How does a hash table work?

Question

I\'m looking for an explanation of how a hash table works - in plain English for a simpleton like me!

For example, I know it takes the key, calculates the hash (I a

Answer 1

All of the answers so far are good, and get at different aspects of how a hashtable works. Here is a simple example that might be helpful. Lets say we want to store some items with lower case alphabetic strings as a keys.

As simon explained, the hash function is used to map from a large space to a small space. A simple, naive implementation of a hash function for our example could take the first letter of the string, and map it to an integer, so "alligator" has a hash code of 0, "bee" has a hash code of 1, "zebra" would be 25, etc.

Next we have an array of 26 buckets (could be ArrayLists in Java), and we put the item in the bucket that matches the hash code of our key. If we have more than one item that has a key that begins with the same letter, they will have the same hash code, so would all go in the bucket for that hash code so a linear search would have to be made in the bucket to find a particular item.

In our example, if we just had a few dozen items with keys spanning the alphabet, it would work very well. However, if we had a million items or all the keys all started with 'a' or 'b', then our hash table would not be ideal. To get better performance, we would need a different hash function and/or more buckets.