What Is Hash Map in Data Structure?


Angela Bailey

A hash map is a data structure that provides a way to store and retrieve key-value pairs efficiently. It is also known as a hash table or associative array. In a hash map, keys are unique and used to access their corresponding values.

How does a Hash Map work?

A hash map uses a technique called hashing to store and retrieve values based on their keys. When a key-value pair is inserted into a hash map, the key is hashed using a hash function. The resulting hash code is used as an index to store the value in an underlying array.

The hash function maps each key to a unique index in the array. However, collisions can occur when two different keys produce the same hash code.

To handle collisions, most hash maps use a technique called chaining. In chaining, each array index contains a linked list of key-value pairs with the same hash code.

To retrieve a value from the hash map, the key is hashed again using the same hash function. The resulting index is used to look up the linked list at that position. The linked list is then traversed until either the desired key is found or the end of the list is reached.

Advantages of using Hash Maps

  • O(1) average time complexity: Hash maps provide constant-time average-case performance for insertion, deletion, and retrieval operations. This makes them efficient for large datasets.
  • Flexible key types: Hash maps can handle various types of keys such as integers, strings, or objects.
  • Fast lookup: Since accessing elements in an array by index has constant time complexity, retrieving values from a hash map is usually faster compared to other data structures.

Disadvantages of using Hash Maps

  • Worst-case time complexity: In rare cases, hash maps can have worst-case time complexity of O(n) for certain operations, such as when there are many collisions.
  • Memory overhead: Hash maps require additional memory to store the underlying array and linked lists.
  • No guaranteed order: The order in which elements are stored in a hash map is not predictable or consistent.

Common Use Cases for Hash Maps

Hash maps are widely used in various applications. Some common use cases include:

  • Caching: Hash maps can be used to cache frequently accessed data, where the keys represent input values and the values represent corresponding output values.
  • Indexing: Hash maps are often used to build indexes in databases, allowing efficient lookup of records based on specific criteria.
  • Data storage and retrieval: Hash maps provide an efficient way to store and retrieve data based on unique identifiers or keys.

In conclusion

A hash map is a powerful data structure that allows efficient storage and retrieval of key-value pairs. It offers constant-time average-case performance for most operations and is widely used in various applications. Understanding how hash maps work can help optimize your code and improve the efficiency of your programs.

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