Pawel Ciucias: Hashtable

Monday 4 May 2015

Hashtable

A hashtable is a data structure that maps key value pairs. A hashtable leverages a hash function to compute an index into an array of slots, from which the value can be found.

You can think of it as an array that uses a function to get it's index from a key. here's a more in depth explanation. now for large data sets especially ones that have a unique key this data structure is perfect.

using System;
using System.Collections;

namespace pc.hashtableexample
{
    class Program
    {
        public class Person
        {
            public string Name { get; set; }
            public DateTime BirthDate { get; set; }
            public Person(string Name, DateTime BirthDate)
            {
                this.Name = Name;
                this.BirthDate = BirthDate;
            }
            public int GetAge()
            {
                DateTime today = DateTime.Today;
                int age = today.Year - BirthDate.Year;
                return BirthDate > today.AddYears(-age) ? --age : age;
            }

            public override string ToString()
            {
                return Name + " is " + GetAge();
            }
        }

        static void Main(string[] args)
        {
            var ht = new Hashtable();

            //add key and item
            ht.Add(2, new Person("Pawel", new DateTime(1984, 01, 31)));
            ht.Add("bla", new Person("Tom", new DateTime(1988, 08, 28)));
            ht.Add(3.0, new Person("Magda", new DateTime(1984, 06, 28)));

            Console.WriteLine(ht["bla"]);
            Console.WriteLine(ht[2]);
            Console.WriteLine(ht[3]);
            Console.WriteLine(ht[3.0]);
        }
    }
}

Above you can see how simple it is to use the hash table data-structure, just keep in mind the following:

calling fr a key that doesn't exist return null,
anything that derives from object can be a key
one last caveat is that the key must be unique