Hash Table

Table hashing is a fundamental data structure used to efficiently store and manage key-value pairs. This is critical for developing high-performance applications that require fast data retrieval and processing. In this article, we’ll explore the concepts, principles of operation, and practical examples of table hashing.

What is table hashing?

Table hashing is a data structure that provides consistent average performance for key-value operations such as insert, delete, and retrieve. It is based on the concept of a hash function that maps keys to specific indexes in an array called a hash table. Using this mapping, table hashing enables efficient data access and manipulation, making it a popular choice for tasks such as database indexing, symbol tables, and caching.

How does table hashing work?

The basic idea behind table hashing is to use a hash function to turn keys into numeric indexes that correspond to positions in a hashed table. This process is called hashing. When a key/value pair is inserted, the hash function calculates the hash value and the pair is placed in the appropriate slot in the hash table. On retrieval, the same hash function is used to determine the key’s position in the hash table, allowing for quick access to the corresponding value.

Dealing with conflicts

Conflicts can arise when two different keys map to the same index in a hash table. Various collision resolution methods are used to solve this problem. A common approach is to use a separate chain, where each hash table space contains a linked list of key-value pairs that have the same index. Another method is open addressing, which uses a predefined sequence of steps to search for the next available slot in the event of a collision.

Example of hashing a table

Let’s look at a simple example of table hashing using JavaScript:

const hashTableSize = 10;
const hashTable = new Array(hashTableSize);
function hashFunction(key) {
  let hashValue = 0;
    for (let i = 0; i < key.length; i++) {
      hashValue += key.charCodeAt(i);
    }
  return hashValue % hashTableSize;
}

function insert(key, value) {
  const index = hashFunction(key);
  if (!hashTable[index]) {
    hashTable[index] = [];
  }
  hashTable[index].push({ key, value });
}

function get(key) {
  const index = hashFunction(key);
  if (hashTable[index]) {
    for (const entry of hashTable[index]) {
      if (entry.key === key) {
        return entry.value;
      }
    }
  }
  return null;
}

insert('name', 'John');
console.log(get('name')); // Output: John