The purpose of this homework is to familiarize yourself with the various tools that will be used throughout the course, and to get a feeling of basic programming in Racket. Note that the submission system will not allow you to submit your code if it does not follow certain requirements. For example, you will not be able to submit code that doesn’t have the required definitions, or doesn’t bind them to appropriate value types, or code that contains lines that are longer than 79 characters. In addition, you are required to have tests that completely cover your code, otherwise your submission will be penalized severely (the server will tell you about it when you submit, and resubmissions are always fine).
In this particular homework, the server will perform additional tests over your code, which will require you to come up with correct solutions to be able to submit. This means that you will generally be graded on contracts and purpose statements, other comments, style, test quality, etc. Correctness will play a very small role here, since everyone is expected to be able to solve these questions.
The first thing you will need to do is to download and install Racket and then the course plugin. When this is done (and you restart DrRacket), you will be able to register for homework submission, as described in the Software Section. (If you work in the lab, Racket should already be installed.)
Shortly after you install the plugin and register, you will be added to the course piazza group, which will allow you to post the required test message (see below). Note: do not email requests to be subscribed to the piazza group, it will be done after you register with the handin server. (It’s done manually, so “shortly” can mean a number of hours, maybe even a day.)
For this problem set (and only for this), you are required to set the
language level to “Intermediate Student”. This will allow you to use
the Stepper to debug your code, and more importantly: learn how Racket
evaluates it. However, this language includes a bunch of things that
we do not know about — not until we learn about them explicitely.
Things that you should not use even if you know about them
require. (If you don’t know about
them then you have no problems, if you do, then pretend that you’ve
never heard about them.)
This homework is for individual work and submission.
Submitted code should have comments that describe each function and
its type, as well as enough test cases for complete coverage (DrRacket
indicates covered expressions with colors for covered and uncovered
source code, unless your code is completely covered). Your tests
should have the form of
(equal? <expected> <expression>), except for
boolean functions (predicates) where they should be either
(not <expression>). (Note that this is
different from what you’ve used in the past; in the next homework we
will switch to the course language that has its own testing facility.)
Important reminder: Your tests should cover your whole code, otherwise the server will penalize your submission heavily. You should not have any uncovered expressions after you hit “Run” — it should keep the same colors, indicating complete coverage. Furthermore, the server will run its own tests over your code, which means that you will not be able to submit code that does not work.
General note: do not duplicate code! If there is an expression that
is used in multiple places, then you should use
Another important note: remember we’re using a computer system for submission. You don’t need to write any meta information at all. Examples of things that we don’t want to see in the submissions because the system already has them: the homework number, your name, your id, the submission date and/or the time, the due date and/or the time. Also, examples of things that we don’t want to see in the submissions because they’re obvious: the course name and/or number, the instructor name, the grader names, the department name, the university name, etc. Again, all of these are things that we don’t need. Save your bits.
Furthermore: grading is done anonymously, so please do not write your name in your submission code!
Read the Academic Integrity page, and indicate your agreement as specified on the page.
Once you’re subscribed to the course piazza group, you will see a test post for this homework. Post a followup note to this post. Make sure that it actually appears, otherwise you will not get the credit for posting. (You only need to make some post, so if you’ve already posted a question, a reply, or a note, then you don’t need to post a note.)
Reminder: the piazza group is
https://piazza.com/northeastern/spring2020/cs44005400/home, and see also the
Piazza Group Section. Note that you will not be able
to view or post on the piazza group until you are subscribed to it, and
you will be subscribed to it only after you installed the course
plugin and created an account — so make sure you do that first.
Once you do this, you will get a notification when you’re on the
piazza group. Again, do not try to subscribe to it by yourself.
Define a function called
bin4-to-num. It interprets four binary
digits (0 or 1) as a single number, with the arguments in the order
of least to most signficant digit. For example, here is a test case
that demonstrates how it works which you can use:
Don’t forget to write a proper contract, a purpose statement, and sufficient tests that cover the whole code and also verify corner cases.
Note that for this homework, test cases are simple toplevel
expressions that should all evaluate to
gcd2 function which takes two non-negative integers a
and b and computes their greatest common divisor. This must be
done with the following algorithm:
a=0, return b.
b=0, return a.
gcd2(a,b) = 2*gcd2(a/2,b/2).
gcd2(a,b) = gcd2(a/2,b).
gcd2(a,b) = gcd2(a,b/2).
gcd2(a,b) = gcd2((a-b)/2,b).
gcd2(a,b) = gcd2((b-a)/2,a).
You should use
odd? to check if an integer number is
even or odd. For example, the expected value of
(gcd2 378 144) is
(gcd2 216 612) is 36. Don’t forget to write a proper
contract and purpose statement, and to properly test the code (the
tests that are given here are useful, but you will need to get
complete coverage, and it is always a good idea to make sure that you
test potential corner cases).
This is called the “Binary GCD” algorithm — if you’re interested, you can read more about it on Wikipedia.
all-even? function, which consumes a list of integers,
#t if all integers are even.
Hint: The common way to deal with an empty input in these kind of
functions is to return
#t since all of the empty list’s items are
Note that the function in this case returns a boolean, so you can simply call it in ways that expect a true result. In other words, write:
(Do not use
One of the most useful sorting algorithms is merge sort.
Implementing it requires a merge function: a function that receives
two sorted lists, and returns a single sorted list with all items
from the two input lists. Implement such a
For simplicity, you are required to deal only with numbers, and
assume that the input lists are (and the output list should be)
sorted in ascending order.
(Do not use
minutes-spent as the number of minutes you spent on your
homework. Please specify a reasonable estimate here and in future
homework, since these values help in determining homework weights.