What Is Hashtable Data Structure?


Heather Bennett

A Hashtable is a data structure that stores key-value pairs. It provides a way to store and retrieve elements based on their keys.

The keys are unique identifiers that can be used to access the corresponding values. Hashtable is also known as a hash map or associative array.

How Does a Hashtable Work?

A Hashtable uses a technique called hashing to store and retrieve values efficiently. Hashing involves applying a hash function to each key, which generates a unique hash code for that key. The hash code is then used as an index to store the value in an internal array called the hashtable.

When you want to retrieve a value from the hashtable, you provide the key, and the hashtable uses the same hash function to calculate the hash code for that key. It then uses this hash code as an index to locate the value in the array.

Collision Handling

In some cases, two different keys may result in the same hash code. This situation is known as a collision. To handle collisions, hashtables use various techniques such as:

  • Separate Chaining: In this technique, each index of the hashtable contains a linked list of key-value pairs with matching hash codes.
  • Open Addressing: In this technique, when there is a collision, the hashtable searches for an alternative empty slot in its array by using probing methods like linear probing or quadratic probing.

Main Advantages of Hashtables

Faster Access: Hashtables provide constant-time complexity for accessing elements by their keys. This means that regardless of the size of the hashtable, accessing elements takes approximately the same amount of time.

Efficient Storage: Hashtables use memory efficiently by storing elements in an array. The size of the array can be adjusted dynamically based on the number of elements in the hashtable.

Flexible Key Types: Hashtables can accommodate a wide range of key types, including integers, strings, and even custom objects. This makes them versatile for different types of applications.

Common Use Cases

Hashtables are commonly used in various scenarios, including:

  • Caching: Hashtables can be used to cache frequently accessed data, allowing for faster retrieval and reducing the load on other systems.
  • Database Indexing: Hashtables are used to index database records based on their unique keys, enabling efficient retrieval of data.
  • Symbols Tables: Compilers and interpreters use hashtables to store symbols and their associated values during code execution.

In conclusion, hashtables provide an efficient way to store and retrieve key-value pairs. They utilize hashing techniques to ensure fast access times and offer flexibility in terms of key types. By understanding how hashtables work and their common use cases, you can leverage this data structure to optimize your applications.

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