Lab 00: Getting Started

Files: lab00.zip

Starter Files

Download lab00.zip. Inside the archive, you will find starter files for the questions in this lab, along with a copy of the Ok autograder.

Introduction

This lab explains how to use your own computer to complete assignments for CS 111 and introduces some of the basics of Python.

If you need any help at any time through the lab, please feel free to come to office hours or post on the course Discord.

This lab looks really long, but it's mostly setup and learning how to use the essential tools for this class; these may seem a bit difficult now, but will quickly become second nature as we move further into the course.

Setup

The terminal

The terminal is a program that allows you to interact with your computer by entering commands. No matter what operating system you use (Windows, macOS, Linux), the terminal will be an essential tool for CS 111.

macOS/Linux

If you're on a Mac or are using a form of Linux (such as Ubuntu), you already have a program called Terminal or something similar on your computer. Open that up and you should be good to go.

Windows

For the purposes of CS 111, you should use Windows PowerShell as your terminal. PowerShell comes pre-installed on Windows and requires no extra setup. You can simply launch it from the Start menu. Most of the Bash commands covered in this course will work in PowerShell; when an alternate command is needed (such as to unzip a compressed folder), the lab instructions will give both the Bash command and the PowerShell command.

If you have difficulties with copy/paste in PowerShell, you can install Windows Terminal from the Microsoft Store using the instructions below.

Install Python 3

Python 3 is the primary programming language used in this course. Use the instructions below to install the Python 3 interpreter. (The instructions may feature older versions of Python 3, but the steps are similar.)

Important: If you already have an older version of Python installed, please make sure to download and install Python 3.9. You can check your Python version by running the command python ––version in the terminal.

Linux

Open the terminal and run the Python installation command for your Linux distribution. If your distribution isn't listed below, consult the internet to find the proper command.

Ubuntu 20.04+ or Debian 11+

sudo apt install python3 python-is-python3

Arch Linux

pacman -S python

macOS

Download the macOS 64-bit installer by selecting the downloadable link under Stable Releases at the page below. https://www.python.org/downloads/macos/

You may need to right-click the download icon and select "Open". After installing, please close and open your Terminal.

Windows

Download the Windows installer (64-bit) by selecting the downloadable link under Stable Releases at the page below. https://www.python.org/downloads/windows/

Run the installer and make sure to check the "Add Python 3.x to PATH" box, which will allow you to execute the python command from your terminal.

After installing, please close and open your terminal. Try running the python command. If this doesn't work, try python3 or py. Windows may use any of these to refer to Python, so you should use the one that works on your machine.

Other

Download Python for your operating system from the download page.

Install a text editor

The Python interpreter that you just installed allows you to run Python code. You will also need a text editor, where you will write Python code.

There are many text editors out there, each with its own set of features. Visual Studio Code (VS Code) is the most popular choice among the staff for this course for writing Python. If you are not already familiar with another editor, this is the one we recommend using for this course. Some other editors that are used among staff are listed below as well.

Students who took CS 110 may already have experience writing Python code in PyCharm. PyCharm will work just as well as VS Code for the purposes of this course.

We highly recommend for this class you use either VS Code or PyCharm.

Warning: Word processors such as Microsoft Word should not be used to edit programs. Word processors can add extra content to documents that will confuse the interpeter. You should never use Microsoft Word or a similar word processor to edit programs for any CS course.

Checkpoint

By this point in the lab, you should have installed a Python interpreter and a text editor. If you had difficulty following any of the steps above, stop here and ask for help before going on.

If your difficulties are caused by your hardware (e.g. you have an iPad or some other device that does not allow you to download software), you should talk to your TA or a professor about your options so that you do not fall behind in the course while you acquire a suitable device.

If you do not want to invest in a laptop right now (e.g. you are planning on serving a mission and getting a laptop when you return, you are not majoring in CS and you do not need a laptop for your other courses, or you do not have the means to get a laptop this semester), BYU has a laptop rental program for students.

Using the terminal

Let's check if everything was installed properly!

First, open a terminal window. If you're on Windows, launch PowerShell from the Start menu. If you're on macOS or Linux, open the Terminal.

We will be using zsh on Linux, but these commands should work on any of these platforms.

When you first open your terminal, you will start in the "home directory". The home directory is represented by the ~ symbol, which you might see at the prompt.

Don't worry if your terminal window doesn't look exactly the same. The important part is that the prompt shows $ (indicating Bash), % (indicating zsh), or PS (PowerShell). If you see C:\Users\Oski> but no PS before it, you're in the Command Prompt! Do not use that! Close it and launch PowerShell instead.

Try running echo "$HOME". Verify that it displays the path to your home directory.

Python Interpreter

We can use the terminal to check if your Python 3 interpreter was installed correctly. Try the following command:

python

If the installation worked, you should see some text printed out about the interpreter followed by >>> on its own line.

Python 3.10.9 (main, Dec 19 2022, 17:35:49) [GCC 12.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>

This is where you can type in Python code. Try typing some expressions you saw in lecture, or just play around to see what happens! You can type exit() or Ctrl-D to return to your command line.

Important: If the output begins with Python 2.7.18, you either installed the wrong version of Python, or you need to use the python3 command instead.

Organizing your files

In this section, you will learn how to manage files using terminal commands.

Make sure your prompt contains a $ somewhere in it and does not begin with >>>. If it begins with >>> you are still in a Python shell, and you need to exit. See above for how.

Directories

The first command you'll use is ls. Try typing it in the terminal:

ls

The ls command lists all the files and folders in the current directory. A directory is another name for a folder (such as the Documents folder). Since you're in the home directory right now, you should see the contents of your home directory.

If ls doesn't work, but dir does: stop! You've mistakenly opened the Windows Command Prompt! Exit it, and run PowerShell instead.

Changing directories

To change directories, use the cd command. Let's try moving into your Desktop directory. First, make sure you're in your home directory (check for the ~ on your command line). If you are not in your home directory, type cd ~ to move into the home directory. Use ls to see if the Desktop directory is present. Try typing the following command into your terminal, which should move you into that directory:

cd Desktop

On some Windows accounts, your actual Desktop folder might be inside OneDrive:

cd OneDrive/Desktop

If you still can't find your Desktop directory, ask for help on Discord or in office hours.

There are a few ways to return to the home directory:

• cd .. (two dots). The .. means "the parent directory". In this case, the parent directory of Desktop is your home directory, so you can use cd .. to go up one directory.
• cd ~ (the tilde). Remember that ~ means home directory, so this command will always change to your home directory.
• cd (cd on its own). Typing just cd is a shortcut for typing cd ~.

You do not have to keep your files on your Desktop if you prefer otherwise. Where you keep your files locally will not affect your grade. Do whatever is easiest and most convenient for you!

Windows PowerShell tip: If you're using PowerShell, you can start it directly by holding Shift, right-clicking inside any folder, and selecting "Open PowerShell Window here". It will automatically cd to that folder, so you'll have to run cd ~ to change to your home directory.

Making new directories

The next command is called mkdir, which makes new directories. Let's make a directory called cs111 on your Desktop to store all of the assignments for this class:

mkdir cs111

A folder named cs111 will appear on your Desktop. You can verify this by using the ls command again or by simply checking your Desktop.

At this point, let's create some more directories. First, we will move to the ~/Desktop/cs111 directory.

cd cs111

Then, let's create folders called proj, lab, and hw inside of the cs111 folder:

cd ~/Desktop/cs111
mkdir proj
mkdir lab
mkdir hw

Now we can list the contents of the directory (using ls), and we will see two folders, proj and lab.

Downloading the assignment

If you haven't already, download the zip archive, lab00.zip, which contains all the files that you'll need for this lab. Once you've done that, let's find the downloaded file. On most computers, lab00.zip is probably located in a directory called Downloads in your home directory. Use the ls command to check:

ls ~/Downloads

If you don't see lab00.zip, ask for help on Discord or in office hours. On some versions of Safari the file may get unzipped for you, in which case you would just see a new directory named lab00.

Extracting starter files

You must expand the zip archive before you can work on the lab files. Different operating systems and different browsers have different ways of unzipping. If the instructions below don't work for you, you can search online or ask a TA.

You can use the terminal to unzip the zip file from the command line. First, cd into the directory that contains the zip file:

cd ~/Downloads

If you are not using PowerShell, you can run the unzip command with the name of the zip file:

unzip lab00.zip

If you are using PowerShell (in Windows), you can run:

Expand-Archive -DestinationPath . -Force lab00.zip

You might also be able to extract files without using the terminal by double clicking (or right-clicking) them in your OS's file explorer.

Once you unzip lab00.zip, you'll have a new folder called lab00 which contains the following files (check it out with cd and ls):

• lab00.py: The template file you'll be adding your code to
• ok: A program used to test assignments
• lab00.ok: A configuration file for ok

Moving files

Move the lab files to the lab folder you created earlier:

mv ~/Downloads/lab00 ~/Desktop/cs111/lab

The mv command will move the ~/Downloads/lab00 folder into the ~/Desktop/cs111/lab folder.

Now, go to the lab00 folder that you just moved. Try using cd to navigate your own way! If you get stuck, you can use the following command:

cd ~/Desktop/cs111/lab/lab00

Summary

Here is a summary of the commands we just went over for your reference:

• ls: lists all files in the current directory
• cd <path to directory>: change into the specified directory
• mkdir <directory name>: make a new directory with the given name
• mv <source path> <destination path>: move the file at the given source to the given destination

Finally, you're ready to start editing the lab files! Don't worry if this seems complicated—it will get much easier over time. Just keep practicing! You can also take a look at our UNIX tutorial for a more detailed explanation of terminal commands.

Python Basics

Expressions and statements

Programs are made up of expressions and statements. An expression is a piece of code that evaluates to some value and a statement is one or more lines of code that make something happen in a program.

When you enter a Python expression into the interactive Python interpreter, its value will be displayed. As you read through the following examples, try out some similar expressions on your own Python interpreter, which you can start up by typing this in your terminal:

python3

Remember, if you are using Windows and the python3 command doesn't work, try using python or py. See the install Python 3 section for more info and ask for help if you get stuck!

You'll be learning various types of expressions and statements in this course. For now, let's take a look at the ones you'll need to complete this lab.

Primitive expressions

Primitive expressions only take one step to evaluate. These include numbers and booleans, which just evaluate to themselves.

>>> 3
3
>>> 12.5
12.5
>>> True
True

Arithmetic expressions

Numbers may be combined with mathematical operators to form compound expressions. In addition to the + operator (addition), the - operator (subtraction), the * operator (multiplication) and the ** operator (exponentiation), there are three division-like operators to remember:

• Floating point division (/): divides the first number number by the second, evaluating to a number with a decimal point even if the numbers divide evenly.
• Floor division (//): divides the first number by the second and then rounds down, evaluating to an integer.
• Modulo (%): evaluates to the positive remainder left over from division.

Parentheses may be used to group subexpressions together; the entire expression is evaluated in PEMDAS (Parentheses, Exponentiation, Multiplication / Division, Addition / Subtraction) order.

>>> 7 / 4
1.75
>>> (2 + 6) / 4
2.0
>>> 7 // 4        # Floor division (rounding down)
1
>>> 7 % 4         # Modulus (remainder of 7 // 4)
3

Assignment statements

An assignment statement consists of a name and an expression. It changes the state of the program by evaluating the expression to the right of the = sign and binding its value to the name on the left.

>>> a = (100 + 50) // 2

Now, if we evaluate a, the interpreter will display the value 75.

>>> a
75

Doing the assignment

Unlocking tests

One component of lab assignments is to predict how the Python interpreter will behave.

Enter the following in your terminal to begin this section:

python3 ok -q python-basics -u

You will be prompted to enter the output of various statements/expressions. You must enter them correctly to move on, but there is no penalty for incorrect answers.

>>> 10 + 2
______

>>> 7 / 2
______

>>> 7 // 2
______

>>> 7 % 2  # 7 modulo 2, the remainder when dividing 7 by 2.
______

>>> x = 20
>>> x + 2
______

>>> x
______

>>> y = 5
>>> y = y + 3
>>> y * 2
______

>>> y = y // 4
>>> y + x
______

Understanding problems

Another component of lab assignments is writing functions. Open up lab00.py in your text editor. You can type open . on MacOS or start . on Windows to open the current directory in your Finder/File Explorer. Then double click or right click to open the file in your text editor. You should see something like this:

def eighteen_seventy_five():
    """Come up with the most creative expression that evaluates to 1875,
    using only numbers and the +, *, and - operators.

    >>> eighteen_seventy_five()
    1875
    """
    return ______

The lines in the triple-quotes """ are called a docstring, which is a description of what the function is supposed to do. When writing code in CS 111, you should always read the docstring!

The lines that begin with >>> are called doctests. Recall that when using the Python interpreter, you write Python expressions next to >>> and the output is printed below that line. Doctests explain what the function does by showing actual Python code. It answers the question: "If we input this Python code, what should the expected output be?"

In eighteen_seventy_five,

• The docstring tells you to "come up with the most creative expression that evaluates to 1875," but that you can only use numbers and arithmetic operators + (add), * (multiply), and - (subtract).
• The doctest checks that the function call eighteen_seventy_five() should return the number 1875.

You should not modify the docstring, unless you want to add your own tests! The only part of your assignments that you'll need to edit is the code unless otherwise specified.

Writing code

Once you understand what the question is asking, you're ready to start writing code! You should replace the underscores in return ______ with an expression that evaluates to 1875. What's the most creative expression you can come up with?

Don't forget to save your assignment after you edit it! In most text editors, you can save by navigating to File > Save or by pressing Command-S on macOS or Ctrl-S on Windows.

Running tests

In CS 111, we will use a program called ok to test our code. ok will be included in every assignment in this class.

For quickly generating ok commands, you can now use the ok command generator.

Back to the terminal—make sure you are in the lab00 directory we created earlier (remember, the cd command lets you change directories).

In that directory, you can type ls to verify that there are the following three files:

• lab00.py: the starter file you just edited
• ok: our testing program
• lab00.ok: a configuration file for Ok

Now, let's test our code to make sure it works. You can run ok with this command:

python3 ok

Remember, if you are using Windows and the python3 command doesn't work, try using just python or py. See the the install Python 3 section for more info and ask for help if you get stuck!

If you wrote your code correctly and you finished unlocking your tests, you should see a successful test:

=====================================================================
Assignment: Lab 0
Ok, version v1.18.1
=====================================================================

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Running tests

---------------------------------------------------------------------
Test summary
    3 test cases passed! No cases failed.

If you didn't pass the tests, ok will instead show you something like this:

---------------------------------------------------------------------
Doctests for eighteen_seventy_five

>>> from lab00 import *
>>> eighteen_seventy_five()
1910

# Error: expected
#     1875
# but got
#     1910

---------------------------------------------------------------------
Test summary
    0 test cases passed before encountering first failed test case

Fix your code in your text editor until the test passes.

While ok is the primary assignment "autograder" in CS 111, you may find it useful at times to write some of your own tests in the form of doctests. Then, you can try them out using the -m doctest option for Python).

Submitting the assignment

Now that you have completed your first CS 111 assignment, it's time to turn it in. You can follow these next steps to submit your work and get points.

Step 1: Submit in Canvas

In your browser, navigate to the CS 111 course page in Canvas. In the menu on the left side of the page, select the "Assignments" tab. This should take you to a list of assignments.

Select "Lab 00" from the assignment list. This will take you to the Lab 00 assignment page. The instructions at the top of the assignment page will tell you what file(s) to submit for the assignment. Beneath those instructions, you should see the Gradescope window.

Gradescope is an autograding tool that you will use throughout the semester to turn in your assignments. If you do not already have a Gradescope account, you should create one now. If you already have a Gradescope account, and if it is set up correctly, it should link automatically with this course. If you have issues with Gradescope, talk to your TA.

Important: When you complete Gradescope setup, you must enter your 9-digit BYU ID number (e.g. 01-234-5678) in the Student ID field. If you use your netid instead, your grades will not link correctly with your Canvas account.

Your Gradescope email address should be [your-netid]@byu.edu; for more information about email services available to BYU students, see this link.

Once your Gradescope account setup is complete, submit the lab00.py file to Gradescope in the window on the assignment page.

Step 2: Verify your submission

Gradescope will automatically run each function that you write against one or more test cases and give you a score. In most cases, this will match the score shown when you ran the test cases using ok in the terminal. If the score displayed by Gradescope does not match what you expected, talk to a TA about troubleshooting options.

Congratulations, you just submitted your first CS 111 assignment!

Appendix: Useful Python command line options

When running a Python file, you can use options on the command line to inspect your code further. Here are a few that will come in handy. If you want to learn more about other Python command-line options, take a look at the documentation.

• Using no command-line options will run the code in the file you provide and return you to the command line. For example, if we want to run lab00.py this way, we would write in the terminal:

  python3 lab00.py

• -i: The -i option runs your Python script, then opens an interactive session. In an interactive session, you run Python code line by line and get immediate feedback instead of running an entire file all at once. To exit, type exit() into the interpreter prompt. You can also use the keyboard shortcut Ctrl-D on Linux/Mac machines or Ctrl-Z Enter on Windows.

  If you edit the Python file while running it interactively, you will need to exit and restart the interpreter in order for those changes to take effect.

  Here's how we can run lab00.py interactively:

  python3 -i lab00.py

• -m doctest: Runs doctests in a particular file. Doctests are surrounded by triple quotes (""") within functions.

  Each test in the file consists of >>> followed by some Python code and the expected output (though the >>> are not seen in the output of the doctest command).

  To run doctests for lab00.py, we can run:

  python3 -m doctest lab00.py