Hash Tables

What is a Hashtable?

1. Hash - A hash is the result of some algorithm taking an incoming string and converting it into a value that could be used for either security or some other purpose. In the case of a hashtable, it is used to determine the index of the array.

2. Buckets - A bucket is what is contained in each index of the array of the hashtable. Each index is a bucket. An index could potentially contain multiple key/value pairs if a collision occurs.

3. Collisions - A collision is what happens when more than one key gets hashed to the same location of the hashtable.

Why do we use them?

1. Hold unique values
2. Dictionary
3. Library

What Are they

Hashtables are a data structure that utilize key value pairs. This means every Node or Bucket has both a key, and a value.

The basic idea of a hashtable is the ability to store the key into this data structure, and quickly retrieve the value. This is done through what we call a hash. A hash is the ability to encode the key that will eventually map to a specific location in the data structure that we can look at directly to retrieve the value.

Since we are able to hash our key and determine the exact location where our value is stored, we can do a lookup in an O(1) time complexity. This is ideal when quick lookups are required.

Hash function

A hash function is any function that can be used to map a data set of an arbitrary size to a data set of a fixed size, which falls into the hash table. The values returned by a hash function are called hash values, hash codes, hash sums, or simply hashes.

Internal Methods

Add() When adding a new key/value pair to a hashtable:

send the key to the GetHash method. Once you determine the index of where it should be placed, go to that index Check if something exists at that index already, if it doesn’t, add it with the key/value pair. If something does exist, add the new key/value pair to the data structure within that bucket. Find() The Find takes in a key, gets the Hash, and goes to the index location specified. Once at the index location is found in the array, it is then the responsibility of the algorithm the iterate through the bucket and see if the key exists and return the value.

Contains() The Contains method will accept a key, and return a bool on if that key exists inside the hashtable. The best way to do this is to have the contains call the GetHash and check the hashtable if the key exists in the table given the index returned.

GetHash() The GetHash will accept a key as a string, conduct the hash, and then return the index of the array where the key/value should be placed.

• In hashing, large keys are converted into small keys by using hash functions. The values are then stored in a data structure called hash table. The idea of hashing is to distribute entries (key/value pairs) uniformly across an array. Each element is assigned a key (converted key). By using that key you can access the element in O(1) time. Using the key, the algorithm (hash function) computes an index that suggests where an entry can be found or inserted.

Hashing is implemented in two steps:

1. An element is converted into an integer by using a hash function. This element can be used as an index to store the original element, which falls into the hash table.

2. The element is stored in the hash table where it can be quickly retrieved using hashed key.

hash = hashfunc(key) index = hash % array_size

In this method, the hash is independent of the array size and it is then reduced to an index (a number between 0 and array_size − 1) by using the modulo operator (%).