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CME 2201 - Assignment 1
In this assignment, you are expected to index words of a document named as ‘story.txt’. You must read this file, split it word by word, and index each word to your hash table according to rules given below.
Requirements
Usage of Java programming language and generic data types are required. You need to implement base functions of a classical Hash Table by yourself (do not extend an available Java Hash Map class directly). Object Oriented Programming (OOP) principles must be applied. Exception handling must be used when it is needed.
1. Main Functionalities
put(Key k, Value v)
Read the given input story.txt file, calculate the number of occurrences of each word as count value, insert this count data into the hash table accordingly.
Value get(Key k)
Search the given word (k) in the hash table. If the word is available in the table, then return an output as shown below, otherwise return a “not found” message to the user. When a word is searched in the hash table; key, count, and index of the word should be printed.
------ Output ------
Search: Ezgi Key: 1243225 Count : 10 Index : 165
Search: Ali Key: 68294842 Count : 3 Index : 132
Note: Results of the words of “Ezgi” and “Ali” were generated as an example. So, you will not obtain the same results by using the ‘story.txt’ file.
resize(int capacity)
Make the hash table dynamically growable. The put method should double the current table size if the hash table reaches the maximum load factor. You should take the initial size of the table as 997 and call the resize method according to two different load factor values (50% and 70%).
2. Hash Function
To specify an index corresponding to given string key, firstly you should generate an integer hash code by using a special function. Then, resulting hash code has to be converted to the range 0 to N-1 using a compression function, such as modulus operator (N is the size of hash table).
You are expected to implement two different hash functions including polynomial accumulation function and your own hash function.
Polynomial Accumulation Function (PAF)
The hash code of a string s can also be generated by using the following polynomial:
where is the left most character of the string, characters are represented as numbers in 1- 26 (case insensitive), and n is the length of the string. The constant z is usually a prime number (33, 37, 39, and 41 are particularly good choices for English words). When the z value is chosen as 33, the string "car" has the following hash value:
Note: Using of this calculation on the long strings will result in numbers that will cause overflow. You should ignore overflows or use Horner's rule to perform the calculation and apply the modulus operator after computing each expression in Horner's rule.
Your Own Hash Function (YHF)
Hash code for converting each word to an integer key must be implemented by yourself. The input value will be the word and the integer key will be returned by your hash code function. The hash (compression) function for converting a key to the index (address calculator) must be implemented by yourself.
3. Collision Handling Approach
You are expected to implement a collision resolution technique based on open addressing. The insertion algorithm is as follows:
Calculate the hash value and initial index of the entry to be inserted. Then search the position linearly. While searching, the distance from initial index is kept which is called DIB (Distance from Initial Bucket). If we can find the empty bucket, we can insert the new entry with the DIB value in here. If we encounter an entry which has less DIB than the candidate entry, swap them.
Step 3: Final state after displacements and insertion
For entry retrieval, entries can be found using linear probing starting from their initial indexes, until they are encountered, or until an empty bucket is found, in which case it can be concluded that the entry is not in the table. The search can also be stopped if during the linear probing of a bucket is encountered for which the distance to the initial bucket is smaller than the DIB of the entry it contains.
4. Performance Monitoring
You are expected to fill the performance matrix (Table 1) by running your code under different conditions including two different load factors (50% and 70%) to decide resizing of hash table and two different hash functions (PAF and YHF).
You should count total number of collision occurrences and measure expended time while indexing words in the "story.txt" under each condition. In addition, you should calculate minimum, maximum and average search times by using the “search.txt” file that contains 100 words to search for (search time means the time expended to find a particular key in the hash table. It does not include the time spent for outputs. To calculate avg. search time, divide the total expended time to the total number of searched keys). You can use System.nanoTime() or System.currentTimeMillis() for time operations.
Load Factor
Hash Function
Collision Count
Indexing Time
Avg. Search Time
Min. Search Time
Max. Search Time
α=50%
PAF
YHF
α=70%
PAF
YHF
Table 1. Performance matrix
Provided Resources
Document to index: story.txt Word list to use in calculation of searching times: search.txt
Due date
December 16, 2020, 23:
Submission
You must upload your all ‘.java’ files as an archive file (.zip or .rar) to the Sakai platform. Your archived file should be named as ‘studentnumber_name_surname.rar.zip’, e.g., 2007510011_Ali_Yılmaz.rar.
Prepare and upload a report with descriptions of your data structure, java code, and performance matrix.
Plagiarism Control
The submissions will be checked for code similarity. Copy assignments will be graded as zero, and they will be announced in the Sakai.
Grading Policy
Job Percentage Usage of Generic, OOP and Try-Catch % 20 Implementation of hash operations and collision handling approach
Performance monitoring %
