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What is Hashtable, And what are features of Hashtable ?

Introduction :

A hashtable is a data structure that allows you to store and retrieve value using a key. It is a widely used data structure in computer science for implementing associative arrays, sets and  caches. The bsic idea behind a hashtable is to use a hash function to map key to indices in an array. This allows for efficient insertion, deletion and retrieval of values.

Here are the Key Features of hashtables:

1. Key-value Storage:

  • A hash Table stores data in key-value pairs.
  • The key is used to uniquely identify each entry in the hashtable.
  • The value is the data associated with the key.

2.  Hash Function:

  • Hash function takes a key as input and produces a fixed size hash code(usually an integer) as output.
  • The hash code is used as an index to store the key-value pair in the array.
  • An ideal hash function distrubutes keys uniformly across the array, minimizing collisions.

3. Array Storage:

  • Hash Table use an array to store the key-value pairs.
  • The array sixe is typically determined based on the expected number of entries and is usually larger than the number of entries to minimize collisions.

4. Collision Handling:

  • Collisions occur when two keys hash to the same index in the Array.
  • Hashtable employ collision resolution techniques to handle collisions:
  • 1. Seprate chaining: Each Array element is linked list, and colllision result in key value pairs being appended to the list.
  • 2. Open Addressing: Colliding keys are placed in the next available (open) slot in the array.

5. Load Factor:

  • The load factor is the ratio of the number of entries to the size of the array.
  • It influences the performance of hashtable.A higher load facctor increases the likelihood of collisions,while a lower load factor increases the memory overhead.

6. Dynamic Resizing:

  • Hashtable often dynamically resize themselves to maintain an optimal load factor.
  • When the number of entries exceds a certain threshold, the array is resized, and all key-value pairs are rehashed to the new array.

7. Fast Lookup, Insertion And Deletion:

  • Due to the use of hash codes and arrays,hashtables provide fast average -case time complexity for operations like lookup,insertion , and deletions.

8. Unordered:

  • The order of elements in a hashtable is not guaranted to be the same as the order in which they where inserted.

9. Common Use Cases:

  • Hashtable are commonly used in scenarios where fast lookup, insertions, and deletions are essential , such as in implementing dictrionaries, caches ,symbol tables,and database indexing .

10. Example Implementation in Programming:

  • Programming Languages often provide hashtable implementations ,such as Python's 'dict', Java's 'Hashmap', Or C++'s 'unordered_map'.

Examples of HashTable in diffrent programming Languages :

1. HashTable implementation in Python's (dict):

Demo_dict={'Java':'James Gosling','Python':'Guido van Rossum','C++':'Bjarne Stroustrup'}

#prints the all Keys using keys()
print(Demo_dict.keys())

# prints the all Values using values()
print(Demo_dict.values())

#Updating Values of dictionary
Demo_dict['Python']='Guido v Rossum'

print(Demo_dict)

OUTPUT:

dict_keys(['Java', 'Python', 'C++'])
dict_values(['James Gosling', 'Guido van Rossum', 'Bjarne Stroustrup'])
{'Java': 'James Gosling', 'Python': 'Guido v Rossum', 'C++': 'Bjarne Stroustrup'}


2. HashTable implementation in Java's (Hashmap):

import java.util.*;

public class HashmapEx{

public static void main(String args[]){

HashMap<Integer,String> map = new HashMap<Integer,String>();//Creating HashMap

map.put(1001,"Shubham");// put() is used to insert element in Map

map.put(1002,"Sam");
map.put(1003,"Grace");
map.put(1004,"Steve");
System.out.println("inserting Hashmap Elements!");

for(Map.Entry m: map.entrySet())
{

System.out.println(m.getKey()+" "+m.getValue());

}
}
}

OUTPUT:

F:\Bloging\Java>javac HashmapEx.java

F:\Bloging\Java>java HashmapEx
inserting Hashmap Elements!
1001 Shubham
1002 Sam
1003 Grace
1004 Steve










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