Can someone explain hash table/hash function to me.... Thanks!

[kkmoslehpour]

Hi, all
I want to get a better understanding of what a hash function is and how it relates to the hash table. I need to implement some sort of word checker that checks a list of words that is read from a text file and stored into the hash table. Then we read from another text file it (containing a couple of sentences some with words that are the same as the ones in the hash table and some are different), if the words that are stored in the hash table do not match the word that is read from the text file sentences, it will output for example, misspellings: word1, word2 .... So it would help if someone can kind of explain this so it will be a bit more clear. Thanks!

[Tolls]

Wiki hash tables.

The hashCode function (in Java) simply maps an object into an int, it's that int that is used by the HashMap/Table/Set for its "buckets".

[kkmoslehpour]

Oh okay so letter like ABC have int values associated with them? that can be mapped into a hash table? If so how do we associate the letters to ints?

[Tolls]

The String class already has a hashCode method.
It looks like this:

    public int hashCode() {
        int h = hash;
        if (h == 0) {
            int off = offset;
            char val[] = value;
            int len = count;

            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }

Slightly random indenting can be blamed on the source code files.

[kkmoslehpour]

can you kind of explain the method above what it is doing thanks!

[Tolls]

It's turning a String into an int, based on the value of each of its chars.

h = 31*h + val[off++];

You'll need to look into the technical aspects of hashing for why exactly it does it this way.

[kkmoslehpour]

How would i represent a string in terms of int values, and also in that code what the offset mean?

[Tolls]

You don't really need to know that sort of detail.
All you need to know is that a hashcode is an integer representation of the object in question.

Since String already has this done then I'm not sure what you are looking for.

[kkmoslehpour]

well we are suppose to implement our own hash function rather than using the one from the java library itself. So that is why I want to understand it..

[Tolls]

In that case you need to read up on it, as I suggested above.
Wiki is a reasonable start point.