lab_hash

Assignment Description

In this lab you will be implementing functions on hash tables with three different collision resolution strategies — separate chaining, linear probing, and double hashing. These hash tables serve an implementation of the dictionary abstract data type.

Lab Insight

Hashing is very powerful as it enables us to build data structure like hash tables and maps. On top of which, there are variations of hashing that can be used to help encrypt data. If you are interested in learning more about the applications of hashing, you can take CS 498 Applied Cryptography, CS 461 Computer Security I, and CS 463 Computer Security II.

Checking Out the Code

All assignments will be distributed via our release repo on github this semester. You will need to have set up your git directory to have our release as a remote repo as described in our git set up

You can merge the assignments as they are released into your personal repo with

git pull --no-edit --no-rebase release main
git push

if you are using multiple machines you may need to use the following to allow them to work correcly.

git pull --no-edit --no-rebase release main --allow-unrelated-histories
git push

The first git command will fetch and merge changes from the main branch on your remote repository named release into your personal. The --no-edit flag automatically generates a commit message for you, and the--no-rebase flag will merge the upstream branch into the current branch. Generally, these two flags shouldn’t be used, but are included for ease of merging assignments into your repo.

The second command will push to origin (your personal), which will allow it to track the new changes from release.

You will need to run these commands for every assignment that is released.

All the files for this lab are in the lab_hash directory.

Preparing Your Code

This semester for MPs we are using CMake rather than just make. This allows for us to use libraries such as Catch2 that can be installed in your system rather than providing them with each assignment. This change does mean that for each assignment you need to use CMake to build your own custom makefiles. To do this you need to run the following in the base directory of the assignment. Which in this assignment is the lab_hash directory.

mkdir build
cd build

This first makes a new directory in your assignment directory called build. This is where you will actually build the assignment and then moves to that directory. This is not included in the provided code since we are following industry standard practices and you would normally exclude the build directory from any source control system.

Now you need to actually run CMake as follows.

cmake ..

This runs CMake to initialize the current directory which is the build directory you just made as the location to build the assignment. The one argument to CMake here is .. which referes to the parent of the current directory which in this case is top of the assignment. This directory has the files CMake needs to setup your assignment to be build.

At this point you can in the build directory run make as described to build the various programs for the MP.

The code for this activity resides in the lab_hash/ directory.

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();

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

Separate Chaining Hash Table

Open your schashtable.hpp. In this file, several functions have not been implemented—your job is to implement them.

insert

• insert, given a key and a value, should insert the (key, value) pair into the hash table.
• 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.

find

• given a key, should return the corresponding value associated with that key
• Here is the Doxygen for find.

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.

Linear Probing Hash Table

Open your lphashtable.hpp. In this file, you will be implementing the following functions.

insert

• 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!

findIndex

• given a key, should return the corresponding index associated with that key
• Here is the Doxygen for findIndex.

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.

Double Hashing Hash Table

Open your dhhashtable.hpp. In this file, you will be implementing the following functions.

insert

• insert, given a key and a value, should insert the (key, value) pair into the hash table.
• Remember the collision handling strategy for double hashing! (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!

findIndex

• given a key, should return the corresponding index associated with that key
• Here is the Doxygen for findIndex.

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.

Grading Information

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

• dhhashtable.hpp
• lphashtable.hpp
• schashtable.hpp

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