CS 115 Lab 2 Practice with Math Operators

95 Points

Due Date: Monday Februay 8 by the end of the lab session

Submit the program you write with the Canvas link on the assignment.

Educational goals of this lab - verify that every student can

work with their team
write an algebraic formula in correct Python syntax
use input and output statements correctly
use test cases to test a program

INSTRUCTIONS:

(10 points) Team Problem #1: Get to know your team

Everyone on the same team should be in the same breakout room. You will be working together. Exchange email addresses so that you can communicate with your team outside of lab. One member emails your TA with a list of the students on your team and their email addresses. Make sure to include which Team number (1-7) you are!

Description of the problem:

You are taking out a loan (principal) of a given amount, at a certain annual interest rate. You are going to be making monthly payments for a given number of years. Calculate what the payment will be, assuming it stays the same for each payment (called a "fixed payment").

This came from the web site: http://www.vertex42.com/ExcelArticles/amortization-calculation.html

(30 points) Team Problem #2: Test Cases (work these out FIRST!, before class if possible)

Create a multi-line comment at the bottom of your program file which contains the answers that go in the blanks below. It should be the expected output from the program described below, the payment, given the inputs in the case. Use a calculator as needed. Here is a online calculator that you can use. You get 15 points for calculating them, 15 points when your program runs and gives the correct answers.

For answer E, give the reason it is an error (you won't have numeric answers for it). Look at the formula, it is dividing by zero. Actually substitute the input into the denominator and show how that is zero. You do NOT NOT want to give the Python error here! You are doing these BEFORE the program is written!

Description Inputs
Principal
Number of Years
Interest Rate per Year (APR) Expected Output
Payment

Normal cases

Normal case with
positive floats for principal and interest rate 220., 3, 20.5 ____A.____

Normal case with
positive ints for all inputs 15000, 4, 10 ____B.____

Boundary cases

Very large number for principal
normal numbers for rate and years 1e9, 5, 10 ____C.____

All inputs are 1 1, 1, 1 ____D.____

Error case

All inputs are 0 0, 0, 0 ____E.____

Description	Inputs Principal Number of Years Interest Rate per Year (APR)	Expected Output Payment
Normal cases
Normal case with positive floats for principal and interest rate	220., 3, 20.5	____A.____
Normal case with positive ints for all inputs	15000, 4, 10	____B.____
Boundary cases
Very large number for principal normal numbers for rate and years	1e9, 5, 10	____C.____
All inputs are 1	1, 1, 1	____D.____
Error case
All inputs are 0	0, 0, 0	____E.____

Remember these answers, A-E, are in a multi-line comment at the bottom of the team .py file.

(15 points) Team Problem #3: Design

In your prolog at the TOP of your .py file, include the names of all team members who are PRESENT at the lab (emails not needed). Also include the purpose, pre- and post-conditions for the program. Remember to give the types of each input and output value.

Paste this design into your .py file and complete the last 2 steps.

    #1. Print title
    #2. Get inputs from user (principal, number years, interest rate)
    #3. Convert rate to percent and to monthly rate
    #4. Convert years to months

    #5. what? (5 points)
    #6. what? (5 points)

(40 points) Team Problem #4: Implementation

Write a program that use the design in problem #3 to tell if you take out a loan of a certain amount, for so many years, with a certain interest rate per year, what the payment amount will be.
You MUST have a main function definition (def main():) and a call to main (main()) at the bottom.
Put spaces around your operators - makes them MUCH easier to read! Both the assignment operator (=) and the math operators.
The design should be interlaced with the code, NOT in the prolog!
Variable names MUST have more than one character. The reason the formula is given with single letters is that mathematicians have to write the formula by hand and the single letters makes it easier. You are typing your identifiers, so you have time to make them a bit longer. Don't use "n", use "months".
The first number input is the loan amount (principal) (float), The second number input is the number of years the loan is for (int), The last number input is the interest rate per year (float).
The user should be allowed to input the interest rate as a percent rate per year, annual interest rate. This input number needs to be converted to a percent (divide by 100) AND converted to a monthly rate (divide by 12).
The number of years needs to be converted to the equivalent number of months. There are 12 months in a year.
Then use the formula given above to find the payment amount.
Display the payment with only two decimal places, using the .format metho. Note that the $ in the test cases is right next to the payment amount.
Test case 1
```
Payment Calculator

Enter initial amount of loan: 25000
Number of years to pay back loan: 30
Interest rate per year (%): 3.25
Payment is $108.80
```
For this test case, the principal is 25000, the number of months is 30 * 12 (to make months from years), the interest rate is 3.25 divided by 100 (for percent) and then divided by 12 (for months).

Test case 2

Payment Calculator

Enter initial amount of loan: 200000
Number of years to pay back loan: 15
Interest rate per year (%): 6
Payment is $1687.71

Make sure to run your test cases from above to see that you are getting the right answers.
Make sure that all members of the team understand the team result.
Submit with the Canvas link.

If you do not attend lab, you will NOT get the points for the Team Problem, even if your team does submit a solution!

Make sure you save your programs somewhere as a backup to the copy on Canvas! Teams forget to submit sometimes!