What Is a Hash Table in Data Structure?


Heather Bennett

A hash table, also known as a hash map, is a popular data structure used in computer science to store and retrieve data efficiently. It provides fast access to stored elements by using a technique called hashing. In this article, we will explore the concept of hash tables and understand how they work.


Hashing is the process of converting an input value into a numerical value called a hash code. The hash code is then used as an index to store and retrieve elements in the hash table. The goal of hashing is to distribute the elements evenly across the available space to ensure efficient retrieval.

To calculate the hash code, a hash function is used. A good hash function should produce unique codes for different inputs and minimize collisions where two different inputs produce the same code. Collisions can lead to performance degradation in accessing elements.

Structure of Hash Tables

A hash table consists of an array or a list of buckets, where each bucket stores one or more key-value pairs. The number of buckets depends on the size of the data set and can vary dynamically as elements are added or removed.

When inserting an element into a hash table, the key-value pair is first hashed using the chosen hash function. The resulting hash code determines which bucket it should be placed in. If there are already elements in that bucket, it may result in a collision.

Collision Resolution

There are various methods to handle collisions:

  • Chaining: In this method, each bucket contains a linked list or another data structure that stores multiple elements with the same hash code.
  • Open addressing: In this method, if there is a collision, an alternative location within the table is found by using a probing sequence. Common probing methods include linear probing, quadratic probing, and double hashing.

Both methods have their pros and cons, and the choice depends on factors such as the expected number of collisions and the cost of memory allocation.

Benefits of Hash Tables

Hash tables offer several advantages:

  • Fast access: Retrieving elements from a hash table is typically faster than other data structures. The average time complexity for accessing an element in a hash table is O(1).
  • Efficient storage: Hash tables can store large amounts of data efficiently by distributing elements across multiple buckets.
  • Flexible key-value pairs: Hash tables allow storing data as key-value pairs, making it easy to associate values with specific keys.

Common Use Cases

Hash tables find applications in various domains, including:

  • Data caching: Storing frequently accessed data to improve performance.
  • Databases: Indexing and quick retrieval of records based on keys.
  • Syntax highlighting: Mapping keywords to their respective styles in code editors.
  • Spell checkers: Efficiently checking if a word is spelled correctly by using a dictionary as a hash table.

In Summary

A hash table is an efficient data structure that allows fast storage and retrieval of elements. It uses hashing to convert input values into unique hash codes, which determine the placement of elements in buckets. Hash tables provide fast access, efficient storage, and flexible key-value pairs, making them widely used in various applications.

If you want to learn more about hash tables or implement one in your code, make sure to explore further resources and practice. Happy coding!

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