Question

Hey guys,

I am currently working on ps1 question 2 under the spring2011 section. I am confused as to why my code doesn't produce the correct answer. Can you please help me? This is the what I have:

balance=float(raw_input("What is the outstanding balance on your credit card?"))
annint=float(raw_input("What is the annual interest rate as a decimal"))
payment=10
newbalance=balance
while balance>=0:
for month in range(1,13):
newbalance=balance*(1+(annint/12))-payment
if newbalance<=0:
break
else
balance=newbalance
payment=payment+10

Thanks!

Answer 1

I put some print commands between your code and I see that the balance does not decrease, also, be sure to use the correct indentation for the code. Let me review it again to find out where you may be having the problem

Answer 2

I see the error in your code, in the for loop, you need newblance to have the new value after each month, you cannot use balance in the newbalance=balance*(1+annint/12) - payment, because you will always use the original balance amount, that's one problem. it should be newbalance=newbalance*(1+annint/12) - payment.
Then in the for you need to have an if statement to break the for loop, if newbalance is <=0.

I hope this will help :), but use print statements in your code to find out where you may have an error (debug).

Answer 3

This is the arranged code
while balance > 0:
for month in range(1,13):
newbalance = newbalance*(1+(annint/12))-payment
#print newbalance
#print payment
if newbalance <=0:
break
#check what happened with the newbalance using payment of 10
#if not less then increase payment and calculate it again
if newbalance <= 0:
balance = newbalance
else:
newbalance = balance
payment = payment+10

Answer 4

Thanks for your response! But I don't completely understand why my balance wouldn't change. Doesn't the last line of else and then the change of balance to newbalance change the value for the loop?

Answer 5

balance is a constant . Lets say for instance the balance entered by user is $5000. This means that balance has a value of $5000. so if you look ar your fo loop below:

for month in range(1,13):
newbalance=balance*(1+(annint/12))-payment
python reads it as
for month in range(1,13):
newbalance=5000*(1+(annint/12))-payment
over and over and over again within the for loop. But remember that the newbalance should change every month for 12 months within the for loop.
I think thats the point that xavigus is making.

Answer 6

ok, thanks for your responses!

Answer 7

Hi Markz, you enter a balance, and you need to find out the best minimum payment so you can totally pay the balance in one year.
Since you enter the balance, you need to keep the original amount somewhere, and you need to use a clone of it to calculate when it is reduced to 0.
Newbalance will be your clone, so you need to use that to calculate the interest and substract the minimum payment, so starting with a payment of $10, you add interest then substract payment, and you have a newbalance after the first month, in the for loop, you need to do that for 12 months, but at the same time you need to check if in any of the 12 months your balance is 0 or negative value, which means you paid off.
If the for loop after 12 months continues with a positive newbalance, that means, your minimum payment needs to change, so the next logic will be under the while loop.
If newbalance is 0 or negative, you assign that value to balance, so the while loop ends, if newbalance continues to be positive, then increase the minimum payment and re-assign to newbalance the original amount you owe so you can test it again, in the for loop, using the new min payment.

Answer 8

This is a descriptive logic of the solution

clonebalance=originalbalance
# initial minimum payment
minimumpayment=10



while originalbalance > 0
for month 1 to 12
newbalance=newbalance*(1+(annint/12)) - minimumpayment
if new balance <=0 #I paid off on or before 12 months
exit the for loop
end if statement
next month in for loop

# after the for loop, need to check if I need
# to continue in the while loop
if newbalance <= 0 #last value from the for loop
originalbalance = newbalance # so the while condition can end
else
#we need to try a new min payment
increase min payment by 10
give newbalance the original balance
end if statement

Hope it helps :)

Answer 9

clonebalance is newbalance, I meant that.

Answer 10

Did not see Shirley's answer, yes, that's the point.

Answer 11

I understand now. Thanks for the description at each step. I think the way I wrote it increased the payment each month rather than running through all 12 months and then increasing if newbalance>0. On to the next lecture now!