What Is Hashing and Hash Table in Data Structure?

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Scott Campbell

In data structure, hashing is a technique used to map data to a fixed-size array called a hash table. This allows for efficient retrieval and storage of data. Hashing involves using a hash function to generate a unique identifier or hash code for each data element.

Hash Function

A hash function takes an input (data element) and produces a fixed-size output, which is typically an integer. The output is deterministic, meaning that the same input will always produce the same output. The goal of a good hash function is to distribute the data uniformly across the hash table.

Hash Table

A hash table is an array that stores key-value pairs. Each key-value pair is stored at a specific index in the array, determined by applying the hash function to the key. The index where an element should be stored is called its hash code.

Hash tables provide constant-time average-case performance for operations such as insertion, deletion, and retrieval. This makes them ideal for applications where fast access to data is required.

Collision Handling

Since the number of possible keys is usually larger than the size of the hash table, collisions may occur when two different keys produce the same hash code. There are several methods for handling collisions:

  • Separate Chaining: In this method, each slot in the hash table contains a linked list of elements that share the same hash code. When there’s a collision, new elements are added to this linked list.
  • Open Addressing: In this method, when there’s a collision, another slot in the hash table is probed until an empty slot is found.
  • Linear Probing: This open addressing technique probes the next slot in the hash table sequentially until an empty slot is found.
  • Quadratic Probing: This open addressing technique probes the next slot using a quadratic formula until an empty slot is found.

Advantages of Hash Tables

Hash tables offer several advantages:

  • Fast Access: Hash tables provide constant-time average-case performance for insertion, deletion, and retrieval operations.
  • Flexible Key Types: Hash tables can handle keys of various types, as long as a hash function can be defined for them.
  • Ease of Implementation: Implementing a hash table is relatively straightforward compared to other data structures.

Conclusion

In summary, hashing and hash tables are fundamental concepts in data structure. They allow for efficient storage and retrieval of data by mapping elements to a fixed-size array using a hash function.

Collisions may occur when two different elements produce the same hash code, which can be handled through techniques such as separate chaining or open addressing. Hash tables provide fast access to data and can handle keys of various types. Their ease of implementation makes them widely used in many applications.

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