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class HashTable:
    # Initialize the hash table
    def __init__(self, size):
        self.size = size
        self.table = [None] * size

    # Hash function that uses the remainder of the key divided by the size
    def hash_function(self, key):
        return 0

    # Insert a key-value pair into the hash table
    def insert(self, key, value):
        index = self.hash_function(key)
        self.table[index] = value
        print(f"Key: {key} -> Index: {index}, Value: {value} stored")

    # Retrieve all entries stored in the hash table
    def retrieve_all(self):
        print("All entries in the hash table:")
        for i, value in enumerate(self.table):
            if value is not None:
                print(f"Index: {i}, Value: {value}")
            else:
                print(f"Index: {i}, Empty")

# Example usage of the hash table
hash_table = HashTable(10)

# Adding data to the hash table
hash_table.insert("apple", "red fruit")
hash_table.insert("banana", "yellow fruit")
hash_table.insert("grape", "purple fruit")

print("=" * 40)
# Retrieve all entries in the hash table
hash_table.retrieve_all()

# Managing Data with Keys and Values using a Hash Table

A `Hash Table` is a data structure that stores data in **key-value pairs** similar to how zip codes are used to look up addresses.

Using a key to quickly find a value makes it efficient to manage large amounts of data.

When storing data, a `Hash Function` is used to generate a specific number (hash value) based on a unique rule, which determines where to store the data.

<br/>

## How Does a Hash Table Work?

A hash table is a data structure that stores key-value pairs.

A hash function takes a key and generates a specific number (hash value) based on a unique rule, which is used to determine the storage location in the table.

For example, the remainder of the input data's length divided by the hash table size can be used as a hash value.

```python title="Example Hash Function"
def hash_function(key):
    table_size = 10
    # Use the remainder of the key's length divided by the table size as the hash value
    return len(key) % table_size
```

For example, if you input the key `"apple"` into a hash function with a table size of 10, the length of `"apple"` is 5, so `5 % 10 = 5` will be returned as the hash value.

This returned hash value means the hash function will store the key `"apple"` and its corresponding value at the 5th position in the hash table.

<br/>

## Implementing a Hash Table in Python

In Python, you can implement a simple hash table using lists and a hash function.

```python title="Simple Hash Table Implementation"
class HashTable:
    # Initialize the hash table
    def __init__(self, size):
        self.size = size
        self.table = [None] * size

    # Hash function that uses the remainder of the key size
    def hash_function(self, key):
        return hash(key) % self.size

    # Store key and value in the hash table
    def insert(self, key, value):
        index = self.hash_function(key)
        self.table[index] = value

        print(f"Key: {key} -> Index: {index}, Value: {value} has been stored in the table.")
```

<br/>

## Code Explanation

- *HashTable class*: Defines the hash table. The `__init__` method sets the size of the hash table and creates a list of that size.

- *hash_function method*: Converts the key to a hash value using Python's built-in `hash` function and determines the storage index by dividing it by the table size.

- *insert method*: Stores the key and value in the hash table. The key is processed by the hash function, and the resulting index stores the value.

- *search method*: Searches for a value in the hash table using the key. It finds the stored index of the key and returns the value.

<br />

## When is a Hash Table Used?

Hash tables are used in situations requiring quick data retrieval and storage, such as in large databases or caches.

A hash table generates a unique hash value using a key, and it uses this hash value to store and retrieve data. This allows for efficient management and fast retrieval of large amounts of data.

A hash table is a data structure that stores data in key-value pairs.