CS 111: Introduction to Computer Science

Winter 2017

HW03: Making change

Due: Friday, 01/13 at 22:00

This assignment will give you a little more practice writing simple Python programs—this time from scratch. It also focuses your attention on how you can do arithmetic in Python using the +, -, *, /, //, and % operators.

The program

Write a program that asks the user for a purchase price and an amount tendered, and then displays the change in dollars, quarters, dimes, nickels, and pennies. The user should enter both numbers in dollars without the dollar sign (e.g., 34.59 or .45), but your output should format with a dollar sign and two decimal points (e.g., $5.41).

Here is how a typical run of your completed program might look, with user input in red and program output in black:

Price? 34.59
Amount tendered? 40

Report
======
Purchase Price: $34.59
Amount Tendered: $40.00

Change:  $5.41
5 one-dollar bills
1 quarter
1 dime
1 nickel
1 penny

Your answer should be the natural one—the one that gives away as little small change as possible. You may assume that both the purchase price and the amount tendered are less than $100, and greater than $0.

A little help

Question #1: How do you turn a string like 34.59 into an integer number of pennies so you can subtract the price from the amount tendered? This is a two-step business:

• First, turn 34.59 into a real number. See the arithmetic.py lab for some ideas on how that goes.
• Second, multiply your real number by 100 and then turn it into an integer.

Question #2: How do you figure out how many quarters there are in 59 cents? The beauty of integer division is that the remainder is thrown away. So if you have an int variable containing 59, and you divide by 25, what do you get?

Question #3: Once you know how many quarters there are in 59 cents, how do you pursue the number of dimes, nickels, and pennies? Well, if you already know there are 2 quarters in 59 cents, how do you determine what's left over? There are two ways to go. You could subtract 2*25 from 59, of course. But you could also just as well ask "what's the remainder when I divide 59 by 25?" Once you have "9 cents" as your answer, you can go back to Question #2 for your dimes, and then again for your nickels. Got the nickels sorted out? Pennies should be easy.

Question #4: Once you have the core of your program written, pause and think about whether it handles all inputs correctly. Surely your program doesn't work if the input price is tofu instead of a number. That's okay. Later on we will learn how to deal with this. However, does your program handle all numeric cases correctly?

Grading

Please name your file change.py. This makes it a little easier for the grader.

• Style/comments [2 points]
  • As always, write a comment at the top of your program with your name, date, and a (short) description of what your program does. Enclose the comment in triple quotes like all our example programs.
  • Avoid using variables such as a or x, since that makes your code harder to read and maintain. Use descriptive names such as pennies or amountTendered.
• User input [2 points]
• The calculation logic [3 points]
  • Did you write code to calculate correctly?
• Program output [3 points]
  • The output doesn't have to match the sample above exactly, but all the elements should be there and things should be displayed in a logical, pleasing way. Remember that a program such as this is communicating with a human user, so guidelines for good writing still applies!

After you're done

Before stepping away from an assignment like this and moving on to other things, take a little time to think about what the assignment was for. Did the assignment increase your mastery of particular programming tools or language constructs? Did it illuminate some important idea? Even a relatively minor assignment (making change is, I admit, not the most stimulating of computational tasks) is likely to have been assigned for a purpose. If you try to put yourself into the professor's mind (a frightening prospect, I know, but be strong!), you can usually get more out of the assignment than if you just get the program working and then forget about it.

This step is known as metacognition, or thinking about thinking. There are a lot of studies about the immensely positive impact for students' learning if students are actively thinking about what is being learned. I encourage you to do these reflective exercise at the end of every assignment, so you know why you were doing the exercise and what you learned from it.

Start early, have fun, and discuss questions on Moodle.