Hash Table is a type of Abstract Data Type (ADT) that allows for the storage and retrieval of data in constant time. This makes it an excellent data structure for applications where the speedy retrieval of data is essential.

Hash Tables are often compared to Dictionaries, because, in essence, they are the same. Dictionaries are in-memory hash tables, optimized to allow fast access to key-value pairs, making them useful in a wide range of applications.

Hash Table: An Array with a Hash Function

A Hash Table is an array of fixed-size buckets, with each bucket having a unique key that is used to access the data stored within it. The magic of Hash Tables comes from the algorithm behind it, called the hash function.

The hash function is responsible for converting the key value into an array index that can be used to access the data. In simple terms, the hash function acts as a mapper between the key and the index location.

## Understand the Hash Function

The hash function takes an input (in this case, the key), and produces a fixed-size output, which is usually referred to as the hash value or the hash code. In the ideal scenario, each key should map to a unique hash value, which maps to a unique index location, thereby allowing for the fast access of data.

However, it’s essential to note that this may not always be the case. If two keys produce the same hash value or index location, it’s called a hash collision.

Examine Python’s Built-in hash()

Python is a popular programming language that comes with a built-in hash() function. The hash() function returns a unique hash value for an object, regardless of its type or contents.

The hash value is fixed in size, which makes it ideal for use in Hash Tables. A significant advantage of Python’s hash() function is that it produces consistent results, meaning that for the same input, it always produces the same output.

Dive Deeper into Python’s hash()

When it comes to the hash() function, there is a lot that goes on under the hood. For instance, the hash() function returns an integer value that has a uniform distribution.

This means that each possible hash value has an equal chance of being returned, making it ideal for use in Hash Tables. Additionally, the hash() function is collision prone, meaning that there is always a chance that two different keys will map to the same index location.

However, when handling collisions, Python follows a strategy called open addressing, where it finds the next available index location to store the data.

## Identify Hash Function Properties

A good hash function should produce uniformly distributed values, meaning that each possible hash value should have an equal chance of being produced. Additionally, it should have cryptographic qualities, making it difficult to generate the same output by looking at different input values.

Finally, it should exhibit an avalanche effect, meaning that small changes in input values should result in significant changes in the output, making it more challenging to recreate the same hash value.

## Conclusion

In conclusion, Hash Tables are efficient data structures that allow for fast storage and retrieval of data. The magic of the Hash Table comes from the hash function, which converts the key value into an index used to store and retrieve data.

Python’s built-in hash() function is an excellent tool for building Hash Tables, especially since it produces consistent values for the same input and has a uniform distribution. Understanding the properties of the hash function is crucial in avoiding hash collisions and delivering the best performance from the Hash Table data structure.

## Implementing a Hash Table in Python

In the previous section, we talked about the Hash Table data structure and how it works. Now, let’s dive into the implementation details of a Hash Table in Python.

## Make Your Own Hash Function

Before we can implement a Hash Table, we need to create a hash function. A hash function should produce a unique hash code for each unique input value.

We can create a custom hash function in Python by utilizing advanced math and prime numbers. Using prime numbers produces the best distribution of hash values, which in turn reduces collisions.

We can utilize the modulo operator to get the final index value from the hash code. Here’s an example to get started:

“`

def custom_hash(key, table_size):

# A simple hash function using ASCII values and modulo operator

hash_value = 0

for char in key:

hash_value += ord(char)

return hash_value % table_size

“`

## Build a Hash Table Prototype in Python With TDD

The next step in implementing a Hash Table is to build a prototype using Test-Driven Development (TDD). The prototype will provide a blueprint for our final implementation, ensuring that the code works as expected with comprehensive testing built-in.

## Define a Custom HashTable Class

The HashTable class is where we define our data structure and hash function. We will create a custom class that allows for the insertion of key-value pairs, retrieval of values with keys, and deletion of key-value pairs.

Here’s an example of how the class can be defined:

“`

## class HashTable:

def __init__(self, table_size):

self.table_size = table_size

self.hash_table = [[] for _ in range(table_size)]

def insert(self, key, value):

hash_key = custom_hash(key, self.table_size)

key_exists = False

bucket = self.hash_table[hash_key]

for i, kv in enumerate(bucket):

k, v = kv

if key == k:

key_exists = True

break

if key_exists:

bucket[i] = ((key, value))

else:

bucket.append((key, value))

def find(self, key):

hash_key = custom_hash(key, self.table_size)

bucket = self.hash_table[hash_key]

for i, kv in enumerate(bucket):

k, v = kv

if key == k:

return v

“`

## Insert a Key-Value Pair

The insert method takes a key-value pair and adds it to the Hash Table. In the example above, we first create a hash code using our custom hash function.

We then check if the key already exists in the Hash Table by iterating over the bucket. If the key exists, we update the value, else we create a new key-value pair and add it to the bucket.

## Find a Value by Key

To retrieve a value from the Hash Table using a key, we first create a hash code using our custom hash function. We then iterate over the bucket, looking for the key.

If the key is found, we return the value associated with it.

## Delete a Key-Value Pair

The delete method takes a key and removes the key-value pair associated with it from the Hash Table. In the example above, we create a hash code using our custom hash function.

We then iterate over the bucket, looking for the key. If the key is found, we remove the key-value pair using Python’s built-in del keyword.

## Update the Value of an Existing Pair

The update method updates the value associated with a key in the Hash Table. It is similar to the insert method, except that we don’t need to check if the key exists since we are guaranteed that it does.

## Get the Key-Value Pairs

The get method returns all the key-value pairs in the Hash Table. In the example above, we first iterate over each bucket in the Hash Table, then iterate over each key-value pair in the bucket, and finally append the key-value pairs to a new list.

## Use Defensive Copying

To prevent unintended mutations, we use defensive copying to create a copy of the bucket list before returning it. This ensures that the original data structure is not modified.

## Get the Keys and Values

The keys and values methods return a list of all the keys and values in the Hash Table, respectively. In the example above, we first obtain all the key-value pairs using the get method, then iterate over each key-value pair and append the key or value to a new list.

Report the Hash Table’s Length

The length method returns the number of key-value pairs in the Hash Table. In the example above, we iterate over each bucket in the Hash Table, then iterate over each key-value pair in the bucket, incrementing a counter variable for each key-value pair we encounter.

## Make the Hash Table Iterable

To make the Hash Table iterable, we define the __iter__ and __next__ methods, which allow us to iterate over each key-value pair in the Hash Table.

## Represent the Hash Table in Text

To represent the Hash Table in text, we define the __repr__ method, which returns a string representation of the Hash Table.

## Test the Equality of Hash Tables

To test the equality of two Hash Tables, we define the __eq__ method, which returns True if all the key-value pairs in both Hash Tables are the same.

## Resolve Hash Code Collisions

Hash collisions are a potential problem when using Hash Tables. To resolve hash code collisions, we can utilize two different methods, namely linear probing and separate chaining.

## Find Collided Keys Through Linear Probing

Linear probing is a technique that allows for the detection of collided keys by scanning for the next available bucket until an empty bucket is found. Here’s an example of how the linear probing technique can be implemented:

“`

## class HashTable:

…

def insert(self, key, value):

hash_key = custom_hash(key, self.table_size)

bucket = self.hash_table[hash_key]

if bucket:

for i, kv in enumerate(bucket):

k, v = kv

if key == k:

bucket[i] = ((key, value))

break

else:

bucket.append((key, value))

self.num_keys += 1

if self.num_keys > self.table_size * 0.75:

self.resize()

else:

self.hash_table[hash_key] = [(key, value)]

self.num_keys += 1

if self.num_keys > self.table_size * 0.75:

self.resize()

“`

## Use Linear Probing in the HashTable Class

In the example above, we first create a hash code using our custom hash function. We then check if the bucket associated with the hash code is empty.

If the bucket is not empty, we iterate over all the key-value pairs in the bucket. If the key already exists, we update the value, else we append a new key-value pair to the bucket.

If the bucket is empty, we create a new key-value pair, thereby resolving the collision.

## Let the Hash Table Resize Automatically

To prevent collisions, we can resize the Hash Table automatically when the load factor exceeds a certain threshold. The load factor is the number of key-value pairs divided by the number of buckets in the Hash Table.

In the example above, we check if the load factor is greater than 0.75, and if it is, we resize the Hash Table.

## Calculate the Load Factor

The load_factor method returns the current load factor of the Hash Table. In the example above, we divide the number of key-value pairs by the number of buckets in the Hash Table to get the load factor.

## Isolate Collided Keys With Separate Chaining

Separate chaining is another technique that allows for the isolation of collided keys by creating a linked list in each bucket. Here’s an example of how the separate chaining technique can be implemented:

“`

## class HashTable:

…

def insert(self, key, value):

hash_key = custom_hash(key, self.table_size)

bucket = self.hash_table[hash_key]

if bucket:

for kv in bucket:

if kv[0] == key:

kv[1] = value

break

else:

bucket.append((key, value))

self.num_keys += 1

if self.num_keys > self.table_size * 0.75:

self.resize()

else:

self.hash_table[hash_key] = [(key, value)]

self.num_keys += 1

if self.num_keys > self.table_size * 0.75:

self.resize()

“`

In the example above, we first create a hash code using our custom hash function. We then check if the bucket associated with the hash code is empty.

If the bucket is not empty, we iterate over all the key-value pairs in the bucket. If the key already exists, we update the value, else we append a new key-value pair to the bucket.

If the bucket is empty, we create a new key-value pair, thereby resolving the collision.

## Retain Insertion Order in a Hash Table

To retain insertion order in a Hash Table, we can implement a custom doubly linked list that stores the keys and values in the order that they are inserted. Here’s an example of how this implementation can be done:

“`

## class HashTable:

…

def __init__(self, table_size):

self.table_size = table_size

self.hash_table = [[] for _ in range(table_size)]

self.doubly_linked_list = DoublyLinkedList()

def insert(self, key, value):

hash_key = custom_hash(key, self.table_size)

bucket = self.hash_table[hash_key]

if bucket:

for kv in bucket:

if kv[0] == key:

kv[1] = value

break

else:

bucket.append((key, value))

self.num_keys += 1

self.doubly_linked_list.append((key, value))

if self.num_keys > self.table_size * 0.75:

self.resize()

else:

self.hash_table[hash_key] = [(key, value)]

self.num_keys += 1

self.doubly_linked_list.append((key, value))

if self.num_keys > self.table_size * 0.75:

self.resize()

“`

In the example above, we first create a hash code using our custom hash function. We then check if the bucket associated with the hash code is empty.

If the bucket is not empty, we iterate over all the key-value pairs in the bucket. If the key already exists, we update the value, else we append a new key-value pair to the bucket, and also append it to the doubly linked list.

If the bucket is empty, we create a new key-value pair, thereby resolving the collision.

## Conclusion

Implementing a Hash Table in Python is a powerful tool that allows for fast access to data. Whether you intend to use it for a small collection of data or a larger project, creating an efficient Hash Table requires a little bit of work upfront.

By implementing a custom hash function, building prototypes using Test-Driven Development, and handling hash code collisions using techniques such as linear probing and separate chaining, you can create a robust and dependable data structure.