Lab 0 Solutions

Solution Files

Introduction

This lab explains how to use your own computer to complete assignments for CS 61A 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 piazza.

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

Register for an account

These accounts allow you to use instructional machines in the CS department, which can be useful if you do not have regular access to a computer. They are not required if you do not plan on remotely accessing the lab computers or printers. If you have a device to save your CS 61A folder, then you do not have to set up an account.

Go to the EECS account site to register for an instructional account. Login using your Berkeley CalNet ID and click the Get a new account button in the row for CS 61A. Your username will be of the form cs61a-xxx. Write down or download your account form so you don't forget it!


Logging into your class account

From your laptop

Most of the work in this class can be done without logging into your account. However, there may be times when you'll find working from an instructional account to be easier.

Let's log in now. Open up your terminal and type in the following command:

ssh cs61a-?@ashby.cs.berkeley.edu

where ? is replaced with the rest of your username.

If you're interested, here's an explanation of what the command does:

  1. ssh is a secure shell (i.e. terminal) that connects to other computers over a network.
  2. cs61a-? is the username on the remote computer.
  3. ashby.cs.berkeley.edu is the domain name of the remote computer. For our purposes it can be any of the servers that belong to Berkeley's CS department.

You can also watch this video for help.

The first time you attempt to ssh to a new server, the following message will appear:

The authenticity of host 'ashby.cs.berkeley.edu' can't be established.
RSA key fingerprint is ...
Are you sure you want to continue connecting (yes/no)?

Type yes. Your computer will remember the remote server, and won't ask you again.

Once you confirm, you will be prompted for your password. If you haven't changed your password yet, use the password you were assigned when you registered for your account.

When you type your password, nothing will show up! This is a security feature, not a bug. Continue typing and press enter to log in.


Changing your password

The temporary password is not the easiest thing to remember. While still logged in, you can change your password by running this command and following the directions on the screen.

ssh update

Registering your account

The first time you log in to your class account, your terminal may ask you some registration questions about the following:

  • Last name
  • First name
  • Student ID
  • Email (please use the same email as above!)
  • Code name (we don't use this information, you can enter anything you want)

If your terminal doesn't prompt you for this information the first time you log in, you can type register to begin the process. You don't need to do this again if you've already registered before.

If you find errors (e.g. you misspelled your name), fix them immediately by running the command:

re-register

Logging out

Once you've registered your account and changed your password, you can log out by pressing Ctrl-D, or with the command exit.

Install a 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 61A.

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

If you're using Windows 10 Update 1909 or later, you can skip this step and just use Windows PowerShell for the purposes of 61A. PowerShell comes pre-installed on Windows and requires no extra setup. Command like cd and ls will work as well (python will work after the setup), which encompasses most of the bash commands you need for this course.

For Windows users, we recommend a terminal called Git-Bash. You can try either method below:

  • Easy (automatic) method: Right-click here, click Save link as..., then save and run our automated installer, following the displayed instructions. (Advanced users: If double-clicking doesn't work, you can try cscript "Install-Python-on-Windows.jse" from the Command Prompt.) If this method succeeds, you can move on and install a text editor; you now run Python inside Git-Bash. (In fact, only inside Git-Bash. So this won't let you run .py files by double-clicking them. But you shouldn't need to do that for this course.) If this method fails, try the alternate method below.

    Anti-virus note: Some anti-viruses mistakenly flag our installer as a virus. We're aware of this, but there's little we can do to prevent it. In the case of Windows Defender, some students have reported success by clicking "See Details", letting it scan the file, and then rebooting. Otherwise, you can try to exclude/whitelist the file from scanning. If neither of these work for you or you aren't comfortable messing with your anti-virus software, you can just use the manual installation method below.

  • Alternate (manual) method: First, if you already tried the automatic installer above, make sure it's fully cleaned up:

    1. Look for Git as an installed program in "Add/Remove Programs", and, if it exists, uninstall it.
    2. Once Git is no longer installed, if a C:\Program Files\Git folder still exists, delete that too.

    Now download and install Git Bash. You can use the default options, with one exception: Select Use Windows' default console window in the Configuring the terminal emulator to use with Git Bash step. This is very important! If you do not select this option, your terminal won't work with Python! Git Bash configuration options

If you're already using Git-Bash from outside this course and reinstalling it isn't an option: Depending on whether you selected the MinTTY option when installing Git, it's possible that typing a command like python won't display anything on the screen. You can fix this by typing winpty python instead (or winpty python3, etc. as described below), but it will be painful, as you will have to remember to do this every time for the rest of the course! Hence, we recommend that you go back and reinstall Git-Bash with the recommended options if possible.

If everything succeeded, you are now able to launch a terminal on Windows by running Git-Bash.

SSL/TLS errors: If you ran into connection security errors, you may need to update your system and/or enable TLS 1.2 (e.g. see here for Windows 7). You can check your TLS version by installing Python first, and running the following in python3:

from json import loads; from urllib.request import urlopen;
loads(urlopen('https://www.howsmyssl.com/a/check').read().decode('UTF-8'))['tls_version']

If you don't see TLS 1.2 or later, that may be why you are encountering problems. If you're on Windows 10, though, the problem may be something else, and you may need to search/ask for help.

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.)

Linux

Run sudo apt install python3 (Ubuntu), sudo pacman -S python3 (Arch), or the command for your distro.

macOS

Download and install Python 3 (64-bit).

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

Windows

If you used our automated installer successfully, skip to the next section—you should already have Python.

If you're using Windows 10 Update 1909 or later, open the Microsoft Store and search for "python." Install Python 3.8 by the Python Software Foundation (this should be the first result). You can then skip the rest of this section. (Note: If you later decide to reinstall Python differently, uninstall it from the Microsoft Store first.)

Otherwise, if you're installing manually, download & run Python 3 (the Windows x86-64 executable 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.

Windows PATH at setup

Other

Download Python 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 editors out there, each with its own set of features. We find that Atom, Sublime Text 3, and Visual Studio Code are popular choices among students, but you are free to use other text editors.

Note: Please, please, please do not use word processors such as Microsoft Word to edit programs.

For your reference, we've also written some guides on using popular text editors. After you're done with lab, you can take a look if you're interested:

Using the terminal

Let's check if everything was installed properly!

First, open a terminal window. If you're on Windows, launch Git-Bash from the Start menu. (Or if you're using PowerShell, run that instead.)

starting the terminal

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) 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 Git-Bash or PowerShell instead. (Do not launch Git-CMD.)

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:

python3

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

Windows troubleshooting:

  • If the python3 command doesn't run at all: Try python, py, or py -3 instead.
  • If Python freezes (doesn't display anything at all): You probably didn't select the "Use Windows' default console window" option when installing Git-Bash manually. Try winpty python, or just uninstall Git-Bash and reinstall it with the correct options.
  • If you see WindowsApps/python: Permission denied: Open Settings (or right-click the Desktop & click Personalize), search for "App Execution Aliases" (or find it by clicking Home→Apps→App execution aliases), and disable the entries that say "python". (Screenshots here.) Alternatively, go to the WindowsApps folder whose path is shown and just rename python.exe and python3.exe to something else, or delete them entirely if you are able to. Then try again.
  • If Python doesn't run at all, and you used our automated installer: Go back and try installing using the manual method.
  • If Python doesn't run at all, and you installed manually: Make sure you set up your "PATH" correctly as shown above.
  • If you mixed multiple versions of Python (e.g. 32-bit and 64-bit, or 3.6 and 3.8, etc.): They may conflict. Occasionally, this becomes extremely difficult to fix—even for instructors. Uninstall them one-by-one (the most recent one first), then reinstall only the latest 64-bit version.
  • If Git Bash shows an empty black screen: You might have had Cygwin previously installed. You can try uninstalling Cygwin (recommended) and following this answer, possibly uninstalling Git-Bash and/or Python as well, and re-running the installer.
  • If you manually downloaded Python from its own home page and don't see 64 in the file name: You probably got the 32-bit version. It might work. But if it doesn't, uninstall it before trying another method. If you need to manually find the 64-bit version, look for a recent stable x86-64 executable installer.

Ask for help if you get stuck!

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 Git-Bash instead.

Changing directories

To move into another directory, 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) and 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

Although, on some Windows accounts, your actual Desktop folder might actually be inside OneDrive:

cd OneDrive/Desktop

If you still can't find your Desktop directory, ask for help on Piazza 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 cs61a 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!

Making new directories

The next command is called mkdir, which makes new directories. Let's make a directory called cs61a on your Desktop to store all of the assignments for this class:

mkdir cs61a

A folder named cs61a 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, make sure you are in the ~/Desktop/cs61a directory. Then, create folders called projects and lab inside of your cs61a folder:

cd ~/Desktop/cs61a
mkdir projects
mkdir lab

Now if you list the contents of the directory (using ls), you'll see two folders, projects and lab.

cs61a directory

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 Piazza or in office hours.

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 you don't know how, you can search online.

Using a terminal, you can unzip the zip file from the command line. First, cd into the directory that contains the zip file:

cd ~/Downloads

Now, run the unzip command with the name of the zip file:

unzip lab00.zip

You might also be able to unzip files without using the terminal by double 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 and submit 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/cs61a/lab

The mv command will move the ~/Downloads/lab00 folder into the ~/Desktop/cs61a/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/cs61a/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. In very simple terms, 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 type a Python expression into the Python interpreter, its value will be printed out. As you read through the following examples, try out some similar expressions in your own Python shell, 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 today's 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

Names are also primitive expressions. Names evaluate to the value that they are bound to in the current environment. One way to bind a name to a value is with an assignment statement.

Arithmetic expressions

Numbers may be combined with mathematical operators to form compound expressions. In addition to + 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 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 and binding its value to the name in the current frame.

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

Note that the statement itself doesn't evaluate to anything, because it's a statement and not an expression. Now, if we ask for a's value, the interpreter will look it up in the current environment and output its value.

>>> a
75

Note that the name a is bound to the value 75, not the expression (100 + 50) // 2. Names are bound to values, not expressions.

Doing the assignment

Unlocking tests

One component of lab assignments is unlocking tests. Their purpose is to test your understanding and make sure you have a good enough grasp on the topic to make progress on the assignment.

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.

The first time you run Ok, you will be prompted for your bCourses email. Please follow these directions. We use this information to associate your code with you when grading.

>>> 10 + 2
______
12
>>> 7 / 2
______
3.5
>>> 7 // 2
______
3
>>> 7 % 2 # 7 modulo 2, equivalent to the remainder of 7 // 2
______
1
>>> x = 20
>>> x + 2
______
22
>>> x
______
20
>>> y = 5 >>> y += 3 # Equivalent to y = y + 3 >>> y * 2
______
16
>>> y //= 4 # Equivalent to y = y // 4 >>> y + x
______
22

Understanding problems

Labs will also consist of function writing problems. 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 twenty_twenty():
    """Come up with the most creative expression that evaluates to 2020,
    using only numbers and the +, *, and - operators.

    >>> twenty_twenty()
    2020
    """
return (1 + 10 ** (1 + 1)) * ((1 + 1) ** (1 + 1 + 1) + 1 + 1) * (1 + 1)

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 61A, 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 twenty_twenty,

  • The docstring tells you to "come up with the most creative expression that evaluates to 2020," but that you can only use numbers and arithmetic operators + (add), * (multiply), and - (subtract).
  • The doctest checks that the function call twenty_twenty() should return the number 2020.

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.

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 2020. 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 61A, 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, you should see a successful test:

=====================================================================
Assignment: Lab 0
Ok, version v1.11.1
=====================================================================

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

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

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

---------------------------------------------------------------------
Doctests for twenty_twenty

>>> from lab00 import *
>>> twenty_twenty()
2013

# Error: expected
#     2020
# but got
#     2013

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

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

Every time you run Ok, Ok will try to back up your work. Don't worry if it says that the "Connection timed out." We won't use your backups for grading.

While ok is the primary assignment "autograder" in CS 61A, 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 61A assignment, it's time to turn it in. Note that it is not sufficient to receive credit for an assignment simply by running the autograder per the last section. You must follow these steps to submit and get points!

Step 1: Submit with ok

In your terminal, make sure you are in the directory that contains ok. If you aren't there yet, you can use this command:

cd ~/Desktop/cs61a/lab/lab00

Next, use ok with the --submit option:

python3 ok --submit

This will prompt you for an email address if you haven't run Ok before. Please follow these directions, and refer to the troubleshooting steps on that page if you encounter issues. After that, Ok will print out a message like the following:

Submitting... 100% complete
Submission successful for user: ...
URL: https://okpy.org/...

Step 2: Verify your submission

You can follow the link that Ok printed out to see your final submission, or you can go to okpy.org. You will be able to view your submission after you log in.

Make sure you log in with the same email you provided when running ok from your terminal!

You should see a successful submission for Lab 0.

Congratulations, you just submitted your first CS 61A assignment!

More information on Ok is available here. You can also use the --help flag:

python3 ok --help

This flag works just like it does for UNIX commands we used earlier.

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.

    python3 
  • -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.

    python3 -i 
  • -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).

     python3 -m doctest