Computer Laboratory 2
CSci 1913: Introduction to Algorithms,
Data Structures, and Program Development
September 17–18, 2019

0. Introduction.

In this laboratory assignment, you will write a Python class called Zillion. The class Zillion implements a decimal counter that allows numbers with an effectively infinite number of digits. Of course the number of digits isn’t really infinite, since it is bounded by the amount of memory in your computer, but it can be very large.

1. Examples.

Here are some examples of how your class Zillion must work. I’ll first create an instance of Zillion. The string gives the initial value of the counter. Blanks and commas in the string are ignored.

z = Zillion('999 999 999 998')

This instance of Zillion contains the number nine hundred ninety nine billion, nine hundred ninety nine million, nine hundred and ninety nine thousand, nine hundred and ninety eight. This is much larger than the maximum number that can be represented in a Java int variable, which is only 2 147 483 647, or around two billion.
      I’ll add 1 to the counter, by calling the method increment, and I’ll print the counter by calling the method toString. I should see 999999999999 (twelve nines) printed.

z.increment()
print(z.toString())

I’ll add 1 to the counter again, and print its digits as before. I should see 1000000000000 (one with twelve zeroes) printed.

z.increment()
print(z.toString())

Finally, I’ll test if the counter contains zero by calling the method isZero. Of course z.isZero() will return False. But Zillion('0').isZero() will return True.

2. Theory.

Your class Zillion must represent a number internally as a list of one or more digits. Each digit d in the list is an integer 0 ≤ d ≤ 9. For example, the number 1234 must be represented as the list [1, 2, 3, 4]. Although Python provides long integers that can have arbitrarily many digits, the class Zillion must not use these long integers. You will receive zero points for this assignment if you use Python’s long integers in any way!
      The method increment must work like this. Starting at the right end of the list, and moving toward the left end, it must change 9’s into 0’s, until it finds a digit that is not 9, or until there are no more digits left to be visited. If it stops because it has found a digit that is not 9, then it must add 1 to that digit. If it stops because there are no more digits left, then it must add 1 to the front of the list. For example, if the list is [1, 2, 9], then increment will first change the 9 at the end of the list to a 0, then add 1 to the digit 2, resulting in [1, 3, 0]. Similarly, if the list is [9, 9], then increment will change both 9’s to 0’s, then add 1 to the front of the list, resulting in [1, 0, 0].
      Hint: unlike the previous lab, you are using lists, which are mutable objects whose elements can be changed. In fact, you must change list elements, or your program will not work correctly.

3. Implementation.

The class Zillion must define the following methods. To simplify grading, your methods must use the same names as the ones shown here. However, they need not use the same parameter names, except for self, which must be unchanged. To demonstrate your understanding of Python, some methods must be implemented in specific ways, as described below.

__init__(self, digits)

The string digits must be a string containing nothing but digits (0 through 9), blanks, and commas. It must contain at least one digit. If digits contains no digits, or if it contains a character that is not a digit, a blank, or a comma, then raise a RuntimeError exception. Convert digits to a list of integer digits as described in section 2, and save it within the instance of Zillion. The list represents the number that is stored in the counter.
      Hints: it is not enough to test if digits is the empty string. For example, a string consisting of nothing but blanks and commas, like ' , ,,' must raise an exception. Also, you may wish to call the built-in function int, which converts a string to an integer. For example, int('0') returns the integer 0.

increment(self)

Increment the counter, using the algorithm described in part 2. Hint: one way to do this is by using a while loop, and another way is to use recursion. There may be still other ways.

isZero(self)

Test if the counter contains zero. That is, test if your list of digits contains nothing but 0’s.

toString(self)

Convert the list of digits to a string, and return the string. Hint: you may wish to call the built-in function str, which converts an integer to a string. For example, str(0) returns the string '0'.

4. Tests.

The file tests2.py on Moodle contains a series of tests. The tests create instances of the class Zillion, call their methods, and print what they return. Some of the tests raise exceptions instead of printing values. They show whether Zillion’s methods raise exceptions correctly.
      To grade your work, the TA’s will run the tests using your functions. If a test behaves exactly as it should, then you will receive all the points for that test. If a test does anything else, then you will receive no points for that test. Your score for this assignment is the sum of the points you receive for all the tests.

4. Deliverables.

Run the tests in the file tests2.py. Then submit the Python code for your class Zillion and the results of the tests. Your lab TA will tell you how and where to turn them in. Your work must be submitted by 11:55 PM on Wednesday, September 25, 2019. (We are experimenting with having the same due date for all labs, no matter what day they meet.)