Introductory Programming
Fall 2004

Homework 1

The reading for this assignment is Chapters 1 and 2 of How to think.... The goal of this assignment is to explore the Python interpreter, to make some syntax and run-time errors, and to write and execute a Python script.

The Interpreter

  1. To start Python, type python in a Unix shell. You should get a startup message and a chevron (»>): 

    Python 2.3.3 (#1, May  7 2004, 10:31:40)
[GCC 3.3.3 20040412 (Red Hat Linux 3.3.3-7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>>
    The chevron is a prompt; it means that the interpreter is waiting for you to type an expression or a statement.

  2. Type 1 + 2 and then hit return. Python evaluates this expression, prints the result, and then prints another prompt.

  3. Type 1 2 and then hit return. Python tries to evaluate the expression, but it can't because the expression is not syntactically legal. Instead, it prints the error message: 

      File "<stdin>", line 1
    1 2
      ^
SyntaxError: invalid syntax
    In many cases, Python indicates where the syntax error occurred, but it is not always right, and it doesn't give you much information about what is wrong. So, for the most part, the burden is on you to learn the syntax rules.

    In this case, Python is complaining because there is no operator between the numbers. If you want to perform multiplication, you have to use the multiplication operator, which is *.

  4. Type print 'hello'. Python executes this statement, which has the effect of printing the letters hello. Notice that the quotation marks that you used to enclose the string are not part of the output.

  5. Type print bob without the quotation marks. The output will look something like this: 

    Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'bob' is not defined
    This is a run-time error; specifically, it is a NameError, and even more specifically, it is an error because the name bob is not defined. If you don't know what that means yet, you will soon.

Running a script

When you are only evaluating a few expressions, or executing a few statements, it is convenient to use the Python interpreter in interactive mode. When you start writing bigger programs, you will want to store your programs in a file called a script and then use the interpreter to execute the script.

  1. An easy way to create and edit files is with emacs. When I am working on a program, I usually have an emacs window to edit the program and a terminal window to run it.

    Make sure that you are in the directory where you want to put a new file and type emacs test.py &. Because the file name ends with py, emacs knows that it is a Python program. The ampersand is there so you can use the emacs window and the terminal at the same time.

  2. Write a line of Python code in the emacs window, maybe a print statement like print 'hello'

  3. Save the file using the File menu, the disc icon or the keyboard shortcut Control-x Control-s.

  4. In the terminal window, type python test.py. You are starting the interpreter and telling it to execute the script test.py. It should print hello.

  5. In the emacs window, change hello to jello. Now go back to the terminal window and run the script again. Did it say hello or jello? If you saved the script before you ran it again, you will see the new version; otherwise you will see the old version.

    This is, frankly, an annoying little GOTCHA! You have to remember to save your script before you execute it.

  6. Add a second line to your script, something like print 3+1. Save the file and then go back to the terminal. Press the Up Arrow key. You should see the previous command again, and you can hit return to execute it again. That should save some typing.

  7. We're almost done, but there is one more GOTCHA! I have to warn you about. Add another line to your script, but instead of a statement (like a print statement), just write an expression (like 5*7). Save and run the script. Did it print 35?

    Probably not. The reason is that when you evaluate an expression in a script, Python doesn't display the result. It you want to display the result, you have to use a print statement.

  8. At this point, you can create and execute Python scripts, and that's pretty much all you need to know for the rest of the semester. Of course, there are other things you might want to know...

Coming attractions

  1. Make sure you are in a directory where you want to put Python code and then use wget to download World.py from the class web page: 

    wget http://wb/ip/code/World.py
    Now press the Up Arrow key and edit the previous command so that it reads 

    wget http://wb/ip/code/AmoebaWorld.py
    Press return to download AmoebaWorld.py.

  2. Run the program you just downloaded by typing 

    python AmoebaWorld.py
    You should see the top view of a microscope slide with an amoeba in the middle.

  3. Press the Run button. The amoeba should move toward the northeast corner of the slide, leaving a trail of slime.

  4. Change the entry marked x(t) so that it reads cos(t) and change y(t) to sin(t). Press run again.

In the next homework we will do something more serious with AmoebaWorld but for now I thought you would like to see it.