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Lab 8: Implementing Hash Tables

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Lab 8: Implementing Hash Tables with different Collision Resolutions
For this lab you will explore implementations of hash tables with the following three collision
resolution strategies:
1. Separate Chaining
a. You will use the linked_list.py you have implemented (Make modifications if
necessary) to implement the separate chaining. You can not use the python list
as the chain.
2. Linear Probing
a. h + s, where s is the step size. use s = 1.
3. Quadratic Probing
a. h + f(i), f(i) = (i + i2
) / 2
In this lab, you are going to store stop words in your hash tables as a test. Stop words are
high-frequency words like the, to and also that we sometimes want to filter out of a document
before further processing. Stopwords usually have little lexical content, and their presence in a
text fails to distinguish it from other texts.
You are going to import stop words from supplied stop_words.txt which contains stop words
separated by a space. You are required to read the stop words contained in the file and create a
hash table of the stop words so that you can use the table to determine if a word is a stop word or
not with ‘in’ operator: e.g. is_stopword = ‘the’ in stop_words. You are going to use each stop
word as a key and 0 as its value and store the word – 0 pairs as key – value pairs.
Use the hash function for hashing string presented in a lecture:
1. def hash_string(string, size):
2. hash = 0
3. for c in string:
4. hash = (hash * 31 + ord(c)) % size
5. return hash
Define classes HashTableSepchain that implements the separate chaining (use linked list as the
list), HashTableLinear that implements the linear probing, and HashTableQuadratic that
implements the quadratic probing.
● __init__(self, table_size=11): this function takes no parameters and returns a hash table
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CPE 202 Spring 2020
object, having initialized an empty hash table ([None] * table_size). The table_size parameter
(default value of 11 for separate chain and linear probing or 16 for quadratic probing) is the
starting size of the table (number of “slots” in the table). This function should initialize the hash
table (use a Python list) and any other instance variables used in your hash table class (e.g.
num_items, num_collisions).
● put(self, key, data): this function takes a key, and an item. Keys are string values. The
function will insert the key-value pair into the hash table based on the hash value of the key (use
the hash_string function shown above). If the key-data pair being inserted into the hash table is a
duplicate key, the old key-value pair will be replaced by the new key-value pair. If the insert
would cause the load factor of the hash table to become greater than a predetermined threshold
value (1.5 for the separate chaining, 0.75 for the linear and quadratic probing), the table size
of the hash table should be increased to twice the old table size plus 1 (new_table_size =
2*table_size + 1) for separate chaining and linear probing. For quadratic probing, the table
size of the hash table should be increased to twice the old table size. After creating the new
hash table, the items in the old hash table need to be re-put into the new table. In this lab, the key
and the value are the same string, which is a stop word. Your linked list implementation needs to
store a string as its payload.
● get(self, key): this function takes a key and returns the value (the value of an
key-value pair) from the hash table associated with the key. If no key-value pair is associated
with the key, the function raises a KeyError exception.
● contains(self, key): this function returns True if the key exists in the table, otherwise
returns False.
● remove(self, key): this function takes a key, removes the key-value pair from the hash
table and returns the key-value pair. If no key-value pair is associated with the key, the function
raises a KeyError exception.
● size(self): this function returns the number of key-value pairs currently stored in the hash
table.
● load_factor(self): this function returns the current load factor of the hash table.
● collisions(self): this function returns the number of collisions that have occurred during
insertions into the hash table. A collision occurs when an item is inserted into the hash table at a
location where one or more key-value pairs has already been inserted. When the table is resized,
do not increment the number of collisions unless a collision occurs when the new key-value pair
is being inserted into the resized hash table.
● Overload following two builtin methods to enable ‘in’ and ‘[]’ operators:
#This is for getting a value with []
def __getitem__(self,key):
return self.get(key)
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CPE 202 Spring 2020
#This is for enabling value assignment with []
def __setitem__(self,key,data):
self.put(key,data)
#This is for enabling in operator on our Hash Tables
def __contains__(self, key):
return self.contains(key)
In addition to classes described above, implement a function import_stopwords(filename,
hashtable) outside of your hash table classes. The argument filename is the name of a file
containing the stop words. The argument hashtable is an object of HashTable classes which can
be either an implementation of the separate chaining, linear probing, or quadratic probing. The
function should return a hash table object even though the object is mutable.
You are free to create helper functions. Try to reuse code by creating common helper
functions that can be commonly used by the three implementations of hash tables.
DO NOT FORGET TO IMPLEMENT ALL THREE BOILERPLATE METHODS IN
ALL CLASSES YOU USE.
Submit a file hashtables.py containing the above, linked_list.py containing your linked list
implementation and a file hashtables_tests.py containing your set of test cases. Zip all three
files as one zip file, and submit it to the grader, Gradzilla. After getting your work graded by the
grader, submit your work to Canvas as well.
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