Lab 2: Linux, Type Conversions, Operators

CSCI 141
Lab 2: Linux, Type Conversions, Operators and Operands, Print
Formatting, Errors
Lab and Homework Assignments
This lab introduces you to the Linux Operating System, and explores errors, printing and
operands. You should have enough time to complete this lab during the lab session. Upload
your submission to Canvas before the deadline. If you have questions, be sure to ask the TA.
Ask questions often. Labs are your opportunity to get personalized help!
This lab is to be done on the Linux operating system. You must use Linux to complete
this lab – basic knowledge of Linux is one of the course outcomes. For the third lab
onward, and for all assignments, you have the option to use either Windows or Linux.
1 Logging into Linux
Linux is an operating system with much of the same functionality as the Windows or Mac
operating systems, but with a slightly different feel. The lab computers are dual boot computers,
and when the computer boots up you can specify which of the operating systems (Windows or
Ubuntu, which is a flavor of Linux) you want to use.
To log into your CS Linux account, reboot your CS computer; when a menu appears, use the
arrow keys to select Ubuntu from the set of options, and press enter to select it. The login
window should look similar to what is shown in Figure 1. You can click on the circular gnome
icon to the right of ”Login” to view a menu where you can customize the look and feel of your
Linux session. For now, we’ll assume that you’ve left it on Ubuntu (Default). Provide your CS
account username and password (the same credentials as you used for your Windows account
in lab 1) to log in.
Once logged in, the overall look and feel of your desktop will vary depending on which environment you have selected. To access system settings for your desktop environment, click on
the power icon in the upper right hand corner and select System Settings... To find Thonny,
click the Ubuntu icon (the top-most icon in the bar on the left side of the screen. If Thonny
doesn’t appear among the applications there, you can type Thonny in the search bar and it
should appear. Once Thonny starts, it should look very similar to the Windows version you
used in last week’s lab.
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Figure 1: A typical linux Login prompt and desktop environment selector
2 Linux Command Line Basics
Windows, Mac OS, and Linux all provide graphical interfaces such as those you’re used to using,
that allow you to open programs and accomplish tasks using the mouse, icons, windows, and
so on. All of these operating systems also provide another way of interacting with them, called
a Command Line Interface (CLI). Although there is a steeper learning curve for knowing how
to accomplish things using the command line, for certain tasks it can be more powerful once
you know how to use it.
In this lab, you will learn the very basic elements of how to interact with the Linux command
line and learn how to run Python code without the help of an IDE such as Thonny. What you
will learn here is only a tiny fraction of the commands available; you can find a helpful ”cheat
sheet” of many commonly used Linux commands on the course website1
i you want to learn
more.
1. Begin by opening a command line window (also called a Terminal). Click on the Ubuntu
icon in the upper left corner and type terminal to initiate a search; click on the Terminal
icon from the results to launch a new terminal window.
2. In the terminal, the $ sign and the space to the right of it where you type is called the
command line. Commands that you issue are interpreted by a program called a shell (the
default shell, or command line language, in the lab machines is bash). It is one of the
many shells available in Linux.
3. You’ll notice that the $ is prepended with your username and an @ followed by the name
of the computer that you are logged into. For example, wehrwes@linux-11:~$ specifies
the user wehrwes logged into the linux-11 machine.
4. All of the things that you can do with the mouse when interacting with a windows
environment you can also accomplish via the command line. In the following steps you
will create a new directory (folder), navigate into that folder, and run python from the
command line. For these instructions, code and/or sample file content or output are
1Direct link: https://facultyweb.cs.wwu.edu/~wehrwes/courses/csci141_19s/labs/linuxref.pdf
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shown in boxes. Type commands EXACTLY as they provided, and press return/enter to
issue the command. For example:
whoami
is instructing you to type the command whoami on the command line and execute it by
pressing return/enter. Try it out. What does the whoami command do?
5. Commands can take arguments, similarly to how functions in Python take arguments,
except here they are not surrounded by parentheses. To create a directory, use the mkdir
command with a single argument that is the name of the directory (folder) that you want
to make. Create the directory Lab2.
mkdir Lab2
6. To confirm that you have made your directory, issue the ls command, which will list all
contents of the directory where you are currently in.
ls
You should see a list with multiple items, which are the files and/or directories in your
account. If done correctly, Lab2 should be listed.
7. In graphical interface, you would double click on a folder to access that folder. In Linux,
you open a directory using the command cd, for change directory. Enter the Lab2 directory.
cd Lab2
8. Launch Thonny via the command line :
thonny &
The & is important: this allows you to continue using the terminal window AFTER you
launch Thonny. Use Thonny to create a new file, helloWorld.py, with a single print
statement : print("hello world"). Save the file in your Lab2 folder. Return to the
terminal, and again issue the ls command, and you should see the just-created file listed.
9. Just as you can run a Python program using Thonny by pressing the green Run button,
you can also run a program from the command line. In the terminal window (make sure
you are in your Lab2 folder, which contains your Python program), run the helloWorld
program by invoking the python3 interpreter :
python3 helloWorld.py
You should see the output of your program printed to the terminal console line.
For this lab, you must invoke the helloWorld.py program from the command
line. To provide proof that you have done so, either upload a screenshot
showing that you invoked helloWorld from the command line (use the screenshot tool available on Linux), or show your TA how you execute the program
helloWorld from the command line so they can give you credit on Canvas for
those points right away.
For the remaining sections of this lab, you can run python either via Thonny or via the
command line.
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3 Errors, comments, and the sep argument of print
Inevitably, you will write code that will have bugs, or errors, no matter how experienced of a
programmer you might be. Knowing how to find and fix bugs is a critical skill for all programmers.
Using a browser such as Firefox, go to the course website:
https://facultyweb.cs.wwu.edu/~wehrwes/courses/csci141_19s/#schedule)
download the file badCode.py, and save it in your Lab2 folder. The contents of that file are
shown in the following figure.
Open the file badCode.py in Thonny. Read over the code, paying particular attention to the
comments. Proper commenting is crucial to writing good code, so that it is easy to read by you
and others. In python all comments begin with the # character. As a general rule:
• Include your name (the author) and a short description at the top of each .py file
• For every few lines of code, include a descriptive comment
• Use white space (blank lines) to chunk your code into logical sections
For all the labs and assignments in this course, commenting is a portion of the rubric. Get into
the habit now of commenting your code well!
Notice that this code contains a brand new way of using print. Adding sep="" (short for
“separator”) as the last argument to the print function will prevent print from adding spaces
between items that you want printed. If you use sep="", the only spaces that are included in
the output are the spaces that you place explicitly into the Strings. For example:
print("Taking", "CSCI", 141, "woo-hoo!")
would print the following:
Taking CSCI 141 woo-hoo!
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but
print("Taking", "CSCI", 141, "woo-hoo!", sep="")
would print the following:
TakingCSCI141woo-hoo!
In general, the print function defaults to " " as the separator between the arguments it prints.
If you want a different separator, you can give any string to the sep argument, such as the empty
string (sep="") or a comma (sep=","), or any other string you’d like (e.g., sep=", uhh, ").
Try out a few calls to print with different separators in the interactive Shell pane in Thonny.
badCode.py has a single intentional error. Run the code to see the error (following Figure).
Look closely at the error message. On what line number is the error? Fix the error. Hint: Refer
to the lecture slides for fixing an error that uses the conversion function int. Be sure that
you modify ONLY the line of code that says numReindeer = int(approxNumReindeer).
You may not modify any other line of code, nor add nor delete any line of code.
4 Additional errors
Download faultyCode.py from the course webpage and save it in your Lab2 folder. The contents
of that file are shown in Figure 2. Take a close look at the code. Notice again the good
commenting. The program faultyCode.py has intentional MULTIPLE errors. Run the code
(either using Thonny or via the command line), and look closely at the error message.
Fix the errors in faultyCode.py. Go back and forth between fixing an error and trying to run
the program. Ultimately, the goal is that the output of the program is as shown in Figure 3.
5 Operands and Operators
In lecture we have discussed operands and operators. Make sure that you know the definition
of both. If you do not recall, review the lecture slides.
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Figure 2: contents of faultyCode.py
Figure 3: Output of the corrected faultyCode.py program.
In this section, you’ll solve the following programming problem: The period key on your
keyboard is broken, but you would like to want to multiply two numbers with a
decimal digit.
Let’s look at an example. Suppose you want to calculate 20.4 × 17.7, but can only enter 204
and 177. The desired result is 361.08. If the user inputs the values 204 and 177, how can
you convert them to 20.4 and 17.7? By using the modular and integer division operators! For
example, 204 modulus 10 has a remainder of 4, which gives the decimal value .4, while 204
integer division 10 gives 20, which is the number before the decimal in 20.4.
The Math: Suppose that for the input values 204 and 177 you have successfully extracted
the whole and decimal values (i.e., 20, 4, 17 and 7). How can you calculate the result of 20.4 x
17.7? Multiplication of decimal values requires you to sum the following four parts:
• The first integer times the second integer
• The first integer times the second decimal
• The first decimal times the second integer
• The first decimal times the second decimal
Notice that for each decimal, we also multiply in a factor of 0.1 to make sure that it is correctly
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weighted in the final product. In our example, the calculation looks like this:
20.4 ∗ 17.7 = (20 ∗ 17)
+ (20 ∗ 7 ∗ 0.1)
+ (4 ∗ 0.1 ∗ 17)
+ (4 ∗ 0.1 ∗ 7 ∗ 0.1)
= 340 + 14 + 6.8 + 0.28
= 361.08
1. Download brokenCalculator.py from the course webpage and save it to your Lab2 folder.
2. That file is incomplete. Only two lines of code have been written, which you are not
allowed to edit. The rest are comments. Lines of code that say # COMPLETE THE CODE
you will need to write. Read the comments for each section to get a sense of what code
you need to write. Also, the number of COMPLETE THE CODE comments in that file
is how many lines of python code I (Scott) wrote in my solution. It’s okay if your solution
uses fewer lines of code, or more, but each block of code should accomplish what the
comment above it specifies.
For the lines of code that you write, you are only allowed to use the print
function, the assignment operator, and the following mathematical operators:
• //
• %
• *
• +
For the lines of code that you write, you cannot use float(), nor int(), nor /.
3. There are 7 parts to the code, labeled A through G. Here are hints for each of them:
• A: This requires a single use of the print function
• B: Use only the // and % operators. Follow the logic in the description above
• C: This requires a single use of the print function
• D: Do the same for the second integer as you did for the first integer (step B). Use
only the // and %operators
• E: The same as step C, but for the second integer
• F: Use the Hint above for explanation on how to do this
• G: This requires a single use of the print function.
In your python code, you CAN make use of periods, but when the program is
RUN, the user CAN ONLY enter integer (non decimal) numbers.
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A sample output for the completed code is shown in the following figure:
Submission
Either show your working code to your TA, who will post your lab grade scores right away, else
upload the following files:
• Screenshot (.png, or .jpg file) showing you invoking your helloWorld program from the
command line
• Your fixed faultyCode.py code file
• The completed brokenCalculator.py file that can reproduce the output shown in the above
figure
Rubric
Screenshot showing execution of helloWorld from the linux command line 5 points
faultyCode.py has been fixed, and is properly commented 3
brokenCalculator.py uses only %, //, + and ∗ operators for the lines of code
that you have written
3
brokenCalculator.py has no syntax errors 3
brokenCalculator.py produces the correct output 3
brokenCalculator.py is properly commented 3
Total 20 points
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