What is the difference between a hash table and an array?

A hash table and an array are both data structures used to store and retrieve elements, but they have some key differences.

1. Structure: An array is a linear data structure that stores elements in contiguous memory locations, while a hash table is a data structure that uses a hash function to map keys to an array index, allowing for more efficient retrieval.

2. Access Time: In an array, accessing an element is done by its index, which takes constant time O(1) as the memory locations are contiguous. In a hash table, accessing an element is done by its key, which involves applying a hash function to calculate the index, resulting in an average access time of O(1) as well. However, in the worst case scenario, when there are many collisions, the access time can degrade to O(n), where n is the number of elements.

3. Key-Value Pair: An array only stores values, while a hash table stores key-value pairs. This allows for efficient retrieval of values based on their associated keys.

4. Dynamic Size: Arrays have a fixed size, meaning they cannot easily grow or shrink. In contrast, hash tables can dynamically resize themselves to accommodate more elements, making them more flexible.

5. Memory Usage: Arrays use memory proportional to the number of elements they store, regardless of whether the elements are sparse or dense. Hash tables, on the other hand, may use more memory due to the need for additional space to handle collisions and maintain the hash function.

6. Collisions: Collisions occur in hash tables when two or more keys map to the same index. Various collision resolution techniques, such as chaining or open addressing, are used to handle collisions and ensure efficient retrieval. Arrays do not have collisions as each element has a unique index.

In summary, while both hash tables and arrays are used for storing and retrieving elements, hash tables provide more efficient retrieval based on keys, dynamic resizing, and the ability to handle collisions. Arrays, on the other hand, have a simpler structure, fixed size, and do not store key-value pairs.