CS 331 Spring 2009
Assignment 4

Assignment 4 is due at 5 p.m. Thursday, April 16. It is worth 25 points.

Procedures

E-mail answers to the exercises below to ffggc@uaf.edu, using the subject “PA4”.

• Your answers should consist of the file pa4.py containing the answers to Exercise A. This file (or an archive file containing it) should be attached to your e-mail message.
• Be sure to include your name in your e-mail.
• I may not read your homework e-mail immediately. If you wish to discuss the assignment (or anything else) with me, send me a separate message with a different subject line.

Exercises (25 pts total)

Exercise A — Basic Python

Purpose

In this exercise, you will write some simple Python functions and classes.

Instructions

Write code meeting the requirements outlined below. Place all of this in the file pa4.py. I prefer that you use Python 3. However, if this is not available to you, some recent version of Python 2 is acceptable (2.5 or 2.6). If you use a version of Python before 3.0, you must let me know this in your e-mail.

• Class vec3. Objects of this class represent 3-dimensional vectors. Essentially, a vec3 is a wrapper around a 3-item list, with various added operations. The items in the list (that is, the coordinates of the vector) are generally expected to be numeric. However, do put place any unnecessary restrictions on their types. The following operations should be available.
  • A constructor taking 3 parameters. This sets the x, y, and z coordinates of the vector to the 3 parameters.
  • The binary + operator. This takes two vec3 objects, and does vector addition.
  • The += operator. Corresponds to the + operator, as usual.
  • The binary - operator. Vector subtraction.
  • The -= operator. Corresponds to the - operator, as usual.
  • Scalar multiplication. Multiply a single value (scalar) by a vec3, as usual.
  • String conversion. Name this according to Python standards. It should return exactly the same string that string conversion of a 3-item Python list would return.
  • Global function dot. This takes two vec3 objects, and returns their dot product as a single value (scalar).

  Class vec3 does not need to do any type checking.

  Example code:

  a = vec3(1,2,3)
  b = vec3(4,5,6)
  print(a)         # Prints "[1, 2, 3]"
  print(a+b)       # Prints "[5, 7, 9]"
  print(dot(a,b))  # Prints "32"
  c = vec3("x", "y", "z")  # Legal, as long as we don't try to add, etc.
  

• Generator function prime3. This function should be a generator that yields, one at a time, the sequence of all prime numbers whose decimal representation ends in “3”: 3, 13, 23, 43, 53, 73, 83, .... It should take no parameters, and it should produce its values in a reasonable time.
• Function foo_maybe. This function takes one parameter, x. It attempts to call x.foo(). If this is successful, it returns the return value of x.foo(). If x.foo() cannot be done, it returns None (this is a special Python no-value value).
• Function get_331_char. This function takes 1 parameter, an integer. It downloads the source for the web page for this class, and returns the character with the given index. It should return a string (not a byte). The first character has index 0. Thus, with the page in its current form, get_331_char(0) should return “<”.

Test Program

I have written a test program: pa4_test.py (Python 3 version) or pa4_test_py2.py (Python 2 version). If you run the test program with your package, then it will test whether your package works properly.

To do the testing, place the test program and your file (pa4.py) in the same directory, and run the test program (e.g., at a Unix prompt, type “python3 pa4_test.py” or “python pa4_test_py2.py”).

Do not turn in the test program.

CS 331 Spring 2009: Assignment 4 / Updated: 15 Apr 2009 / Glenn G. Chappell / ffggc@uaf.edu