Hashed Table


A hashed table, more commonly known as a hash table, is a data structure that allows efficient storage and retrieval of key-value pairs. It uses a hash function to map a key to an index in an array, where the corresponding value is stored. This structure enables fast look-ups and supports insertion and deletion operations with average-case time complexity of O(1).


The phonetics of the keyword “Hashed Table” is:/ˈhæʃt ˈteɪbəl/with “hashed” being /ˈhæʃt/ and “table” being /ˈteɪbəl/.

Key Takeaways

  1. Hashed tables, also known as hash tables or hash maps, are data structures that provide fast insertion, deletion, and retrieval of values based on their associated keys by using a hash function.
  2. Hash functions are responsible for converting the input key into an index in the underlying array. Properly designed hash functions should distribute keys uniformly to avoid collisions and minimize the need for rehashing.
  3. Techniques like chaining and open addressing help to resolve collisions that occur when multiple keys produce the same index. This ensures that hashed tables maintain their efficient performance even when they contain a large number of entries.


Hashed tables, or hash tables, are a crucial data structure in computing due to their exceptional efficiency in storing, searching, and retrieving data.

Their importance lies in the ability to reduce the time complexity of operations, such as insertion, deletion, and searching, to near-constant O(1) time, making them ideal for managing large datasets and time-sensitive tasks.

By employing a hashing function to convert keys (unique identifiers) into hash values or index positions, hash tables avoid conventional searching methods, which can be significantly slower.

This offers significant advantages in various applications, including database indexing, caching, and real-time data processing, thus solidifying the hashed table’s importance to the field of technology.


A hash table is an essential data structure that serves as the backbone for many software applications and systems, playing a vital role in ensuring efficient data storage and retrieval. As its primary purpose, a hash table allows for speedy access and manipulation of data through the implementation of a strategic hash function. This hash function translates complex data into simplified keys, which are then responsible for identifying a specific index in the table where the data is stored.

Thanks to this uniquely generated key-value pair system, hash tables dramatically reduce the time spent searching for data points and the resources required for the process, consequently fostering a streamlined, performance-oriented data management setting. Practical applications of hash tables abound across various industries, often requiring efficient organization and retrieval of vast data sets. For example, in database management, hash tables are integral in executing speedy query operations and indexing massive volumes of information.

Further, in computer programming languages, hash tables facilitate the creation of dictionaries and associative arrays that effectively manage key-value pairs. Even the realm of cybersecurity derives value from hash tables, as they lay the structural foundation of cryptographic hash functions that ensure the secure transmission of data online. Overall, the usefulness and adaptability of hash tables to distinct situations contribute to its persistent and widespread presence in today’s rapidly evolving technological landscape.

Examples of Hashed Table

Hash tables are widely used in various real-world applications due to their efficiency in searching, insertion, and deletion operations. Here are three examples of real-world implementation of hash tables:

Database Indexing: Hash tables are commonly used to create indexes in databases. An index in a database is similar to an index in a book, allowing quick look-up of relevant data. By using a hashed key and running it through a hashing function, the database can find the appropriate data by looking up the corresponding address in the hash table. This significantly improves search efficiency within the database, especially when dealing with large volumes of data.

Cache Implementation: Caches are temporary storage for frequently accessed data, usually used to reduce access time and server load. Many caching systems utilize hash tables to store key-value pairs, where the key is the unique identifier and the value is the actual data itself. In-memory caching systems, such as Memcached and Redis, use hash tables to store frequently requested data while reducing access times. This helps improve performance by limiting the number of times the same data must be fetched from the original storage system.

Spell Checkers and Auto-completion Features: Applications, such as Microsoft Word and Google Docs, use hash tables to store dictionaries of valid words to implement their spell-checking and auto-completion features. When the user begins typing, the application hashes each input word and checks it against the hash table of valid words. If a match is found, the spell checker knows the word is spelled correctly. If not, the application suggests alternative words that have similar hashes. This is an efficient way to check input against a large dictionary of words quickly.

FAQ – Hashed Table

1. What is a Hashed Table?

A Hashed Table, also known as a hash table or hash map, is a data structure that stores key-value pairs. It uses a hash function to compute an index or address into an array of buckets or slots, from which the desired value can be found. This allows for fast and efficient look up and insertion operations.

2. How does a Hashed Table work?

A Hashed Table works by taking a key input and passing it through a hash function. The output of the hash function determines the index where the key-value pair will be stored in the table. If a collision occurs (i.e., two keys have the same hash value), various methods such as chaining or open addressing can be used to resolve the collision and store the data.

3. What is a good hash function?

A good hash function has the following characteristics: it distributes the keys uniformly across the table, minimizes collisions, is deterministic (i.e., it always gives the same output for the same input), and is fast to compute. Cryptographic hash functions, such as SHA-256, are considered to be strong hash functions, but they may not be necessary for all use cases, as they can be slower than other simple hash functions.

4. What are the advantages of using a Hashed Table?

Hashed Tables offer several advantages, including efficient insertion, deletion, and search operations, often with average-case time complexity of O(1). They are also useful for implementing associative arrays, dictionaries, or sets, and can be easily resized when needed. Hashed Tables also do not require keys to be ordered, allowing for more flexibility in key selection.

5. What are some common use cases for Hashed Tables?

Hashed Tables can be used in various scenarios, such as implementing caching systems, counting the frequency of items in a collection, checking for duplicates, implementing symbol tables in compilers and interpreters, and managing user sessions in web applications, among others.

6. What are the possible drawbacks of using a Hashed Table?

Some drawbacks of using Hashed Tables include the potential for collisions, which can reduce performance or cause unexpected behavior. Hashed Tables may also require more memory due to the overhead of the hash function and data structures used to handle collisions. Additionally, if the hash function is poorly designed or if a fixed table size is used without resizing, performance can degrade significantly.

Related Technology Terms

  • Collision Resolution
  • Hash Function
  • Load Factor
  • Key-Value Pair
  • Rehashing

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