What is hash map?

person shubham sharmafolder_openFlutterlocal_offeraccess_time October 15, 2024

A hash map (also known as a hash table) is a data structure that stores key-value pairs, allowing for efficient data retrieval based on keys. It is widely used for fast lookups, insertions, and deletions.

Key Concepts of a Hash Map:

  1. Key-Value Pair: Each element in a hash map is stored as a pair, consisting of a key and a value. The key is unique, and it is used to access the corresponding value.

  2. Hash Function: A hash function is used to convert the key into a hash code (an integer). This hash code determines where the key-value pair should be stored in the map (its index).

  3. Buckets/Slots: The hash map uses an array of “buckets” or “slots” to store the key-value pairs. The hash code determines which bucket the data goes into. Ideally, each key gets mapped to a unique bucket.

  4. Collision Handling: Sometimes two keys may produce the same hash code (known as a collision). There are various strategies to handle this:

    • Chaining: Store multiple key-value pairs in the same bucket using a linked list or another data structure.
    • Open Addressing: Find another empty slot in the array when a collision occurs (linear probing, quadratic probing, etc.).
  5. Time Complexity:

    • Lookups (searching for a key): On average, this is O(1) (constant time), meaning that it’s extremely fast.
    • Insertions: On average, this is also O(1).
    • Deletions: On average, O(1) as well.

Example in Dart (Using Map):

In Dart, the Map class is an implementation of a hash map. Here’s a simple example:

Key Features of a Hash Map:

  • Fast Access: Hash maps provide constant-time access to data in ideal conditions.
  • Dynamic Size: They can resize dynamically as the number of key-value pairs grows, although this may incur occasional performance overhead (when resizing occurs).
  • Unordered: The order of key-value pairs is not guaranteed; the internal order is based on the hash codes.

Use Cases:

  • Dictionary-like structures: Storing and retrieving items by key.
  • Counting occurrences: Hash maps are often used for counting occurrences of items (e.g., words in a document).
  • Caching: Quick lookup of previously computed results.

Limitations:

  • Hash Collisions: Although hash maps are designed to handle collisions, poor hash functions or excessive collisions can degrade performance.
  • Memory Overhead: Hash maps can consume more memory than other data structures like arrays or lists due to internal structures (e.g., linked lists in chaining).

In summary, a hash map is an efficient and widely used data structure for storing and retrieving key-value pairs quickly, with average time complexities of O(1) for most operations.

warningComments are closed.