2010 September 14 / jj|d|a|v|i|s|@|c|a|r|l|e|t|o|n|.|e|d|u

Assignment: Introduction

Carleton College CS 201, Fall 2010, Prof. Joshua R. Davis

Introduction

This assignment is short and simple, I hope. It is due by noon on Wednesday.

Read the Syllabus

We did not go over the syllabus in class, but you are responsible for knowing everything on it. For example, how do you activate your late pass? What task must you complete sometime during the first two weeks of class?

Test Your Computer Setup

Go to a Carleton CS lab such as CMC 306 or CMC 304. Start a computer into Mac OS X (if you can't find one already running it) and log in. There are four applications that we'll use heavily. All four should be in your Dock; if not, then find them and put them there. Ask for help if you need it.

Safari or Firefox, for reading the assignments as you work on them.
TextWrangler, for editing Python programs.
Terminal, for running Python programs.
Finder, for manipulating files.

First we're going to edit and save a Python program. Open TextWrangler and enter the following Python code into a new text document.

# 2010 August 29 by Joshua R. Davis

import random

def factorial(n):
    """The input n is a nonnegative integer. Returns the factorial n!."""
    if n == 0:
        return 1
    else:
        return n * factorial(n - 1)

if __name__ == "__main__":
    for i in range(6):
    	# Generate a somewhat random integer between 0 and 20 (more like 0 and 19).
        n = int(20.0 * random.random())
        print factorial(n), "=", n, "!"
    # Launch the help viewer for factorial (press Q to exit).
    help(factorial)

Near the bottom of the code is a loop that fires 6 times. Change 6 to some other small integer, such as the number of letters in your first name. At the top of the file is a comment giving the revision date and a list of authors. Change the date to today's date, and add yourself as an author. Save the document as intro.py.

Now we're going to run the program. Open Terminal. Navigate to the directory (folder) where you saved intro.py, using these basic Unix shell commands:

pwd tells you what directory you're currently in, in the hierarchy of all directories and files on the computer.
ls lists the contents of the current directory.
cd directory (where directory is the name of some directory inside your current directory) changes your current directory. That is, it takes you "down", deeper into the hierarchy.
cd .. takes you "up" to the directory that contains your current directory.
cd takes you back to your home directory; use this if you get lost.

Once you've navigated to the correct directory, enter python intro.py to run your Python program. It should print out some numbers and then launch the help viewer. When you want to exit the help viewer, press Q; then you can see the numbers again. Make sure that you understand what every line of code does. If you don't, then ask someone for help.

Finally, submit your work using the following steps. Warning: Once you've submitted a file, you cannot edit or delete it.

In the Finder program, select the Go menu, then Connect to Server....
Type courses.its.carleton.edu/COURSES and press Return.
Enter your username and password; the COURSES file server should open in a new Finder window.
Navigate to the student work section of our course — that's f10, then cs, then cs201-02-f10, then Student Work.
Select your directory, then the hand-in directory inside it.
Drag your file intro.py into your hand-in directory.

Understand Good Python Style

The code I gave you above exhibits what I call Good Python Style:

At the top of the file is a comment that includes the revision date and the names of all of the authors.
After the date/author comment come all of the import statements. That is, import statements aren't sprinkled throughout the file; they're all at the top. This is so that anyone who reads this file can quickly determine which other modules it uses.
All functions, methods, and classes have docstrings. A docstring is a string, enclosed in tripled double quotation marks """, that comes immediately after the function, class, or method declaration and describes how to use that function, class, or method. Like a comment, a docstring tells the reader how to use the function/class/method. Unlike a comment, a docstring is viewable in the Python help system.
The code is well-commented. Commenting is a subtle art; to comment well, you have to ask, "What would a reader of this code want me to tell her?" Here are some guidelines. A comment describes what the code is intended to do — what its purpose is. It does not give a blow-by-blow description of what the code is literally doing — e.g. "Return n times the factorial of n - 1." Tricky bits of code are always explained. The comment precedes the code that it describes and is indented exactly as far as that code.
Each module (file) ends with a demo section that shows how to use the most important features of the module. The demo code is enclosed in if __name__ == "__main__":. The demo code runs if the user executes the module directly, but does not run if the user imports the module into another module.
Except for import statements, all top-level code is in the demo section. In other words, the only stuff allowed outside the demo section are comments, import statements, function definitions, and class definitions.

In this course, every piece of code that you write is required to exhibit Good Python Style.

Send Me E-Mail

Please send me a brief e-mail message with the following information. (My address is at the top of the page.)

name
home town/country
expected year of graduation
possible major(s)
Why are you taking this course?
Have you taken CS 111 at Carleton? Which programming languages do you know?
Did you have any problems with the earlier parts of this assignment?
Anything else you want me to know?