# Hash Structure

Explore Hash Structures in Computer Science for an insightful perspective into this abstract and crucial data handling concept. This exploration focuses on demystifying the complex algorithmic feature that is hashing, elucidating its integral role in successful data management and manipulation. Navigate your way through the intricacies of hashing, understanding its robust presence in various lingo, with Python being a poignant example. Grasp the workings of diverse hashing methodologies, analysing the strengths and pitfalls of each as you equip yourself with comprehensive knowledge of hash functions. Challenging preconceived notions, there's an attempt to bust common myths associated with hashing. The ultimate goal is to deepen your understanding of hash structures in an accessible, engaging and fact-rich environment. From theoretical foundations to practical applications, this is a complete package, addressing the varied facets of hash structures in computer science.

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## Define Hashing in Computer Science

Hash structures are an integral part of computer science, aiding in efficient data manipulation and management. Their powerful capability to enhance the performance of data structures is what makes them so crucial in this field.

Hashing is a technique that is used to uniquely identify a specific value from a collection of values. It's a process that helps retrieve the value of a variable directly without searching the entire set.

1. Hashing involves the use of a 'hash function' to generate a unique 'hash code' or 'hash value' for a given input value.
2. The input to the hash function can be of any length but the output (hash value) is always of a fixed size.
3. The hash function ensures that even a minor change in input value would result in a major change in the output hash value.
In the realm of Computer Science, a hash structure or 'hash table' is a data structure that implements an associative array abstract data type, essentially a structure that can map keys to values.

Assume we have data pairs where the first element is the name of a student and the second element is their phone number. A hash table could be used to save this data and allow us to rapidly look up the phone number associated with any student's name.

### Importance of Hashing in Data Handling

Hashing serves an indispensable role in data handling. It offers swift data retrieval, making it beneficial for database indexing, cache storage, and data retrieval operations in large databases.
• In database management, hashing can be used as an index mechanism. This allows data to be retrieved without having to scan every single record — a process that would be highly time-consuming in large databases.
• In cache storage, data can be distributed across multiple storage buckets using a hash function. This leads to efficient data access and storage management.
• Another use case for hashing is in data encryption. Hash functions, especially cryptographic hashing, can be used to ensure data security and integrity.

Even password management systems make use of hashing. When a user creates an account with a password, the password is hashed and the hash value is stored. When they log in, the password is hashed again and checked against the stored hash value. This ensures that even if someone can access the stored hashes, they cannot reverse engineer the original password.

While hashing is an important technique, there are some common misconceptions to be aware of.
1. Contrary to popular belief, hashing is not encryption. Hashing is a one-way function - i.e., a function which is practically infeasible to invert. You can't get back the original data from the hashed value.
2. It is a common mistake to think that similar input data will result in similar hash values. A good hash function will produce drastically different results even for minutely varied input data. This property is known as the 'avalanche effect'.
3. Another misunderstanding is that hash values are unique. In reality, multiple different inputs may yield the same hash value, an event known as a 'hash collision'. However, a good hash function will minimise this probability.
These clarifications should aid you in understanding and optimally using hash structures in computer science. Hashing techniques are indeed powerful, but like any tool, you need to know how to use them properly to reap the maximum benefits.

## Deep Dive into Hash Function in Data Structure

Understanding the structure of data is of vital import when tackling hash function methodologies. Without this, fully comprehending how the hash function operates becomes significantly more difficult.

### Understanding the Role of the Hash Function

In hashing, the hash function is the core player. It acts as a bridge between the input data and the hash structure or hash table. The key role of a hash function in a hash table is to compute an index into an array of buckets or slots, from which the desired value can be found. Essentially, given a key, the hash function produces an integer, which can then be used as an index to locate the associated value.

A hash function is any function that can be used to map data of arbitrary size to fixed-size values. The values returned by a hash function are often called hash codes, hash values, hashes, or simply indices.

The primary goal of a hash function is to distribute the keys uniformly across the hash table. A hash function does this optimally if each hash is independent of other hashes and each is provided roughly equally. There are a few crucial properties of a good hash function:
• It should be deterministic, meaning the same input will always produce the same hash.
• It should be fast to compute the hash value for any given input.
• It should evenly distribute hash values across the hash table (uniformity).
• It should ensure a drastically changed output even for a minutely changed input (avalanche effect).

### Different Hash Function Methodologies

Various methodologies encode different characteristics in a hash function, each suited to specific types of data and use cases. Some common hash function methodologies include:
1. Division Method: In this method, the hash function is defined as $$h(k) = k \mod p$$, where $$k$$ is the key, $$p$$ is a prime number and $$mod$$ signifies the modulus operation. This method works best when the choice of $$p$$ is not close to a power of 2, given binary representations of keys. This is to avoid generating the same hash for keys that are multiples of each other.

If we consider $$p$$ as 7, the hash function will distribute the keys uniformly as the prime number 7 doesn't align with powers of 2. So, for keys 15 and 22, $$h(15) = 15 \mod 7 = 1$$ and $$h(22) = 22 \mod 7 = 1$$. This scenario shows a hash collision, where two different keys resolve to the same index.

2. Multiplication Method: This method works by multiplying the key $$k$$ with a constant $$A (\ 0 < A < 1$$ ), extracting the fractional part of $$kA$$, and then multiplying this by $$m$$, the size of the table, with the result taken as the floor value. The beauty of this method is that the value of $$A$$ doesn’t have to be a prime number and it has the flexibility of setting the table size $$m$$ to be any convenient size.
3. Universal Hashing: This method effectively randomises the process of hashing. Instead of a single hash function, it utilises a collection of hash functions chosen in a random manner.

## Application of Hash Function in Various Scenarios

Hash functions find broad applications due to their efficiency in data handling and retrieval. These real-world applications help you see how abstract concepts are implemented in actual scenarios:
• Data Retrieval: In databases, hash functions are used to retrieve data without having to search the entire database. The hash code of an item is used to identify the item's location.
• Cryptography: Cryptographic hash functions are used extensively in information security applications such as password storage and data integrity checks. These functions take an input and return a hashed output of a fixed size, making them ideal for generating unique identifiers.
• Cache Function: In memory cache systems like MemCached or Redis, hash functions allocate data across multiple storage buckets for efficient data access.
• Load Balancing: Hash functions are used in designing load balancers for distributed systems. By hashing incoming requests, the load balancer can determine the appropriate server for each request, ensuring an even distribution of load.

In the area of big data, hash functions play a monumental role. They are used as a checksum to verify data integrity while transferring large amount of data. These functions are also instrumental in MapReduce frameworks like Hadoop to partition, shuffle and sort data.

We have now embarked on a thorough exploration of hash functions in data structures, understanding their role, different methodologies, and wide array of applications. Each understanding deepens your grasp of the puzzle that is hashing in Computer Science.

## Mastering Hashing Algorithm in Data Structure

In the world of computer science, mastering the concept of hashing algorithms is a crucial step forward. These algorithms underpin the swift and efficient access, storage, and retrieval of data across several applications.

### Working Principles of Hashing Algorithm

The essence of the hashing algorithm lies in the hash function that it utilises. Each time an input key is 'hashed', the hash function generates a specific index or 'hash value'. This value is then used to locate the storage position in the hash table for the respective data record. The uniqueness of each hash value facilitates direct data access within the collection, thereby bypassing the need for time-consuming search operations. So, let's delve deeper into the core principles behind a hash function:
1. Determinism: For any given input, the function must consistently yield the same output, assuming no alteration of the input.
2. Fixed Size: Regardless of the input size, the function results in a fixed-size hash value.
3. Every output (hash) is equally likely: A key characteristic of a good hash function is the even distribution of hash values. Keys must not cluster under certain indexes but should scatter uniformly over the entire table.
4. Avalanche Effect: Even a slight modification in the input must lead to a drastic change in the output, implying the hash values are highly sensitive to the input values.

### Algorithm Design Considerations for Hashing

The design of the hashing algorithm is pivotal in maintaining integrity and avoiding constraints of the hash functions. Here are few considerations to maintain an effective design:
1. Avoidance of Collision: A collision occurs when two distinct keys give rise to the same hash value. Although it's impossible to avoid collisions completely, a good hashing algorithm strives to minimise them. Handling strategies like chaining, open addressing, or double hashing could be employed to manage collisions when they occur.
2. Load Factor: The 'Load factor' ($$\lambda$$) is defined as the number of items stored in the hash table divided by the capacity of the hash table. $\lambda = \frac{n}{k}$ where $$n$$ is the number of keys and $$k$$ is the size of the hash table. The load factor helps keep track of the space usage and when it crosses a predefined threshold, it indicates it's time to resize the hash table.
3. Choice of Hash Function: The choice of hash function primarily depends on the data. Numeric keys may use division or multiplication, while string-based keys often use polynomial methods.
4. Table Size: Size of the hash table plays a vital role in hashing. It is generally preferred to be a prime number to facilitate uniform distribution and reduce the likeliness of collision.

### Pros and Cons of Different Hashing Algorithms

Different scenarios call for different hashing algorithms and each one has its own pros and cons. Here's a brief look at a few of the most commonly used hashing algorithms:
Hashing AlgorithmProsCons
Division HashingRelatively simple, works well for numeric keysSensitive to the choice of the divisor; risk of clustering
Multiplication HashingAble to handle any kind of input data, less clusteringComputationally intensive due to multiplication and extraction of the fractional part
Universal HashingRandomised approach reduces the risk of clustering, ideal for keys that follow a patternRequires a good random number generator, might be computationally intensive
Appreciating the intricacies of different hashing algorithms and their respective highs and lows aids in making judicious choices while designing data structures. Whether your priority is search speed, or saving memory, understanding these concepts helps you craft data structures to best serve your needs.

## Hashing Data Structure in Python

Python as a high-level programming language provides direct support for data structures such as hash tables, also known as 'dictionaries'. These pre-packaged data handling tools make Python an excellent choice for hashing applications in computer science.

### Implementing Hash Structures in Python

Python implements hash structures through a built-in data type called 'dictionary'. A dictionary in Python is an unordered collection of items and is defined within curly brackets { }. Each pair in the dictionary is separated by a colon (:), where the first element is known as 'key' and the second as 'value'. The items are separated by commas, and the whole thing is enclosed in curly braces. An example of dictionary representation in Python:


student = {
'name': 'John',
'age': 20,
}

In this instance, 'name', 'age', and 'grades' serve as keys, and 'John', 20, and [88, 76, 92] are the corresponding values. Key points to note while implementing hash structures in Python:
• The keys in a dictionary are unique and immutable, meaning they cannot be changed. They are also hashable, allowing a hash value for each key to be calculated and stored along with the item.
• Values associated with the keys can be of any Python data type, and they can be modified at any point.
• Values can be accessed, removed or modified directly through their unique keys.
Python dictionaries offer various in-built methods that simplify operations like searching, updating, and removing items.

### Explaining Python’s Built-in Hash Function

Python comes with a built-in hash() function, which accepts a single immutable object (like numbers, strings, tuples) as input, and returns a fixed-size integer value. This function is essential for maintaining the integrity and efficiency of Python dictionaries. Let's consider an example of the built-in hash function:
hash_value = hash("Python")
print(hash_value)

The output will be a unique integer value that represents the hash code for the string "Python". It's important to note a few things about Python's built-in hash() function:
• The hash function can only accept an immutable type as input. Attempting to hash mutable items such as lists or dictionaries will lead to a TypeError.
• The function returns a hash value which is a transparent object. While Python guarantees that for an object $$x$$, $$hash(x)$$ will always yield the same results throughout the program lifecycle, the result may vary across different runs of the program or different versions of Python.
• The hash function in Python is deterministic, meaning it will return the same hash value for the same input across all Python environments and platforms (within the constraints of the same Python version and bitwise architecture).

### Practical Python Code Examples for Hashing

To illustrate some practical use-cases of hash structures in Python, let's delve into some code examples. Example 1: Create a dictionary of items, access values and modify a value.

product = {
'name': 'Laptop',
'price': 800,
'quantity': 5
}

print(product['name'])  # prints: Laptop
print(product['price'])  # prints: 800

# Changing price value
product['price'] = 900

print(product['price'])  # prints: 900

Example 2: Implementing a simple password storage and verification system using hash.

import getpass
import hashlib

# Create a dictionary to store users and their hashed passwords
users = {}

# Create a hash of the password

# Store the user and hashed password in the dictionary

# Check if the user exists and the hashed password match
print("Access granted.")
else:

This Python script asks for a username and password, hashes the password and stores it with the username in a dictionary. It then asks for the username and password again before performing the hash function and checking it against the stored hash value. This is a simplified example of how hashed data structures are used to maintain user privacy and security.

## Types of Hashing in Data Structure

Hashing in data structures can be implemented through various techniques. These numerous approaches can seem overwhelming at first, so having an in-depth understanding of each type is critical in using them efficiently in computer science.

### Overview of Different Hashing Types

At a high level, hashing techniques can be divided into a few prominent types:
1. Static Hashing: In static hashing, the hash function allocates data to a fixed number of predefined buckets. This means the size of the hash table is fixed and doesn't change with the increase or decrease in the number of entries.
2. Dynamic Hashing: Contrary to static hashing, dynamic hashing adapts to changes in the size of the hash table. It allows the hash function to add or remove buckets dynamically according to the volume of entries.
3. Linear Hashing: This is a hybrid of static and dynamic hashing, best suited for database applications. It allows records to be added and removed one bucket at a time, while maintaining a linear hash function.
4. Distributed Hashing: In this method, the hash table is divided into several nodes. Each node is responsible for managing a portion of the hash table. It is often used in distributed storage systems.
Each of these types possesses their own sets of advantages and trade-offs, making them suitable for certain specific scenarios.

### Comparative Analysis of Various Hashing Types

Let's dive deeper into their differences, focusing on their key characteristics and advantages:
Static HashingFixed number of buckets, use of simple hash functionSimple implementation and predictable memory usage
Dynamic HashingVariable bucket count, resizing of hash table according to load factorHighly scalable, supports large volumes of data
Linear HashingIncremental growth of hash table, addition or removal of one bucket at a timeOptimal for databases, smoother transition during rehashing
Distributed HashingDivided hash table, different nodes manage different partsFits distributed storage systems, enhances data availability and resilience
By contrasting various types of hashing, you can establish a better understanding of which type is most suitable according to the nature and size of the data you are dealing with.

### Real-world Examples for Different Hashing Types

Real-world examples can often clarify abstract concepts. So here are a few use-cases for each type of hashing:
• Static Hashing: Mail departments utilise static hashing to sort mails into predefined pigeonholes based on the first digit of pin codes.
• Dynamic Hashing: E-commerce platforms maintain user session data using dynamic hashing. As the number of active users fluctuates dynamically, the method efficiently handles the scaling in/out of session data.
• Linear Hashing:Databases in a flight reservation system can utilise linear hashing to handle tickets bookings and cancellations. The handling of one bucket at a time ensures smooth capacity transitions.
• Distributed Hashing: Distributed file systems such as the Hadoop Distributed File System (HDFS) make use of Distributed Hashing for dividing data across multiple nodes for achieving fault tolerance and load balancing.
These practical applications underline the vital role of hashing in managing and manipulating data in a wide range of scenarios.

## What is Hashing? - Key takeaways

• Hash structures are an essential part of computer science, aiding in efficient data manipulation and management.

• Hashing in data structure refers to the technique used to uniquely identify a specific value from a collection of values.

• The input to the hash function can be of any length but the output (hash value) is always of a fixed size.

• Hashing in data handling offers swift data retrieval, beneficial for database indexing, cache storage, and data retrieval operations in large databases.

• A hash collision occurs when different inputs yield the same hash value, a good hash function will minimise this probability.

Is hash table a linear data structure?

No, a hash table is not a linear data structure. In linear structures, data items are arranged in a sequence, one after the other, whereas in a hash table, data is not ordered. Instead, it uses a hash function to compute an index into an array in which an element will be inserted or searched.

What is a hash table data structure?

A hash table is a data structure that implements an associative array abstract data type, a structure capable of storing key-value pairs. Keys are hashed into a set of integer indices that correspond to buckets where the corresponding values are stored. It uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found. In essence, it allows access to data in constant time, making it highly efficient for large scale data sets.

What is hash function in data structure?

A hash function in data structure is a special function that is used to map data of arbitrary size to fixed size values. These resultant fixed size values are known as hash codes or hash values. It is primarily used in hash tables to quickly locate a data record given its corresponding unique key. The efficiency of a hash table depends largely on the efficiency of the hash function.

What is hashing in data structure with example?

Hashing in data structure is a technique used to directly map data to a specific location in a memory block. It helps in quick data retrieval by utilising a predefined unique index (hash key). For example, when searching for a book in a library database, instead of going through each record, the hashing function can map "book title" directly to its data location, significantly reducing search time.

What is hash collusion in data structure?

Hash collision in a data structure occurs when two different keys produce the same hash value. This creates a problem because each unique key is supposed to map to a distinct location in the data structure. Hash collisions can degrade the performance of the hash table by forcing it to manage multiple keys at the same location. There are techniques to handle these collisions such as chaining and open addressing.

## Test your knowledge with multiple choice flashcards

What is an example of the use of Dynamic Hashing in real-world applications?

What are the applications of hashing in data handling?

What are some common hash function methodologies?

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