CSE 333 18au Homework #1
Out: Friday, September 28, 2018
Due: Thursday, October 11, 2018 by 11:00 pm.
For homework #1, you will finish our implementation of two C data structures: in part A, a doubly-linked list, and in part B, a chained hash table.
Please read through this entire document before beginning the assignment, and please start early! This assignment involves messy pointer manipulation and malloc/free puzzles, and these can cause arbitrarily awful bugs that take time and patience to find and fix.
|Part A -- doubly linked list|
If you've programmed in Java, you're used to having a fairly rich library of elemental data structures upon which you can build, such as vectors and hash tables. In C, you don't have that luxury: the C standard library provides you with very little. In this assignment, you will add missing pieces of code in our implementation of a generic doubly-linked list.
At a high-level, a doubly-linked list is incredibly simple; it looks like this:
Each node in a doubly-linked list has three fields; a payload, a pointer to the previous element in the list (or NULL if there is no previous element), and a pointer to the next element in the list. If the list is empty, there are no nodes. If the list has a single element, both of its next and previous pointers are NULL.
So, what makes implementing this in C tricky? Quite a few things:
- First, we want to make the list useful for storing arbitrary
kinds of payloads. In practice, this means the payload element in
a list node needs to be a pointer supplied by the customer of the
list implementation. Given that the pointer might point to
something malloc'ed by the customer, this means we might need to
help the customer free the payload when the list is destroyed.
- Second, we want to hide details about the implementation of
the list by exposing a high-level, nicely abstracted API. In
particular, we don't want our customers to fiddle with next and
previous pointers in order to navigate through the list, and we
don't want our customers to have to stitch up pointers in order to
add or remove elements from the list. Instead, we'll offer our
customers nice functions for adding and removing elements and
a Java-like iterator abstraction for navigating through the list.
- Third, C is not a garbage-collected language: you're responsible for managing memory allocation and deallocation yourself. This means we need to be malloc'ing structures when we add nodes to a list, and we need to be free'ing structures when we remove nodes from a list. We also might need to malloc and free structures that represent the overall list itself.
Given all of these complications, our actual linked list data structure ends up looking like this:
Specifically, we define the following types and structures:
- LinkedList: a pointer to a LinkedListHead
structure. When our customer asks us to allocate a new, empty
linked list, we malloc and initialize a LinkedListHead
structure, and return a pointer to that malloc'ed structure to
- LinkedListHead: this structure contains bookkeeping
information about an entire linked list, including the number of
nodes in the list and pointers to the head and tail nodes of the
- LinkedListNodePtr: a pointer to a LinkedListNode
- LinkedListNode: this structure represents a node in a
doubly-linked list. It contains a field for stashing away
(a pointer to) the customer-supplied payload, and fields pointing
to the previous and next LinkedListNode in the list. When a
customer requests that we add an element to the linked list, we
malloc a new LinkedListNode to store the pointer to that element, then we do
surgery to splice the LinkedListNode into the data structure and
we update the LinkedListHead as well.
- LLIter: sometimes customers want to navigate
through a linked list. To help them do that, we provide them with
an iterator. LLIter points to a structure that keeps the
state of an iterator. When a customer asks for a new iterator, we
malloc a LLIterSt, and return a pointer to it to the customer.
- LLIterSt: this structure contains bookkeeping associated with an iterator. In particular, it tracks the list that the iterator is associated with and the node in the list that the iterator currently points to. Note that there is a consistency problem here: if a customer updates a linked list by removing a node, it's possible that some existing iterator becomes inconsistent by pointing to the deleted node. So, we make our customers promise that they will free any live iterators before mutating the linked list. (Since we are generous, we do allow a customer to keep an iterator if the mutation was done using that iterator.)
What to do.
You should follow these steps to do this assignment:
- Make sure you are comfortable with C pointers, structures,
malloc, and free. We will cover them in detail in lecture, but you
might need to brush up and practice a bit on your own; you should
have no problem Googling for practice programming exercises on the
Web for each of these topics.
- Get the source files for hw1. Navigate to a directory that
contains a checked-out copy of your cse333 Git repository and run
git pull. (See the CSE 333 Git Tutorial for some tips if the
pullcommand fails because you have unstaged changes or other problems.) After the
pullcommand finishes you should see the following directories and files in your repository:
(You can delete the local copy of the
bash$ ls clint.py gtest hw0 hw1
hw0directory if you want once you have pushed your final version of hw0 to GitLab, but everything else needs to remain in place for this and later parts of the project over the rest of the quarter.)
- Look inside the hw1 directory. You'll see a number of files and
subdirectories, including these that are relevant to Part A:
- Makefile: a makefile you can use to compile the
assignment using the Linux command make all.
- LinkedList.h: a header file that defines and documents the API to
the linked list. A customer of the linked list includes this
header file and uses the functions defined within in. Read
through this header file very carefully to understand how the
linked list is expected to behave.
- LinkedList_priv.h, LinkedList.c: LinkedList_priv.h is a
private header file included by LinkedList.c; it defines the
structures we diagrammed above. LinkedList.c contains the
partially completed implementation of our doubly-linked list.
Your task will be to finish the implementation. Take a minute
and read through both files; note that there are a bunch of
places in LinkedList.c that say "STEP X:" these labels identify
the missing pieces of the implementation that you will finish.
- example_program_ll.c: this is a simple example of how a
customer might use the linked list; in it, you can see the
customer allocating a linked list, adding elements to it,
creating an iterator, using the iterator to navigate a bit, and
then cleaning up.
- test_linkedlist.cc: this file contains unit tests that we wrote
to verify that the linked list implementation works correctly.
The unit tests are written to use the Google Test unit
testing framework, which has similarities to Java's JUnit testing
framework. As well, this test driver will assist the TA in
grading your assignment: as you add more pieces to the
implementation, the test driver will make it further through the
unit tests, and it will print out a cumulative score along the
way. You don't need to understand what's in the test driver for
this assignment, though if you peek inside it, you might get
hints for what kinds of things you should be doing in your implementation!
- solution_binaries: in this directory, you'll find some Linux
executables, including example_program_ll and test_suite.
These binaries were compiled with a complete, working version of
LinkedList.c; you can run them to explore what should be displayed
when your assignment is working!
- Makefile: a makefile you can use to compile the assignment using the Linux command make all.
- Run "make" to verify that you can build your own versions
of example_program_ll and test_suite. Make should print out a
few things, and you should end up with new binaries inside
the hw1 directory.
- Since you haven't yet finished the implementation of
LinkedList.c, the binaries you just compiled won't work correctly
yet. Try running them, and note that example_program_ll halts with
an assertion error or a segfault and test_suite prints out some
information indicating failed tests, and may crash before
- This is the hard step: finish the implementation of LinkedList.c.
Go through LinkedList.c, find each comment that says "STEP X", and replace
that comment with working code. The initial steps are meant to
be relatively straightforward, and some of the later steps are
trickier. You will probably find it helpful to read through
the code from top to bottom to figure out what's going on.
You will also probably find it helpful to recompile frequently
to see what compilation errors you've introduced and need to fix.
When compilation works again, try running the test driver to
see if you're closer to being finished.
- Note: You may not modify any header files or
interfaces in this or later project assignments. We may test your
code by extracting your implementations and compiling them with the
original header files or in some other test harness where they are
expected to behave as specified. You certainly are free, of
course, to add additional private (
static) helper functions in your implementation, and you should do that when it improves modularity.
- Debugging hint: Verify333 is used in many places in the code to check for errors and terminate execution if something is wrong. You might find it helpful to discover the function that is called when this happens so you can place a debugger breakpoint there.
- Note: You may not modify any header files or interfaces in this or later project assignments. We may test your code by extracting your implementations and compiling them with the original header files or in some other test harness where they are expected to behave as specified. You certainly are free, of course, to add additional private (
- We'll also be testing whether your program has any memory
leaks. We'll be using Valgrind
to do this. To try out Valgrind for yourself, do this:
- from the hw1 directory run the following command:
valgrind --leak-check=full ./solution_binaries/example_program_llNote that Valgrind prints out that no memory leaks were found. Similarly, try running the test driver under Valgrind:
valgrind --leak-check=full ./solution_binaries/test_suiteand note that Valgrind again indicates that no memory leaks were found.
- now, in the hw1 directory, compile your versions of the example_program_ll and test_suite binaries, and try running them under Valgrind. If you have no memory leaks and the test_suite runs the linked list tests to completion, you're done with part A!
- from the hw1 directory run the following command:
|Part B -- a chained hash table|
A chained hash table is also a fairly simple data structure. It consists of an array of buckets, and each bucket contains a linked list of elements. When a user inserts a key/value pair into the hash table, the hash table uses a hash function to map the key into one of the buckets, and then adds the key/value pair onto the linked list. (As an important corner case, if the key of the inserted key/value pair already exists in the hash table, the hash table replaces the existing key/value pair with the new one, and returns the old key/value pair to the customer.)
So, over time, as more and more elements are added to the hash table, the linked lists hanging off of each bucket will start to grow. As long as the number of elements is a small multiple of the number of buckets, lookup time is small: you hash the key to find the bucket, then iterate through the chain (linked list) hanging off the bucket until you find the key. As the number of elements gets longer, lookup gets less efficient, so our hash table includes logic to resize itself to maintain short chains.
As with the linked list in Part A, we've given you a partial implementation of a hash table. Our hash table implementation looks approximately like this:
Specifically, we defined the following types and structures:
- HashTable: a pointer to a HashTableRecord
structure. When our customer asks us to allocate a new, empty
hash table, we malloc and initialize a HashTableRecord
structure (including malloc'ing space for the bucket array
inside it, and allocating LinkedLists for each bucket), and return
a pointer to that malloc'ed structure to the customer.
- HashTableRecord: this structure contains bookkeeping
information about the hash table.
- HTIter: (not shown in the diagram) sometimes
customers want to iterate through all elements in the hash table.
To help them do that, we provide them with an iterator.
HTIter points to a structure that keeps the state of a
hash table iterator. When a customer asks for a new iterator
we malloc an HTIterRecord and return a pointer to it.
- HTIterRecord: this structure contains bookkeeping associated with an iterator. In particular, it tracks the hash table that the iterator is associated with, the bucket within the hash table the iterator currently points at, and uses linked list iterator to track which element in the chain linked list the hash table iterator is pointing at. As well, we use a field to track whether or not the iterator is currently valid (i.e., whether it is pointing somewhere in the table, or whether it has been run off the end of the table).
What to do.
You should follow these steps to do this assignment:
- The code you fetched in Part A also contains the
files you'll need to complete your hash table implementation and
test it. Similar to the linked list, the hash table implementation
is split across a few files: HashTable.c contains the
implementation you need to finish, HashTable.h contains the public
interface to the hash table and documents all of the functions
and structures that customers see, and HashTable_priv.h contains
some private, internal structures that HashTable.c uses.
- Read through HashTable.h first to get a sense of what the hash
table interface semantics are. Then, take a look at
example_program_ht.c; this is a program that uses the hash table
interface to insert/lookup/remove elements from a hash table,
and uses the iterator interface to iterate through the elements
of the hash table.
- test_hashtable.cc contains our Google Test unit tests for the
hash table. As before, run this (on its own, and using valgrind)
to see how close you are to finishing your hash table
- Look through HashTable.c, find all of the missing pieces
(identified by STEP X comments, as before), and implement them.
- As before, in solution_binaries, we've provided you with linux
executables (example_program_ht and the same test_suite as before)
that were compiled with our complete, working version of
HashTable.c You can run them to explore what should be displayed
when your part B implementation is working
and look at the source code for examples of how to use the data structures.
You'll note that we provided a second Makefile called "Makefile.coverage". You can use it to invoke the "gcov" code coverage generation tool. Figure how to (a) use it to generate code coverage statistics for LinkedList.c and HashTable.c, (b) note that the code coverage for HashTable is worse than that for the LinkedList, and (c) write additional HashTable unit tests to improve HashTable's code coverage.
The bonus task is simple, but we're (deliberately) providing next to no detailed instructions on how to do it -- figuring out how is part of the bonus task!
Please make sure your additional unit tests don't change the scoring mechanism that we use, obviously. (We'll be checking that.) Place your additional unit tests in a separate file from the original test suite. That will make it easier for us to find and evaluate your tests.
|What to turn in|
When you are ready to turn in your assignment, you should follow
exactly the same procedures you used in hw0, except this time tag
the repository with
hw0-final). Remember to clean up and commit and
push all necessary files to your repository before you add and
push the tag.
After you have created and pushed the tag, be
absolutely sure to test everything by creating a new
clone of the repository in a separate, empty directory, checkout
hw1-final tag, and verify that everything works
as expected. Refer to the hw0 turnin instructions for details,
and follow those steps carefully.
If you fail to check your work and your project doesn't build properly when the same steps are done by the course staff to grade it, you may lose a huge amount of the possible credit for the assignment even if almost absolutely everything is actually correct.
We will be basing your grade on several elements:
- The degree to which your code passes the unit tests contained
in test_linkedlist.cc and test_hashtable.cc. If your code fails a
test, we won't attempt to understand why: we're planning on just
including the number of points that the test drivers print out.
- We have some additional unit tests that test a few additional
cases that aren't in the supplied test drivers. We'll be checking
to see if your code passes these as well.
- The quality of your code. We'll be judging this on several
qualitative aspects, including whether you've sufficiently
factored your code and whether there is any redundancy in your code
that could be eliminated.
- The readability of your code. While we don't
have detailed, formal coding style guidelines that you must follow, you
attempt to mimic the style of code that we've provided you.
Aspects you should mimic are conventions you see for
capitalization and naming of variables, functions, and arguments,
the use of comments to document aspects of the code, and how code
We strongly suggest using the clint.py tool to check your code for
You also will find it useful to refer to the Google C++ Style Guide
(linked from the course web site). Much of this guide applies equally
well to C.