Question

What are prefix hash functions used for? I have created chaining, quadratic, and linear hash tables. I was given simples, prefix, and full-length hash methods which i am not sure what they are used for??

Here is the code:

int HashTable_qp::preHash(string & key, int tableSize )
{
    string pad = "AA";
    //some words in the input are less than 3 letters
    //I choose to pad the string with A because all padded characters 
    //have same ascii val, which is low, and will hopefully alter the results less
    if (key.length() < 3)
    {
        key.append(pad);
    }
    return ( key[0] + 27 * key[1] + 729 * key[2] ) % tableSize;
}

collision detection:

while(i != DataArray.size())
{
    tStart = clock();

    if(QuadraticProbingHT.preHash(DataArray[i],101) == QuadraticProbingHT.preHash(DataArray[i],101) )
    {
        collision_count++;
    }
    tStop = clock();
    total_c += tStop - tStart;
    i++;
}

Answer 1

A prefix hash hashes a string by its first few characters (the prefix).

Note that in the implementation you give, it hashes a string using the first three characters (if existent; it pads with AA if necessary). Thus, ass and associate has the same hash value under this specific prefix hash.

A full-length hash uses every character in the string to determine the hash value.