Question

I am trying to solve the second problem on the first problem set ("Paying Debt Off In a Year"). Apparently there's some error in my code. Can anyone please check the code and tell me the flaw? Thanks in advance.

Answer 1

balance = float(raw_input('Enter the outstanding balance on your credit card: '))
ann_rate = float (raw_input('Enter the annual credit card interest rate as a decimal: '))
month_rate = ann_rate/12
i = 0
t = balance
while t > 0:
t = balance
i += 10
for j in range (1, 13):
t = t * (1 + month_rate) - i
print 'RESULT'
print 'Monthly payment to pay off debt in 1 year: ', i
print 'Number of months needed: ', j
print 'Balance: ', t

Answer 2

I think I figured this on my own. The inner for loop didn't check for the value of t. So whenever the t became negative, it would continue counting until the end of the loop. I rewrote this using two nested while statements that check the value of t:

balance = float(raw_input('Enter the outstanding balance on your credit card: '))
ann_rate = float (raw_input('Enter the annual credit card interest rate as a decimal: '))
month_rate = ann_rate/12
i = 0
t = balance
while t > 0:
j = 0
t = balance
i += 10
while (j < 12) and (t > 0):
t = t * (1 + month_rate) - i
j += 1
## print 'balance: ', t
## print 'minimum pay: ', i
## print '-----'
print 'RESULT'
print 'Monthly payment to pay off debt in 1 year: ', i
print 'Number of months needed: ', j
print 'Balance: ', round(t, 2)

Answer 3

please use a code pasting site and selct python syntax highlighting

Answer 4

code pasting sites
1. pastebin.com
2. dpaste.com

Answer 5

Thanks. So after the second one, I am working on the 3rd problem and I am badly stuck this time (same problem, but using bisection method):

http://dpaste.com/1007290/

Can anyone please read the code and give me hints ? I wanna know what goes wrong here.

Answer 6

why are you using twelve times ans in all of your tests. A good way to start tackling a problem is to write down the steps you need to perform to solve the problem.

1 - initialize parameters
2 - establish an initial upper and lower bound
3 - calculate a guess that is half-way between the bounds
3 - run the guess through 12 payment cycles
4 - test the result
5 - if the guess was too big, make the upper bound equal to the guess and repeat at 3
6 - if the guess was too small, make the lower bound equal to the guess and repeat at 3
7 - if the guess was just right, stop

it seems as if you should be testing updating_bal against the tolerance:
this condition would stop the iteration: -epsilon <= updating_bal <= epsilon
this condition indicates guess is too big: updating_bal < -epsilon
this condition indicates guess is too small: updating_bal > epsilon

you need to be careful where you put things, you might want to look at the order of line 11 and 12-15. print statements help a lot when trying to debug: http://dpaste.com/1007732/

are all thos /12's and *12's an attempt at optimization? early optimization is the root of all evil. keep it simple, get it to work, see if it is fast enough - only then try to determine what part of the code can be optimized

Answer 7

Thank you so very much, bwCA! Your insight really helped. I edited the code and now in the test runs it seems to be working:

http://dpaste.com/hold/1008381/