A hash table, or a hash map, is a data structure used in computer science to store and retrieve values based on a unique key. Hash tables offer an efficient implementation method for associative arrays or dictionaries, which involve storing data in the form of key-value pairs. The primary idea behind a hash table is to use a hash function to map keys to indices in an array, allowing for quick retrieval of values associated with those keys.
Here’s how a hash table works:
- Hash Function: A hash function takes an input (the key) and produces a fixed-size value (the hash code or hash value) representing the key. The goal of a good hash function is to evenly distribute keys across the available indices in the underlying array.
- Indexing: Subsequently, the hash code comes into play to ascertain the precise index for storing the related value. This operation commonly involves employing the modulo operation on the hash code, using the size of the collection. Which wraps the hash code into the valid index range.
- Collision Handling: Since multiple keys could potentially hash to the same index (collision), hash tables need to address collisions. Various collision resolution techniques exist, such as chaining and open addressing. Chaining involves maintaining linked lists at each array index to store multiple values with the same hash code. Open addressing involves searching for the next available slot in the array when a collision occurs.
It’s important to note that the effectiveness of a hash table depends on the quality of the hash function and the chosen collision resolution strategy. A poor hash function or inefficient collision resolution can lead to performance degradation and reduced efficiency.