CSCE 146

Lab 12

In this lab we’ll examine Hashing techniques and how to resolve collisions during hashing.

1. Write a class PrimeTest, that tests whether a given number is prime or not. Implement a method with the following signature :

Public Boolean isPrime(int n)

{

//method implementation

}

A simple method of determining the primality of a number N is to check whether it is divisible by any number between 2 and the ÖN. In case N is not divisible by any such number, it is a prime.

2. Download the Table class from Michael Maine’s website. This class implements a Hash Table based on Open Address hashing with Linear Probing. Also, make sure that you have the file ‘newfile.dat’ containing student data, in the same folder. Now, write a class TableTest, in which you invoke the constructor of the Table class to create a Table object. Use the PrimeTest class that you wrote in step 1 to set the size of the table to a prime number p, such that :

1000 < p < 1500 and p = 4k + 3 where k is an integer.

Now write a method to read in student data from the file, and insert student objects into the Hash Table. The pseudo-code for this is given below:.

while (there is more data in the file ‘newfile.dat’)

{

Read data corresponding to the next student.

Instantiate a Student object with this data.

Add the Student object to the Hash Table.

}

3. Solve the programming project # 6 on page 565 of your text book. Add suitable class variables and methods to the Table class so that you can obtain the statistics mentioned in the project. In the TableTest class, search the Table for a student that already exists in the Table, and print out the statistics obtained.

4. Now, modify the Table class to implement Double Hashing. To do so, we first we need to find a pair of twin primes in the range (1000 - 1500), using the PrimeTest class. Next, replace the private hash() method of the Table class with two methods hash1() and hash2(). Define these methods as follows :

hash1(key) = Math.abs(key.hashCode()) % data.length

hash2(key) = 1 + Math.abs(key.hashCode()) % (data.length – 2)).

You will also need to modify the put() method of the Table class to use double hashing to resolve collisions.

Use the same tests that you ran in step 3 and print out the statistics for Double Hashing. Compare these with the statistics that you obtained previously for Linear Probing.

Enjoy!!