Description
Question 1 (10 points):
Purpose: To work with the concept of references a bit more carefully.
Degree of Diculty: Easy. If you understand references very well, this is not dicult.
In class (and in the readings) we saw a version of Selection sort. As described in the readings, our implementation of selection sort works by repeatedly removing the smallest value from an unsorted list and
adding it to end of a sorted list until there are no more values left in the unsorted list (see Chapter 1 of the
readings).
✞ ☎
1 unsorted = [3 , 2 , 5 , 7 , 6 , 8 , 0 , 1 , 2, 8 , 2]
2 sorted = list ()
3
4 while len( unsorted ) > 0:
5 out = min ( unsorted )
6 unsorted . remove ( out )
7 sorted . append ( out )
8
9 print ( sorted )
✝ ✆
One problem with this implementation is that it modies the original list (line 6). Unless we know for sure
that a list will never be needed in the future, removing all contents is a bit drastic. To address this problem,
we can change the code so that a copy of the original list is made rst. Since we make the copy, we can
say for sure that the copy will never be needed in the future, and so removing all its contents is absolutely
ne.
The le a1q1.py is available on Moodle, and it contains a function called selection_sort() which is very
similar to the above code:
✞ ☎
1 def selection_sort ( unsorted ):
2 “””
3 Returns a list with the same values as unsorted ,
4 but reorganized to be in increasing order .
5 : param unsorted : a list of comparable data values
6 : return : a sorted list of the data values
7 “””
8
9 result = list ()
10
11 # TODO use one of the copy () functions here
12
13 while len ( acopy ) > 0:
14 out = min ( acopy )
15 acopy . remove ( out )
16 result . append ( out )
17
18 return result
✝ ✆
On line 11, there is a TODO item, which is where we will add code to create a copy of the original unsorted
list.
Also in the le are 5 dierent functions whose intended behaviour is to copy a list. Your job in this question
is to determine which, if any, of these functions does the job right.
Note: There is a Python list method called copy(), which we are not using on purpose. We need to
understand references, and we must not side-step the issue.
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CMPT 145
Principles of Computer Science
For each of the 5 copy() functions in the le a1q1.py, do the following:
• Determine if the function makes a copy of the list. Hint: some do not!
• Determine if the function is suitable for use in selection_sort(), that is, if it works, would you use this
function? Do you think it is a good function? Hint: some are not!
Do not change the function selection_sort() except to make use of one of the versions of the copy()
function on line 11. Do not change the code for any of the copy() functions, except to add a doc-string.
What to Hand In
Your answers to the above questions in the text le called a1_reflections.txt (PDF, rtf, docx or doc are
acceptable). Please help the marker by clearly indicating the question number.
You might use the following format:
✞ ☎
Question 1
copy1 ()
– makes a copy
– is suitable
copy2 ()
– makes a copy
– is not suitable
✝ ✆
The above example does not necessarily reect the right answers!
Be sure to include your name, NSID, student number, section number, and laboratory section at the top of
all documents.
Evaluation
Each of the copy functions is worth 2 marks; 1 mark for each question.
Page 3
CMPT 145
Principles of Computer Science
Question 2 (10 points):
Purpose: To work with the concept of references a bit more carefully. To debug a program that uses
references incorrectly.
Degree of Diculty: Moderate. If you understand references very well, this is not dicult.
In the le named histogram-broken.py, you’ll nd code to display a primitive histogram on the console
based on integers typed on the console by the user. Here is an example of how it might work:
How many data values? 10
Enter integer data
1 2 3 4 5 4 3 2 3 4
———– Histogram —————-
1: *
2: **
3: ***
4: ***
5: *
The Histogram is supposed to print one * for each occurrence of the value in the input. Histograms are
useful for visualizing the distribution of data. For example, from this histogram, we can see that 3 and 4
were the most common values in the input (there were three of each).
Unfortunately, the implementation in the le we’ve provided has several errors, as if written by a novice
programmer who didn’t take the time to design the algorithm carefully, and so did not quite have enough
time to nish.
Your task is as follows:
1. Study the functions carefully. While there are some errors, there are parts of the functions that are
very close to working. The purpose of this question is not to get you to invent anything tricky, but to
get you to practice debugging.
2. Figure out what each part of the functions is supposed to do. The programmer left no helpful comments to assist your study. Sorry.
3. Add doc-strings appropriate to the functions. You might have to modify the doc-strings if you change
the functions.
4. Fix the obvious errors, and then try to get the implementation working by debugging and testing.
5. Make a list briey mentioning every change you make to the program. For example you might have a
list that starts like this:
1. Deleted line 7 of the original program.
2. Changed function isNegative() to return a Boolean value instead of a string.
Hints: We created these functions by starting functions that worked perfectly, and then we added errors
to it, similar to the errors novices might make. Most of the errors are related to the material we’ve covered
in lecture so far, including review and the unit on references. Careful study should be enough to x this
program. You won’t have to add much code at all. Just x what’s there.
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What to hand in:
• The list of changes you made to the program, in the text le called a1_reflections.txt (PDF, rtf, docx
or doc are acceptable). This is the same document you used earlier. Please help the marker by clearly
indicating the question number.
• Hand in your working program in a le called a1q2.py.
• A text-document called a1q2_demo.txt that shows your program working on a few examples. You
may copy/paste to a text document from the console.
• If you wrote a test script, but this is not necessary, hand that in too, calling it a1q2_testing.py
Evaluation
• 4 marks. Your list of changes includes the bugs we know about in the original program.
• 3 marks. Your program works.
• 3 marks. Your functions have appropriate doc-strings, and other appropriate documentation.
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Question 3 (10 points):
Purpose: To build a slightly more complex program and test it. To get warmed up with Python, in case
you are using it for the rst time.
Degree of Diculty: Moderate. There many ways to complete this problem, but don’t leave this to the last
minute. The basic functionality is easy. There are aspects of the problem that you won’t appreciate
until you start testing.
A mathematician named Blaise Pascal has been accused of murder, and Phoenix Wright, ace attorney,
has taken the case as the defence attorney. A piece of paper with what appears to be a Pascal’s Triangle
written on it was found at the crime scene. If the paper does indeed have a correct Pascal Triangle on it
(EvidenceTriangle.txt from moodle), then the defendent will be found guilty (Prosecutor: 1. Only Pascal
could successfully write such a large Pascal’s triangle. 2. The triangle is named after Pascal, which is pretty
incriminating if you ask me). However if the found Pascal’s Triangle turns out to be incorrectly constructed,
Pascal will be found innocent (Phoenix: Pascal would NEVER make a mistake creating his triangle!). Phoenix
isn’t great at math, and needs your help for this case!
A Pascal’s triangle is a triangular array of numbers. Typically, the top row is row 0, and for each row there
are (row number) + 1 numbers. Each number is the sum of the number above it and to the left, and the
number above it and to right. The top row is special, and contains only a 1. See https://en.wikipedia.
org/wiki/Pascal%27s_triangle for more information.
In this question you will implement a program that checks whether a series of numbers constitutes of a
Pascal’s Triangle or not. Your program should work by reading input from the console, and sending an
answer to the console, as in the following example:
• It reads a number N on a line by itself, where N > 0. This will be the number of rows.
• It reads N lines of numbers, with each line needing (row number) + 1 positive numbers (Remember row
numbers start at 0).
• It checks whether the sequence of numbers is a Pascal Triangle or not. Your program should display
the message “yes” if it satises the above criteria, or “no” if it does not.
4
1
1 1
1 2 1
1 3 3 1
5
1
1 1
1 2 1
1 4 3 1
1 4 6 4 1
The example above shows input entered by the user, with each newline being a new row in the table. The
input from the left table should output yes whereas from the right should output a no, as on row 3 there’s
a 4 instead of a 3 (remember rows start at 0).
There are many ways to solve this problem. You are responsible for the design of your program.
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Note: You may complete this question using any method you see t, but you will have to reect on your
method in the next question.
Note: One option is to use an array of arrays, or list of lists. Before accessing a list or array, you may need
to check that the index you are using is valid and within your bounds.
What to Hand In
• Your implementation of the program: a1q3.py.
• A text document called a1q3_demo.txt, showing at least six (6) demonstrations of your program working on 3 dierently sized examples that are true Pascal Triangles, and 3 dierently sized examples that
are not Pascal Triangles. This is a demonstration, not testing. Finally, at the end of a1q3_demo.txt show
the input and output of running the numbers from EvidenceTriangle.txt from moodle, and declare
whether or not the defendant is GUILTY or NOT GUILTY! Note, depending on the text editor/line endings, you may be able to copy paste the input from EvidenceTriangle.txt into your command line.
• If you wrote a test script, hand that in too, calling it a1q3_testing.py.
Be sure to include your name, NSID, student number, course number and laboratory section at the top of
all documents.
Evaluation
• 5 marks: Your program works.
• 1 marks: Your program outputs the correct answer when fed in the input from EvidenceTriangle.txt
(found on moodle).
• 4 marks: Your program is well-documented.
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Question 4 (10 points):
Purpose: To reect on the work of programming for Question 3. To practice objectively assessing the
quality of the software you write. To practice visualizing improvements, without implementing improvements.
Degree of Diculty: Easy.
Answer the following questions about your experience implementing the program in Question 3. You may
use point form, and informal language. Just comment on your perceptions; you do not have to give really
deep answers. Be brief. These are not deep questions; a couple of sentences or so ought to do it.
1. (2 marks) Comment on your program’s correctness. How condent are you that your program (or the
functions that you completed) is correct? Would it work on extremely large Pascal Triangles, or would
it fail due to hard-coding?
2. (2 marks) Comment on your program’s eciency. How condent are you that your program is reasonably ecient?
3. (2 marks) Comment on your program’s reusability. For example, if another student from the class
read your code, would s/he understand it in a reasonable time? Have you organized your code into
functions?
4. (2 marks) Comment on your program’s robustness. Can you identify places where your program might
behave badly, even though you’ve done your best to make it correct? You do not have to x anything
you mention here.
5. (2 marks) How much time did you spend writing your program? Did it take longer or shorter than you
expected? If anything surprised you about this task, explain why it surprised you.
You are not being asked to defend your program as being good in all these considerations. For example,
if your program is not very robust, you can say that; you don’t need to make it robust.
What to Hand In
Your answers to the above questions in the text le called a1_reflections.txt (PDF, rtf, docx or doc are
acceptable). This is the same document you used for Q1 and Q2. Please help the marker by clearly
indicating the question number.
Be sure to include your name, NSID, student number, course number and laboratory section at the top of
all documents.
Evaluation
Each answer is worth 2 marks. Full marks will be given for any answer that demonstrates thoughtful re-
ection. Grammar and spelling wont be graded, but practice your professional-level writing skills anyway.
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