CS 151: Lab 1

Lab Exercise 1: Equipping yourself for the projects

Main course web page

The purpose of this lab time is to give you an introduction to the basic tools we will be using this semester. These tools include a terminal, a text editor, and the Python interpreter.

Labs and projects are tightly coupled. Each project is initiated with a set of lab exercises. In the projects, you will write code that expands on the code written in lab. For each project, you will be writing Python code and running it. The programs typically create images. You will be posting some of these images, along with a description of the project, on a Wiki page. You will then turn in your code by putting it in a special directory on one of the campus computer servers. Today's lab will allow you to practice each of these tasks.


Tasks

  1. Introduction to the Terminal

    Open up the Terminal application, which is a text-based method of using your computer.

    A terminal is simply a text-based interface to the computer. In a terminal, you can type commands, manipulate files, execute programs, and open documents. Anything you can do in the Finder, you can do in a terminal (the reverse is not true).

    When working in a terminal, the current directory is called your working directory. A terminal will usually start in the home directory of your account.

    In a terminal we often need to specify a file or directory on the hard drive. The complete description of where a directory or file on a computer is located is called its path.

    On a Unix style terminal, directories in a path are separated by a forward slash /. The topmost directory on the filesystem is indicated by a single slash. The directory tree for your computer is all relative to the top level directory. In other words, starting with a /, you can type the complete path to any file on your computer. For example, the following is the path to my home directory.

    /Users/srtaylor

    Most of the time, however, you will be specifying paths relative to your current directory. Any path that starts without a slash is relative to your current working directory. Note that paths are case-sensitive, so be sure to use proper capitalization.

    You will use paths, and filenames to change directories, list the contents of directories, and run Python programs.

    Some important terminal commands are the following. We'll walk through how to use them together in lab.

    • ls - with no argument, ls lists the files in the current directory. If you put a path after the ls command, it will list the contents of the specified path or file.
    • cd <directory> - change directory. If you use the command with no arguments, it changes the directory back to your top-level home directory. The current directory is specified as . and the parent directory of the current working directory is specified using ..

      cd Set the current directory to your home directory
      cd ~ Set the current directory to your home directory. (The ~ character is shorthand for your home directory.)
      cd .. Move up to the parent directory of the current directory
      cd blah     Move into the sub-directory blah
      cd - Move to your last working directory

    • mv <from> <to> - move a file, including just renaming it in the current directory. The mv command will remove the file in the old location after copying it to the new location.
    • cp <from> <to> - copy a file to a new location or name
    • rm <filename> - remove the file (will not remove a directory). Not that this is not the same as dragging a file to the trash. Once the filed as been removed, it is gone for good.
    • rm -r <directory> - remove the directory and all the files and subdirectories in it

    Some useful terminal properties are the following.

    • Wildcard characters: the star character * is a wildcard character. If you want to see all the files in a directory that start with A, you can type:

      ls A*

      A possibly bad thing to do is to type rm *

    • Tab completion: when you have typed part of the name of a file or program, pressing the tab key will complete the filename as far as possible while the choice is unique. For example, if you have only one file that starts with the letter b, then typing b and then the tab key will complete the filename.

      If you have two or more files that start with b, tab completion will beep at you. If you press tab again, then the terminal will show you the options (all the files that start with b).

    • Hitting return - you don't have to be at the end of the line to press return. If you go back and edit something in a terminal command, you can press return and execute the whole line no matter where the cursor is located. Try it.

    There are many useful terminal commands (which we will also refer to as Unix commands), many of which we will introduce in the project. They include

    • pwd - tells you the complete pathname of the current directory
    • less <filename> - scroll through a file
    • cat <filename> - send the file to standard output
    • echo "a string" write the text within the string to standard output
    • touch <filename> touch either updates the modification date on a file or creates an empty file if the named file does not exist. This can be useful in various situations, like when you are learning to create, rename, and delete files using a terminal.
    • cut - process a stream of standard output, returning a specific piece of each line of input. This will be covered in the project.
    • curl <url> - transfer data from a url to this computer. This will also be used in the project.


  2. Introduction to the file servers

    A fileserver is a central file system you can access from any computer. It's like having a virtual USB key you can plug into a computer to store your files. Colby maintains two different fileservers we use in this course. One (the Personal server) is for you to store the code you are working on. The other (Courses) is for you to turn in code. We will give instructions regarding the Courses directory in the project instructions. For now, let's focus on where you should keep code that is work in progress.

    There are two ways to mount the file server.

    • Option 1: Load the root server directory and navigate to your directory.

      You can mount the Colby fileserver root directory by going to the Finder and typing cmd-K, or selecting 'Connect To Server...' from the Go menu. It will bring up a dialog box, into which you want to enter the following.

      Mac OSX:   smb://filer.colby.edu
      Windows: \\filer.colby.edu

      and then click on the appropriate directories until you find your personal directory.

    • Option 2: Mount your directory directly.

      You can mount your personal directory explicitly using the the following path in the 'Connect To Server...' dialog.

      smb://filer.colby.edu/Personal/<username>

    This system is backed up regularly, and you can access it from any computer on the Colby network. We strongly suggest you store all of your work for this course into your personal directory. You can, in fact, work directly from your personal directory. Let's practice doing that for the first project.

    Project Prep:

    1. Mount your personal directory.
    2. In the terminal, navigate to it. Type cd followed by the path to the directory. E.g. for Bruce, it may look like this:

      cd /Volumes/personal/bmaxwell

      Or, use a shortcut. If you type cd in the terminal, then drag the icon for your directory onto the terminal, then the path to it will automatically be pasted.

    3. Make a directory (in your personal folder) named Project1. You can do this using the Finder, or you can type mkdir Project1 into the terminal.


  3. Introduction to text editing

    Text editors are the workhorse programs for writing code. You don't want fancy fonts or WYSIWYG layouts, you just want to see lines of text, preferably with syntax highlighting, which means that special words in a language are highlighted to make it easier to read the code. There are many editors to choose from.

    In lab, I will be using TextWrangler. It is a free text editor for the Mac and it has all the features I need for writing programs. If you would like to install it on your personal computer, go to the TextWrangler web site and follow their instructions.

  4. Project Prep:

    1. Open TextWrangler
    2. Set a couple of preferences.
      1. Set it up to display line numbers. This will help later, when you encounter error messages that tell you where an error is by supplying the file's name and the line's number.
    3. Create a new file (Cmd-N). Save it as smart.py in your Project1 directory.
    4. Put in the line:

      print 'You are smart'

    5. Then save the file (Cmd-S).
    6. Now, you can instruct Python to run the program. In the terminal type

      python smart.py

      Congratulations, you just ran your first Python program. (Python humor)

      Note that it works only if the current working directory is the one that contains the file smart.py. So, one thing you need to do when working on a project is to ensure that your Terminal is in the right directory. Think of it as making sure TextWrangler and Terminal agree about where the programs are. Note that, in general, you run Python programs by typing

      python <filename>

    For your information

    Here are some other text editors you should know about:


  5. Introduction to Python

    Python is a simple yet powerful interpreted language for making computers do stuff.

    We generally write code using a text editor and then run the code using the command line, just as in the prior example. Now we're going to write a program that takes in three numbers and calculates their sum and average.

    Project Prep

    1. Create a new file in your text editor. Save the file as addthree.py
    2. Type your name, a date, and the course and project in a comment at the top of the file. Every Python file you ever write should start with these three lines.
    3. Type the following three lines of code.

      print 'version 1'
      print 'sum', 42 + 21 + 5
      print 'avg', (42 + 21 + 5) / 3

    4. Run the program from the terminal.

      python addthree.py

    5. What does the program do? What happens if you change the 5 to a 6 on the last two lines of code and run it again?

      Remember, you have to save the file before you run it again on the Terminal.


  6. Introduction to the Wiki

    Colby Wiki - a Wiki is a convenient way to put content up on the web quickly and easily. We'll watch a video on how to create your personal space and then go through the process of creating a new page in your space.

    Tutorial Videos

    Log in to the wiki, but wait until Stephanie says "Your turn" before clicking on the "Create your personal space" link. The system can't handle more than one space-creation at a time.

    For each project, you will create a new page in your wiki space. You can edit any page in your space by going to the Edit tab. There you will find one or two options for editing your page. The Rich-text option (available when using FireFox) lets you do the normal select and point and click type formatting. The wiki markup lets you use the wiki markup language to make your page.

    Project Prep:

    1. Make a page for Project 1. Go to the Add menu in the upper right corner, and select the Page option. Replace the "New Page" title with something meaningful, like "Stephanie's CS151 Project 1" so that the page content is clear from the title.
    2. Now save your page. Even though it has no content, let's save it -- it is a good idea to save often. After saving your page, it will show up in the list of child pages on the bottom of your home page. You can link to it one your home page by editing the home page, pressing the link button (it looks like a chain) and searching your space for it. But it is also perfectly acceptable to simply leave it as a child page.
    3. In all of you project wiki pages, you will need to include pictures of the output your program created (which you may want to capture in an image). Since this project is an introduction to writing Python code, the first required picture is of the tools you need to write and run the code. Get your screen set up: Run your addthree.py program and leave the output visible in the Terminal. Then make sure the Terminal and TextWrangler are all visible (and make sure TextWrangler has addthree.py displayed). Take a picture of these two windows by pressing Shift-Command-4. A cursor will pop up. Draw a box around TextWrangler and the Terminal. The computer will take a picture of it and place that picture on your desktop. I have found that it is a good idea to make your picture as narrow as possible. Otherwise, it can be hard to view it on the wiki page. Also, you should change the name of the file containing the image. The Wiki tends to have problems with dates within filenames.
    4. Now let's edit the content of the project 1 wiki page. Click on the link to it, then press Edit. Write a stub for your write-up saying "These are the tools we use." and place the image in the page (press the mountain button and upload the image from your Desktop).
    5. Label the page. The label field is a text field at the bottom of the page, and is different from the title of your page. Each assignment will specify a label, which we will use to find your pages so we can show off your work and so we can grade it. For this project, add the label cs151sf15project1.
    6. Save it, sit back and think about how much you have already accomplished.

    For your information

    If you ever need to remove a page, go to that page and select "Remove" from the "Tools" menu in the upper right.

    Remember to save your wiki page before navigating away from it. The wiki won't detect it and remind you.

When you are done with the lab exercises, you may begin the project.