COS 100: Introduction to Programming

Interim 2021

HW Project 06: Handling money

Project goal

Prerequisites

• Finish lab07 first: it is designed to prepare you for this project.

Project specification

1. Write a function called ask_for_currency with the following specification:
   • Parameter: a string to be used as a prompt.
   • Ask the user for a number using the prompt passed to this function.
   • This number is a currency amount (such as $34.59) but without the dollar sign in front (such as 34.59).
   • Convert this to an integer number of pennies (such as 3459).
   • Return: the integer number of pennies.
2. Write a function called format_currency with the following specification:
   • Parameter: a currency amount represented as an integer number of pennies.
   • Convert this number (such as 3459) to a string in the standard way we write currency (such as $34.59), with a dollar sign in front and two decimal places.
   • Return: the string representing the amount.
3. Write (and call) a main function that asks the user for:
   • the number of items to purchase,
   • the price for each item,
   • the amount tendered,
   and report to the user:
   • the total purchase price,
   • the amount tendered, and
   • the change that's due.

Notes and hints

• Name your project source code change.py.
• You must use functions properly. If your main function does not use the two other functions you wrote, you will lose points.
• You may assume that the numbers the user enter are positive.
• If the amount tendered is less than the total purchase price, do something reasonable.
• The user should enter currency amounts in dollars without the dollar sign (e.g., 34.6 or .45), but your output should format with a dollar sign and two decimal places (e.g., $5.40).
• Try to always display two decimal places. This should be achieved by techniques you learned involving arithmetic. Do not use string slicing.

Suggested order of development

• As always, try to code this up a little bit at a time, testing after each feature you've added.
• Specifically, write one function at a time and test that function thoroughly.
• For example, to test the format_currency function, you can write, at the end of your file, things like print(format_currency(3459)) to see if the function is working.
• If you see None printed to the screen, it may mean that format_currency is not returning any value.
• Get this fully working before you even start to write a main function.
• For the main function, start by just asking for the price of one item, amount tendered, and the report.
• Then finally add the ability to purchase multiple items.

Sample run

Number of items? 2
Price? 20
Price? 14.59
Amount tendered? 40

Report
======
Purchase total: $34.59
Amount tendered: $40.00
Change due: $5.41

Grading

• Total points: 20
• ask_for_currency function: 5
  • Make sure the parameter is a string and the return value is an integer.
• format_currency function: 5
  • Make sure the parameter is an integer and the return value is a string.
• main function: 6
• Style: 4
  • Follow all the previous style guidelines, and also:
  • Remember to document the purpose of each function with a triple-quoted docstring (see lab for more explanations).

Optional challenge

• Also report the change in dollars, quarters, dimes, nickels, and pennies. Your answer should be the natural one—the one that gives away as little small change as possible.
• Sample run:

  Number of items? 2
  Price? 20
  Price? 14.59
  Amount tendered? 40
  
  Report
  ======
  Purchase total: $34.59
  Amount tendered: $40.00
  
  Change due: $5.41
  5 one-dollar bills
  1 quarter
  1 dime
  1 nickel
  1 penny
  

• If you want, you may submit this extension to Moodle as detailed.py, but it will not be graded.
• This assignment is inspired in part by one created by Jeff Ondich.

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