Question

I am writing a program that will hash words from a document along with their frequency of use and line numbers. I thought that I had finished it when I was told that you have to create a hash table from scratch. I do not know where to begin. Any suggestions as to where and how to start would be appreciated.

Answer 1

A hash table is an important and fundamental data structure. You can read more about it at Wikipedia’s Hash table article. Fortunately, they are fairly easy to implement.

Basically a hash table is a data structure that takes a key and returns or stores a value for that key.

At their core, they are usually implemented using an array, which we shall call arr such that key-value pairs are stored at arr[key.hashCode()%arr.length]. Notice that since your array is not of infinite length, and that hashCode is not guaranteed to produce unique values you will eventually end up with keys that map to the same index of the array. This is called a collision.

One way of resolving these collisions is to store a linked list for each member of arr. Then the definition of arr would look like this

LinkedList<Object> arr[];

All objects which map to arr[key.hashCode()%arr.length] will be added to the list at that position. When you want to retrieve an object from the hashtable, jump to the linked list found at arr[key.hashCode()%arr.length] and iterate through each member of the linked list until you find a key-value pair where the keys are .equals.

A good hash table implementation might do things like resize arr once it gets too full.