Introduction to Hash Table

Hash Table and Hash Function

A hash table is a data structure that provides efficient insertion, deletion, and retrieval of key-value pairs. It uses a technique called hashing to map keys to indices in an array, where the values associated with the keys are stored.

The main idea behind a hash table is to use a hash function that takes a key as input and generates a unique index in the underlying array. This index is used to store and retrieve the corresponding value associated with the key. The hash function should ideally distribute the keys uniformly across the array to minimize collisions.

Some of the advantages of using a hash table include:

Fast Lookup: Hash tables provide fast retrieval of values based on keys. On average, the time complexity for search, insertion, and deletion operations is O(1) (constant time), making it efficient for large datasets.

Flexible Key Types: Hash tables can handle various types of keys, including strings, numbers, and custom objects. As long as a suitable hash function is defined, any type can be used as a key.


However, hash tables also have some drawbacks:

Hash Collisions: Hash collisions occur when two different keys generate the same hash value, causing them to be mapped to the same array index. Collisions can lead to performance degradation as additional operations are required to handle them. Techniques like chaining (using linked lists or other data structures to handle multiple elements at the same index) or open addressing (probing for an empty slot) can be used to resolve collisions.

Memory Overhead: Hash tables require additional memory to store the underlying array and handle collisions. If the hash table is not carefully designed or the hash function produces poor distribution, it can result in increased memory usage.

Hash Function Dependency: The efficiency of a hash table relies heavily on the quality of the hash function. A poor hash function can lead to more collisions, impacting the performance of the hash table. Choosing or designing an appropriate hash function is crucial for achieving good performance.


Despite these drawbacks, hash tables remain widely used due to their efficient look-up operations and flexibility in handling different key types. They are commonly implemented in programming languages and utilized in applications that require fast access to data based on keys, such as databases, caches, and symbol tables.