A hash table, also known as a hash map, is a popular data structure that provides efficient insertion, deletion, and retrieval operations. It is based on the concept of hashing, which involves mapping key-value pairs using a hash function. The primary purpose of a hash table is to store and retrieve values based on their associated keys in constant time on average.
Here’s a step-by-step explanation of how a hash table works:
- Hash Function: A hash function takes a key as input and computes a hash code or hash value, which is typically an integer. The goal of a good hash function is to distribute the keys uniformly across the available hash table slots, minimizing collisions (more on this later).
- Hash Table Structure: A hash table is typically implemented as an array of fixed size, known as buckets or slots. Each slot can hold a key-value pair or, in some cases, a linked list or other data structure to handle collisions.
- Insertion: When inserting a new key-value pair into a hash table, the hash function is applied to the key to determine the index (slot) where the pair will be stored. If the slot is empty, the pair is simply stored there. If the slot is occupied, a collision occurs.
- Collision Handling: Collisions happen when two or more keys hash to the same index. To handle collisions, there are several techniques:
- Separate Chaining: Each slot in the hash table contains a linked list or some other data structure. Colliding key-value pairs are stored in the same slot, forming a chain.
- Open Addressing: When a collision occurs, the hash table searches for the next available slot, following a predetermined probing sequence (linear probing, quadratic probing, etc.), until an empty slot is found.
- Robin Hood Hashing: A variant of open addressing where the hash table rearranges colliding elements to minimize the length of the probing sequence.
- Cuckoo Hashing: Two hash functions are used, and each key is assigned to one of the two possible slots. If a collision occurs, one of the keys is evicted and moved to its alternative slot.
- Retrieval: To retrieve a value associated with a key, the hash function is applied to the key to determine the index. If there are no collisions, the value is found directly in the slot. If there are collisions, the search follows the same collision resolution mechanism used during insertion until the key is found or determined to be absent.
- Deletion: Deleting a key-value pair involves locating the key in the hash table, which is similar to the retrieval process. Once found, the slot is either marked as empty (in separate chaining) or marked with a special marker to indicate a deleted entry (in open addressing).
The efficiency of a hash table depends on the quality of the hash function, the load factor (the ratio of occupied slots to the total number of slots), and the chosen collision resolution technique. With a well-designed hash function and appropriate handling of collisions, hash tables can provide constant-time average case performance for insertion, deletion, and retrieval operations.
I hope this explanation helps you understand the mechanism of a hash table!