Question

Python: Why can't I insert print statements to debug this?

Answer 1

balance = 320000
annualInterestRate = 0.2
epsilon=.01
low=balance/12.0
monthlyInterestRate=annualInterestRate/12.0
high=balance*(1+monthlyInterestRate)**12/12.0
payment=(high+low)/2.0
newBalance=balance
while abs(balance-newBalance)>epsilon:
newBalance=balance
for count in range(12):
newBalance=(newBalance-payment)*(1+monthlyInterestRate)
if balance>newBalance:
low=payment
else:
high=payment
payment=(high+low)/2.0
print('Lowest Payment: '+str(round(payment, 2)))

Answer 2

the output is coming out too high, but I can't debug it because it only will let me put print statements in at the end for some reason. I would like to put one up between lines 9 and 10 to track the balance, and another at the end to track payment. Any ideas?

Answer 3

Why can't you debug? I'm not too sure about Python nor the editor you are using, but if you want to do things like debug you will need a good IDE, which I also recommend. I'll take a look later to see if I find a mistake tho.

Answer 4

high=balance*(1+monthlyInterestRate)**12/12.0 what's this **?

Answer 5

Also I would surround by brackets if that is what should happen. (balance*(1+monthlyInterestRate)**12)/12??

I mean I shouldn't have to tell a math wizz like you that, so I'm assuming you did this on purpose, but who knows... that ** is something I'm more konfused about :P

Answer 6

The program is to use bisection search to find the minimum monthly payment required to pay off some debt "balance" within one year
.
high=balance*(1+monthlyInterestRate)**12/12.0

this is the maximum yearly payment could be imagining we didn't pay off anything at any month and the interest accrues by a factor of 1+monthlyInterestRate each month, then the monthly payment is given by dividing by 12 months.
And yeah, I tried the parentheses bit.

Answer 7

what is an IDE ?

Answer 8

** raises to a power btw

Answer 9

IDE = Integrated Development Environment

Answer 10

Problem solved, I had all sorts of horrible things going on. the worst being that I defined
newBalance=balance
at the beginning, so the next line
while abs(balance-newBalance)>epsilon
was never true, so the thing wasn't even running the body of the program :P

I won't type the corrected program as it is an answer to the MITx thing, but I did get it :)