program辅导 、讲解 C/C++编程设计
Introduction
In this lab, you will deepen and solidify the concept of pointers through the concept of a
doubly-linked list data structure. You will implement a library to create, modify, and delete linked
lists. You will also learn to allocate and deallocate space during runtime. Finally, you will use
your library to compare two algorithms for counting unique words in a list. This lab will be
completed on both the Nucleo development kit and Linux.
Reading
● K&R – Chapters 5, 6.7, 7.8.5, appendix B5
Concepts
● Doubly-linked lists
● Memory allocation
● Sorting
● Pointers (including NULL)
● Algorithmic analysis
Required Files:
● LinkedList.c
● LinkedListTest.c
● sort.c
● README.pdf
BOARD OF STUDIES IN COMPUTER ENGINEERING
Linked Lists
20 Points
Lab Files:
● DO NOT edit these files:
o LinkedList.h – Contains the spec and prototypes for the linked-list functions you
will implement.
o BOARD.c/h – Standard hardware library for ECE013.
o stopwatch.c/h – Tools for benchmarking the execution time of your program.
o Lab06_main.c – Default Makefile target; contains the main() function that you
will use to benchmark the algorithms that you develop in sort.c.
o GNUmakefile — Used for compiling this lab on Linux.
● Edit these files:
o sort.c – This file contains some starter code for a demonstration of the testing
for the sorting algorithms along with a function to generate a word list for the
sorts to operate on.
▪ It is included as sort_template.c
● Create these files:
o LinkedList.c – Implement the library described in LinkedList.h.
o LinkedListTest.c – A test harness for your LinkedList library. Should include a
main().
Assignment requirements
● Your program will implement the functions whose prototypes are provided in
LinkedList.h. The functions all have appropriate documentation and describe the
required functionality.
● Within LinkedListTest.c you will:
o Create a test harness that tests your linked list functions and ensures
that they return the correct values. As in previous labs, at least two tests
are required per function.
o Once you are done testing the functionality of LinkedList.c, you will need
to exclude LinkedListTest.c from the project.
● Within sort.c you will:
o Implement two algorithms for sorting linked lists inside the functions
SelectionSort() and InsertionSort(). You will use these with the
provided main() code to perform timing experiments that will
(hopefully) demonstrate the usefulness of linked lists. You should not
modify either main() or CreateUnsortedList() in this file.
● Add inline comments to explain your code.
● Create a readme file and export it to PDF named README.pdf containing the
following items. Spelling and grammar count as part of your grade so you'll want
to proof-read this before submitting. This will follow the same rough outline as a
lab report for a regular science class. It should be on the order of three
paragraphs with several sentences in each paragraph.
o First you should list your name & the names of colleagues who you have
collaborated with.
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o Report the results of the timing experiment in sort.c. Which was faster,
SelectionSort() or InsertionSort()? Explain why. Was this what you
expected? How does performance compare on Linux vs your Nucleo?
o In the next section you should provide a summary of the lab in your own
words. Highlight what you thought were the important aspects of the
lab. If these differ from how the lab manual presents things, make a
note of that.
o The following section should describe your approach to the lab. What
was your general approach to the lab? Did you read the manual first or
what were your first steps? What went wrong as you worked through it?
What worked well? How would you approach this lab differently if you
were to do it again? How did you work with other students in the class
and what did you find helpful/unhelpful?
o The final section should describe the results of you implementing the
lab. How did it end up finally? How many hours did you end up spending
on it? What did you like about it? What did you dislike? Was this a
worthwhile lab? Do you have any suggestions for altering it to make it
better? What were the hardest parts of it? Did the points distribution for
the grading seem appropriate? Did the lab manual cover the material in
enough detail to start you off? Did examples or discussions during class
help you understand this lab or would more teaching on the concepts in
this lab help?
1 NOTE: collaboration != copying. If you worked with someone else, be DETAILED in your
description of what you did together.
● Make sure that your code triggers no errors or warnings when compiling as they
will result in a significant loss of points.
● Follow the style guidelines.
Doing this Lab on Linux:
For this lab we will continue using the make system. The commands as before are
below. For this to work the file ‘GNUmakefile’ needs to be in the same directory as
the rest of your files, it is only used when compiling on Linux.
● $ make
o This command will create the final executable named Lab06
● $ make LinkedListTest
o This command will compile your linked list test harness into the
executable LinkedListTest
Grading:
This assignment consists of 20 points:
● 8.5: Automated Components:
o 6.5 – LinkedList library
▪ 3.5 = 0.5 points per function for each of LinkedListNew(),
LinkedListCreateAfter(), LinkedListCreateBefore(), LinkedListRemove(),
LinkedListSize(), LinkedListGetFirst(), LinkedListGetLast(), and
LinkedListData().
▪ 1.0 point, all functions handle null pointer inputs
▪ 1.0 point, all functions that allocate can handle malloc() failures
▪ 0.5 no functions reference deallocated data
▪ 0.5 no functions allocate unnecessary memory
o 2.0 – Sort.c
▪ 1.0 SelectionSort() gives correct results, and does not print.
▪ 1.0 InsertionSort() gives correct results, and does not print.
● 11.5: Human components:
o 0.5 -- LinkedListPrint()
o 4.0 -- Test harness for LinkedList
▪ 1 point for testing each function.
▪ 1 point for thorough and diverse tests.
▪ 1 point for readability of output.
▪ 1 point for correctly performing the timing experiment on
SelectionSort() and InsertionSort(), then reporting your results in
README.pdf.
o 1.0 sort.c:
▪ 1 – explanation in README.pdf is clear, accurate, and demonstrates
understanding of why linked lists are useful.
o 1.5 points – Readability and code style
o 4.5 points – Lab writeup
● Deductions:
o NO CREDIT for sections where required files don't compile
o -1 changing the name of a prototyped function in sort.c (this will break our
autochecker scripts!)
o -2: At least one compilation warning
o -2: Extremely bad coding practice (Used extern or goto, extreme inefficiency or
unreadability)
o Other deductions at grader’s discretion.
o It must compile within Visual Studio Code/PlatformIO and Linux using the C11
standard. Code that does not compile will receive no credit.
Pointers
Pointers are covered thoroughly in the required reading for this lab, so if you are having
trouble, refer back to chapter 5 of K&R. However, there is one concept about pointers
that is not directly addressed in the reading: null pointers. A null pointer is a pointer to
nothing. These pointers must be handled as a special case if they are passed to a
function that expects non-null data pointers. This is one of the major sources of crashes
in programs.
The main problem with null pointers arises from when you try to dereference them
(assuming x is an int pointer and equal to NULL): *x = 6;
The reason for why becomes clear when you think about what memory location x points
to. 0, or NULL, is an invalid memory location, and a "null pointer dereference" error
occurs because there is no memory location to write to, so an error occurs. This is a
"fatal" error, which means that the program has no way to handle it, so the only thing it
can do is crash! This is a common cause of Windows blue-screen-of-death errors (when
this dereferencing happens in kernel space
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). The solution to this is to check for null
pointers before dereferencing. An especially important case for checking to see if a
pointer is null is after any call to malloc() or calloc(), which we will cover a little later.
Doubly-linked lists
In computer programs, much as in real life, keeping a list of things can be useful. Usually
the number of items that will be in this list is known ahead of time and so in a computer
program this list could be kept in a standard C array. There will be occasions, like when
processing user input, when the number of items to be stored in a list is not known
ahead of time. This is a problem with C’s statically-allocated arrays. The common
solution is to use another data type called a linked list.
Linked lists are exactly what they sound like: a collection of objects that are all linked
together to form a single list. As each item is linked to at least one other item in the list
there is a set ordering to the list: from a “head” item at the start to a “tail” item at the
end. Since these items are all connected it is easy to access any item from any other item
by just traversing or “walking” through the list.
For this lab you will be implementing a doubly-linked list, the more useful sibling of the
linked lists. A doubly-linked list is also straightforward: each item is linked to both the
item before it and after it. This allows for traversal of the list from any element to any
other element by walking along it, which makes using the list very easy.
2Note that kernel space vs user space is a key concept in protected mode multitasking operating systems
such as Linux and WinNT derivatives. Using protected mode, a misbehaving user program cannot (in
theory) crash the entire system. The absence of protected mode is a key property of embedded systems.
The items in the list you are implementing are stored as structs in C because they will
be storing a few different pieces of data. Specifically it holds a pointer to the previous
ListItem, which will be NULL if it is the head of the list; a pointer to the next
ListItem, which will be NULL if it’s at the end of the list; and a pointer to any kind of
data (NULL if there’s no data). The typedef and the name after the “}” let you refer to
the struct in a similar fashion to any other data type, by using the single name
“ListItem” instead of the longer “struct ListItem”.
The definition of the ListItem struct in LinkedList.h:
typedef struct ListItem {
struct ListItem *previousItem;
struct ListItem *nextItem;
char *data;
} ListItem;
Now that you understand the structure of a linked list we will introduce the various
operations that can be performed upon a list. The standard operations are creating a
new list, adding elements to a list, finding the head of a list, and removing elements
from a list.
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Creating a new list: A new list is created by just making a single ListItem. As this ListItem
is both the head and tail of the list there is no item before it or after it in the list.
Adding to a list: Now that you have a list, how do you add more elements to it? With the
arrays that you are familiar with, you need to know two things: the position to insert into
and the data that will be inserted. With linked lists it’s a little different because there’s
never a “free spot” to insert a new item into. What is done instead is that the position of
the new list item is relative to an existing item, generally the item before it in the list. So
to insert an item into the list, that item is inserted after an existing item. If the list went
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It is incredibly useful to your understanding to draw this out on a piece of paper or a white board. Make
boxes for each member of the struct, and use arrows to point to the next list element and the previous
ones (in other words, use arrows to show what the pointers point to). Go through all of the functions and
make sure you understand what you need to do. Once you understand it conceptually, coding it up is very
simple.
A <-> B <-> C and you want to insert D after B then the list would become A <-> B <-> D
<-> C. So that means that the previous item and next item pointers of both B and C will
need to change to accommodate the new item D.
Finding the head: The head of a list is a special item because it has no preceding
element (represented by a NULL pointer). Since all the elements in a list are connected,
finding the head merely requires traversing the list until a list item is found with no
preceding element. A function that finds the head of the list has one odd scenario; see if
you can figure out what it is.
Removing an element: Removing an element from a list is the opposite of adding to it.
Following the example above you’d go from a list like A <-> B <-> D <-> C to A <-> B <-> C.
The pointers of B and C both need to be modified to account for the removal of D.
Generally the data that was stored within D is also desired after the removal of the item
and should be returned.
malloc(), calloc(), and free()
This lab also relies on the use of memory allocation using malloc() (and/or calloc())
and free(). These are discussed somewhat in chapter 5 of K&R. As they are standard
library functions they are documented thoroughly online or in the Linux man-pages.
Refer to those resources to understand them.
It should be emphasized here that after any call to malloc() or calloc() you should
always check for NULL pointers! Memory allocation relies on the heap, which PlatformIO
specifies as 8192 bytes (0x200) for our Nucleo development kit by default. This project
requires at least a couple hundred bytes for malloc() and calloc() to work, so we can
proceed with the default heap size.
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4The linker script used by PlatformIO (which defines heap size) is auto-generated for all
supported chip architectures. You can find the location of the linker script for STM32
architectures under the “~/.platformio/packages/tool-ldscripts-ststm32/” path on
Linux/UNIX-like operating systems.
Note that this makes it easy to test that your code is properly checking for NULL
pointers: if you set the heap to 0, ALL calls to malloc()/calloc() will fail; if your code
doesn't crash, it's working!
Sorting
Sorting is an incredibly important function in computer programming. While you may
not think it is used a lot, it is quite common within a program to have the need to sort a
series of numbers. Sorting is an entire field of study within computer science and so
there are a huge number of algorithms that do just that. In this lab, you will be focusing
on two: Selection Sort is simple, intuitive, and easy to implement on a static array, but it
is slow. Insertion Sort is usually significantly faster, but it relies on a data structure that
allows insertion.
Selection sort partitions the list into two partitions. The first partition is sorted, while
the second partition is unsorted. At the start of the algorithm, the sorted partition is
empty. With each iteration, the sorted partition grows by one element, and the
unsorted partition shrinks, until there are no remaining unsorted items. This is achieved
by finding the smallest element in the unsorted partition and moving it to the sorted
partition.
Pseudo-code for selection sort is provided below. This pseudocode is written in a way
that makes it easy to apply to linkedLists, but note that in this case the pointers could
easily be replaced with indexes to a static array. We use two pointers: “FU” stands for
“First Unsorted”, representing the first item in the unsorted portion of the list. “S”
stands for “Scan”, since it “scans” through the unsorted partition, looking for the
smallest item.
FU is pointer to first item
while FU is not tail:
S is pointer to FU’s nextItem
while S is in list:
if FU > S:
swap FU and S contents
advance S
advance FU
The outer for loop effectively tracks the right-most element of the sorted array filling up
the left portion of the array. This means that for each iteration of the outer-loop, the
inner-loop can perform many element swaps.
Below is an example of Selection Sort in action:
D A C E B //
^FU ^S // D > A, swap and advance S
A D C E B //
^FU ^S // A < C, advance S
^FU ^S // A < E, advance S
^FU ^S // A < B, advance S
// S is at end of list, advance FU
A D C E B //
^FU ^S // D > C, swap and advance S
A C D E B //
^FU ^S // C < E, advance S
^FU ^S // C > B, swap and advance S
A B D E C //
// S is at end of list, advance FU
A B D E C //
^FU ^S // D < E, advance S
^FU ^S // D > C, swap and advance S
A B C E D //
// S is at end of list, advance FU
A B C E D //
^FU ^S // E > D, swap and advance S
A B C D E //
// S is at end of list, advance FU
// FU is at tail, return
Insertion Sort operates in a similar way to selection sort. Like Selection Sort, it partitions the list
into a sorted and unsorted portion, and uses a double-loop structure to move items from the
unsorted portion into the sorted portion. Unlike its slower cousin, insertion sort leverages an
“insert” operation to reduce the average time spent in the inner loop. Rather than “scanning”
through the unsorted partition in search of the smallest element, it scans through the sorted
portion to find the best place to insert the next item. It has the advantage that this scan does
not need to cover the entire sorted partition, instead stopping as soon as it finds the appropriate
place to insert.
Pseudocode for an insertion sort algorithm is given below. We use three pointers: “FS” stands
for “First Sorted”, and represents the first item in the sorted partition of the array. “LU” stands
for “Last Unsorted,” and represents the last item in the unsorted partition. Again “S” stands for
“scan,” since its job is to scan through the sorted partition to find the appropriate insertion
point.
FS = tail of list
while FS is not head of list:
LU = FS’s previous item
if LU < FS:
FS = LU
else:
S = FS
while (S is not tail of list):
if S's next item is greater than LU:
break
else:
S = S's next item
remove LU item
re-insert after S
Insertion Sort is slower than Selection Sort in a static array, but can be very quick in a linked list
data structure.
Note that LinkedList.h does not have a true “insert” function. You can achieve something similar
by removing an item and then using CreateAfter() or CreateBefore() to insert it back in
5
. You will
also need to account for inserting items at the beginning and end of your list.
Below, you can see the InsertionSort in action:
5 Don’t forget to save the pointer to the ListItem’s data member before you remove!
D A C E B //
^LU ^FS // E > B, make an S pointer
^S // B < E, advance S
// S is at end of list, insert
D A C B E //
^LU ^FS // C > B, make an S pointer
^S // C < E, insert
D A B C E //
^LU ^FS // A < B, advance FS and LU
D A B C E //
^LU ^FS // D > A, make S pointer
^S // D > B, advance S
^S // D > C, advance S
^S // D < E, insert
A B C D E // FS is head, return
^FS
You will write code for both of these algorithms inside of the SelectionSort() and InsertionSort()
functions in sort.c.
Evaluating SelectionSort() and InsertionSort() in Lab06_main.c
Although we have stated that InsertionSort() is faster than SelectionSort(), we should
test that claim experimentally. To do this, we will use the “stopwatch” library [included
in this assignment’s directory] to start, stop, and report the differences between their
execution times.
To measure the time required to execute SelectionSort(), we need to track the system
time immedIately before and after SelectionSort(), and then run our code. To do this
using the stopwatch.c library included with this lab:
1. Stopwatch_Init() – This function sets up your system’s clock; only needs to be
called once.
2. Stopwatch_StartBenchmark() – Starts or restarts the stopwatch timer.
3. Stopwatch_StopBenchmark() – Stops the stopwatch timer and saves the results
in its local scope.
4. Stopwatch_PrintBenchmarkResults() – Displays information about the cycles
required/time past for the last benchmark run. NOTE: You will need to call this
function to see the results after running “Stopwatch_StopBenchmark()”!
Approaching this lab
Like all labs for this class, you should first start with implementing the LinkedList library.
Be sure to handle when malloc() returns NULL, NULL pointers as arguments to functions,
and whether the function expects the head of the list or not.
1. Implement LinkedListNew().
Test this by writing code to create a new list of size 1. Manually inspect the
resultant struct that is created using the Variables window in VS
Code/PlatformIO to see that it is correct while running on your Nucleo.
2. Implement LinkedListCreateAfter() and LinkedListCreateBefore().
Test these by creating a list of multiple sizes greater than 1. Manually inspect the
resultant list using the Variables window in VS Code/PlatformIO.
3. Now that you can create lists of a multitude of sizes, implement
LinkedListGetFirst(). This function will be helpful for implementing the other
functions.
Test this function by creating a few different lists, storing the pointer to the head
node. Pass a non-head node to GetFirst() and see if it matches the memory
address of the head node.
4. Implement LinkedListGetSize().
Run it on the different size lists you created earlier and confirm that results are
as expected.
5. Implement LinkedListPrint() and LinkedListSwapData().
These should be straight-forward to test.
6. At this point you are now ready for implementing SelectionSort() and
InsertionSort in sort.c. The debugger will be very useful here. You may find it
convenient to use a shorter list during testing.
7. Once SelectionSort() and InsertionSort() are functional, use the stopwatch tool
and the debugger to measure the time required for each sort. Record the
results in README.pdf and explain them.
A note on using GNUmakefile
One beautiful perk of using Makefiles is easy cleanup. While in the previous lab, you had to
remove every object (.o) file and executable individually, the GNUmakefile provided for you in
this lab assignment can take care of this mess for you. Simply type into your terminal:
Unset
$ make clean
This is just one of many useful features that you can reap from having a well-built Makefile,
especially when dealing with larger projects. We encourage you to read more about building
Makefiles.
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A note on testing with PlatformIO/VS Code
While Makefiles (e.g. GNUmakefile) allow you to specify the target that you would like to
compile and run on your microcontroller, for the purposes of our course you will only be able to
build a PlatformIO project with one “main()” function declared within its scope. For example in
this lab assignment, you will need to move either your LinkedListTest.c or your Lab06_main.c to a
directory outside of the “src/” directory of your project for it to compile. We suggest leaving any
files that you do not want to build in your “Lab06/” directory for safe-keeping, then swapping
them between there and the “Lab06/src/” directory depending on which main() function you
would like to run.
How your code will be tested
Like previous labs, your code will be tested using your code as a library where we will
test with different programs to exercise your library on both Nucleo and Linux. Your
LinkedListTest.c program will also be run so that we can observe your testing output.
6 A thorough (but perhaps overwhelming) guide to writing Makefiles is the GNU Make manual. This guide
contains a comprehensive description of the features offered by Makefiles, and it is not terribly-written;
however, it is very long. A more practical approach, as with much software development, is to read and
analyze other developers’ code when you encounter it (key word: “analyze”).