lab_hash Hellish Hash Tables

Assignment Description

In this lab you will be implementing functions on hash tables with two different collision resolution strategies — separate chaining and linear probing. These hash tables serve an implementation of the dictionary abstract data type. In the second part of the lab you will use the dictionary to solve some problems.

Note that there are a LOT of files in this lab — don’t get too intimidated as you won’t be modifying the vast majority of them.

Getting Set Up

From your CS 225 git directory, run the following on EWS:

git fetch release
git merge release/lab_hash -m "Merging initial lab_hash files"

If you’re on your own machine, you may need to run:

git fetch release
git merge --allow-unrelated-histories release/lab_hash -m "Merging initial lab_hash files"

Upon a successful merge, your lab_hash files are now in your lab_hash directory.

The code for this activity resides in the lab_hash/ directory. Get there by typing this in your working directory:

cd lab_hash/

There is also a separate lab_hash_data directory which must be downloaded separately. Do so by running

make data

from your lab_hash directory.

Here is the list of all of the files in this lab. (YOU WILL BE FINE. IT IS OK!) There are a lot of files, but you only will need to modify five of them. They have been marked in the above table, and the spec guides you through which functions you have to implement. Don’t panic!

In your directory there should be two folders as well: lab_hash_data and soln. The lab_hash_data folder contains text files used in the applications later, and the soln folder contains sample output to diff your code’s output against.

Notes About list Iterators

When you are working with the Separate Chaining Hash Table, you will need to iterate over the linked list of a given bucket. Since the hash tables are templatized, however, this causes us a slight headache syntactically in C++. To define a list iterator on a given bucket, you will need to declare it as follows:

typename list< pair<K,V> >::iterator it = table[i].begin();

Also note that if you do not insert your data at the head of the bucket, you may end up with your results being in a different order than ours. This is fine, but if you want to diff with the solution output (in the soln) folder, you will want to ensure that you always insert your data at the head of the list in each respective bucket.

std::list<std::pair<K, V>> it = table[i].begin();
table[i].erase(it);
it++;

remove and resizeTable on a Separate Chaining Hash Table

Open your schashtable.cpp. In this file, the above two functions have not been implemented—your job is to implement them.

remove

• Given a key, remove it from the hash table.
• If the given key is not in the hash table, do nothing.
• You may find the Doxygen for remove helpful.

resizeTable

• This is called when the load factor for our table is $$\ge 0.7$$.
• It should resize the internal array for the hash table. Use the return value of findPrime with a parameter of double the current size to set the size. See other calls to resize for reference.
• Here is the Doxygen for resizeTable.

insert on a Linear Probing Hash Table

Open your lphashtable.cpp. In this file, you will be implementing the insert function.

• insert, given a key and a value, should insert the (key, value) pair into the hash table.
• Remember the collision handling strategy for linear probing! (To maintain compatibility with our outputs, you should probe by moving forwards through the internal array, not backwards).
• You do not need to concern yourself with duplicate keys. When in client code and using our hash tables, the proper procedure for updating a key is to first remove the key, then re-insert the key with the new data value.
• Here is the Doxygen for insert.

You MUST handle collisions in your insert function, or your hash table will be broken!

Test Your Code Using charcount

You can read the Doxygen for the CharFreq class here. It has already been implemented for you.

Type the following into the terminal:

make charcount

This will make the charcount executable. This file takes two arguments: the first is the file to analyse, and the second is the frequency for which to return characters. For example, a run on lab_hash_data/sherlock_holmes.txt for characters with frequency greater or equal to 10000 might look like:

$ ./charcount lab_hash_data/sherlock_holmes.txt 10000 schash
Finding chars in lab_hash_data/sherlock_holmes.txt with frequency >= 10000 using SCHashTable...
54459   e
40038   t
35944   a
34570   o
30921   i
29449   h
29436   n
27805   s
25298   r
18981   d
17548   l
13495   u
12060   m
11432   w
10847   c

Implement the WordFreq Class

Now we will write our own application for a hash table! Open word_counter.cpp. You will be writing the getWords function. This function, given an integer threshold, should return a vector of (string, int) pairs. Each of these pairs is a string and its frequency in the file. Your vector should only contain those pairs whose frequencies are greater than or equal to threshold. Look at char_counter.cpp if you are stuck. You may find the TextFile class defined in textfile.h helpful.

Helpful Doxygen links: Doxygen for WordFreq, Doxygen for TextFile.

Test Using the wordcount Executable

When you’re done with the above, type the following into your terminal:

make wordcount

This will build the wordcount executable, which uses the WordFreq class to determine word frequencies in a file. It, like charcount, takes two arguments: the first is the path to the file to be analyzed, and the second is the threshold for which to grab words. If a word occurs >= threshold times, it will be printed to the console.

For example, a run of wordcount on lab_hash_data/metamorphoses.txt with a frequency 1000 might look like:

$ ./wordcount lab_hash_data/metamorphoses.txt 1000 lphash
Finding words in lab_hash_data/metamorphoses.txt with frequency >= 1000 using 
Table...
10473   the
5753    of
4739    and
3078    to
2246    a
1989    in
1706    was
1572    his
1548    that
1500    her
1456    with
1306    is
1241    he
1185    by

You should verify your solution works with both kinds of hash tables.

Implement the AnagramFinder Class

You can read the Doxygen for AnagramFinder here.

Finding anagrams is another clever use of hash tables. Open anagram_finder.cpp. You will be writing the checkWord function, which simply returns a boolean value. It should return true if the string test is an anagram of word, and false otherwise.

An anagram of a given word is a word which has the same letters. For example, “retinas” and “stainer” are anagrams of each other. Think carefully about what it means for a word to be an anagram of another. Is there something we can use a hash table to keep track of? Does counting letter frequency help you?

Test Using the anagramtest Executable

When you’re done with the above, type the following into your terminal:

make anagramtest

This will make the anagramtest executable, which you can use to test your AnagramFinder class. It can be run with or without arguments. Without arguments will test a very simplistic example—with arguments is more interesting.

anagramtest can also take two arguments: the first is the path to the file to be analyzed, and the second is the word to find anagrams for. For example, a run of anagramtest on lab_hash_data/words.txt looking for anagrams of retinas might look like:

$ ./anagramtest lab_hash_data/words.txt retinas schash
Checking file lab_hash_data/words.txt for anagrams of retinas using SCHashTable...
anestri is an anagram of retinas
asterin is an anagram of retinas
eranist is an anagram of retinas
nastier is an anagram of retinas
ratines is an anagram of retinas
resiant is an anagram of retinas
restain is an anagram of retinas
retains is an anagram of retinas
retinas is an anagram of retinas
retsina is an anagram of retinas
sainter is an anagram of retinas
stainer is an anagram of retinas
starnie is an anagram of retinas
stearin is an anagram of retinas

You should verify your solution works with both kinds of hash tables to verify their correctness as well.

Implement the LogfileParser Class

This task is a bit more abstract, but is highly applicable for companies. Given a logfile with the following content:

[customer_name]:    url    date

We want to answer the following questions (quickly!):

• What unique URLs have been visited?
• Given a customer and a product, have they visited that product? If so, when was the most recent time they did so?

In order to do so, you will have to parse the logfile and do something with hash tables. The parsing occurs in the constructor for the LogfileParser and the querying happens in the other two functions. You should design your constructor so that you can run the two helper functions as quickly as possible (that is, I should be able to do $$n$$ queries on your LogfileParser in $$\mathcal{O}(1)$$ time per query after the constructor has finished).

Read the Doxygen for LogfileParser here.

You are safe to assume that customers have distinct names, and that URLs always follow the pattern /product/XX/ where XX is an integer. Products (and thus URLs) have distinct integer IDs.

Test Your LogfileParser with lfparse

When you are done with the above, type the following into your terminal:

make lfparse

This will build the lfparse executable, which will test your LogfileParser class. It takes a single argument: the data file to be processed. For example, a run of lfparse on lab_hash_data/log2.txt might look like:

$ ./lfparse lab_hash_data/log2.txt
Parsing logfile: lab_hash_data/log2.txt...
Number of unique URLs: 10
Printing unique URLs:
        /product/5/
        /product/0/
        /product/7/
        /product/1/
        /product/6/
        /product/8/
        /product/2/
        /product/3/
        /product/4/
        /product/9/
Running sample visited queries...
        chase visited /product/0/ on Tue Mar 27 20:15:05 2018
        chase visited /product/1/ on Tue Mar 27 21:12:07 2018
        chase visited /product/2/ on Tue Mar 27 18:46:05 2018
        chase visited /product/3/ on Sat Mar 24 19:57:35 2018
        chase visited /product/4/ on Mon Mar 26 21:05:44 2018
        chase visited /product/5/ on Tue Mar 27 20:12:16 2018
        chase visited /product/7/ on Mon Mar 26 20:06:51 2018
        chase visited /product/8/ on Sun Mar 25 19:27:13 2018
        chase visited /product/9/ on Mon Mar 26 19:29:58 2018

You should verify your solution works for both kinds of hash tables to verify their correctness as well.

genlog More Logfiles!

If you would like, you can also generate more random logfiles to throw at your lfparse executable. To do so, type the following into your terminal:

make genlog

genlog takes three arguments: filename, products, and lines (in that order).

• filename is the path to the output file. If the output file exists, it will be overwritten!
• products is the number of unique products there are.
• lines is the length of the logfile in lines to generate.

Committing Your Code

When you are finished with the lab, be sure to commit your code back to subversion—this lab will be autograded.

Grading Information

The following files (and ONLY those files!!) are used for grading this lab:

• lphashtable.cpp
• schashtable.cpp
• word_counter.cpp
• anagram_finder.cpp
• logfile_parser.cpp

If you modify any other files, they will not be grabbed for grading and you may end up with a “stupid zero.”