What Is Rehashing Data Structure?


Larry Thompson

Rehashing is a technique used in data structures to resolve collisions that occur when multiple keys are mapped to the same index in a hash table. It involves finding an alternative index for the key by applying a secondary hashing function.

Understanding Hash Tables

Before diving into rehashing, let’s first understand what a hash table is. A hash table, also known as a hash map, is a data structure that allows efficient insertion, deletion, and retrieval of key-value pairs. It works on the principle of hashing, where each key is mapped to a unique index in an array called the hash table.

Hash functions play a crucial role in determining the index for each key. Ideally, these functions should distribute the keys uniformly across the array to minimize collisions. However, collisions are inevitable due to limited array size and an infinite number of possible keys.

The Need for Rehashing

Collisions occur when two or more keys are mapped to the same index in a hash table. This can lead to performance degradation as it increases the time complexity of operations like insertion and retrieval. The primary goal of rehashing is to reduce collisions and improve overall efficiency.

Rehashing involves finding an alternative index for a key when a collision occurs at its original index. This is done by applying another hashing function to the key until an empty slot or an available index is found.

The Process of Rehashing

The process of rehashing can be summarized in the following steps:

  • Additional Hash Function: A secondary hashing function is applied to calculate an alternative index for the collided key.
  • New Index Calculation: The new index is calculated using this secondary hashing function, taking into consideration the size of the hash table.
  • Collision Resolution: If the new index is already occupied by another key-value pair, a collision has occurred again. In this case, the process is repeated until an empty slot or an available index is found.
  • Reinsertion: Once a suitable index is found, the collided key-value pair is reinserted at this new index in the hash table.

Benefits of Rehashing

Rehashing offers several benefits in terms of improving the performance and efficiency of hash tables:

  • Reduced Collisions: By finding alternative indices for collided keys, rehashing helps minimize collisions and distribute keys more evenly across the hash table.
  • Faster Operations: With fewer collisions, operations like insertion and retrieval can be performed more efficiently as they require fewer comparisons and iterations.
  • Better Space Utilization: Rehashing allows for better utilization of space within the hash table by reducing the number of empty slots caused by collisions.


In conclusion, rehashing plays a vital role in resolving collisions that occur in hash tables. By applying secondary hashing functions to collided keys, rehashing helps distribute them more uniformly across the hash table.

This results in improved performance, faster operations, and better space utilization. Understanding rehashing is essential for anyone working with hash tables to ensure efficient data storage and retrieval.

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