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

Here is, in short, how a hash table works.

Imagine you have a library, full of books. If you were to store the books in an array, you would put each book on a spot on a shelf, and then when someone asked you to find a book, you'd look through all the shelves -- pretty slow. If someone said "book #12345", you could find it pretty easily, though.

Let's say instead you say, if the book title starts with 'A', it goes in row 1. If the second letter is 'B', it goes in row 1, rack 2. If the third letter is 'C', it goes in row 1, rack 2, shelf 3... and so on until you identify the book position. Then, based on the title of the book, you could know exactly where it should be.

Now, there are some problems in the simplistic "hashing" algorithm I described -- some shelves are going to be way overloaded while others stand empty, some books will be assigned to the same slot.. so the real hash functions are carefully constructed to try to avoid such problems.

But this is the basic idea.