What Is a Hash Table Data Structure?


Larry Thompson

A hash table is a data structure that allows for efficient retrieval and storage of key-value pairs. It is also known as a hash map or dictionary in other programming languages.

The main idea behind a hash table is to use a hash function to convert keys into an index or address in an array. This allows for constant-time average retrieval and insertion operations, making it one of the most popular data structures used in computer science.

How Does a Hash Table Work?

At the core of a hash table is an array, which serves as the underlying data structure. Each element in the array is called a bucket or slot, and it can store one or more key-value pairs. The size of the array determines the number of buckets available for storing data.

When inserting a key-value pair into a hash table, the hash function takes the key as input and computes an index within the range of the array size. This index is then used to determine which bucket in the array should hold the data. The value associated with the key is stored in that bucket.

The process of retrieving a value from a hash table follows a similar pattern. The hash function calculates an index based on the provided key, and then this index is used to locate the corresponding bucket where the value is stored.

Collision Handling

A potential issue with using arrays and hash functions is that multiple keys can map to the same index, causing collisions. There are several techniques for handling collisions:

  • Separate Chaining: Each bucket contains its own linked list or other data structure to store multiple values that map to the same index.
  • Open Addressing: When there’s a collision, an alternative empty slot in the array is found by probing or searching sequentially for the next available slot.

Both techniques have their advantages and trade-offs, and the choice between them depends on the expected usage patterns and requirements of the hash table.

Benefits of Using a Hash Table

Hash tables offer several benefits:

  • Fast Retrieval: Retrieving a value from a hash table has an average time complexity of O(1), making it highly efficient.
  • Flexible Key Types: Hash tables can handle various types of keys, not just integers but also strings, objects, and more.
  • Dynamic Size: Hash tables can dynamically resize themselves to accommodate more data without a significant performance impact.


A hash table is a powerful data structure that provides fast retrieval and storage of key-value pairs. By using a hash function to map keys to indices in an array, hash tables can achieve constant-time average performance for common operations.

Handling collisions is an important consideration when implementing a hash table. By understanding how hash tables work and their benefits, you can leverage this versatile data structure to improve the efficiency of your algorithms and applications.

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