The fundamentals of Hash tables?

Question

I\'m quite confused about the basic concepts of a Hash table. If I were to code a hash how would I even begin? What is the difference between a Hash table and just a normal

Answer 1

Hashtables are associative. This is a huge difference from arrays, which are just linear data structures. With an array, you might do something like this:

int[] arr = ...
for (int i = 0; i < arr.length; i++) {
    System.out.println(arr[i] + 1);
}

Notice how you are getting an element out of the array by specifying an exact memory offset (i). This contrasts with hashtables, which allow you to store key/value pairs, later retrieving the value based on the key:

Hashtable table = new Hashtable();
table.put("Daniel", 20);
table.put("Chris", 18);
table.put("Joseph", 16);

With the above table, we can make the following call:

int n = table.get("Chris");

...and be assured that n will be valued at 18.

I think this will probably answer most of your questions. The implementation of a hashtable is a fairly interesting topic, one which Wikipedia addresses passably well.