Question

Help with Program

Answer 1

http://ocw.mit.edu/images/jquerybubblepopup-theme/grey/tail-bottom.png

is the assignment

-The problem on the assignment is Problem 2, Paying off Debt in a year.
-This is what I currently have:

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance_Credit_Card = raw_input("Outstanding Balance on Credit Card:")
annual_Interest_Rate = raw_input("Annual Interest Rate:")

month = 0
<= 0

monthly_Interest_Rate = annual_Interest_Rate / 12.0
updated_Balance_Each_Month = previous_Balance * (1 + monthly_Interest_Rate)

Answer 2

your link doesn't work

Answer 3

http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/unit-1/lecture-4-machine-interpretation-of-a-program/MIT6_00SCS11_ps1.pdf

Answer 4

ok firstly, let's look at what you have so far:
what is this line:
```
month = 0
<= 0
```
?

Answer 5

well I was initiating a loop but, I incorrectin git

Answer 6

Incorrectly wrote it*

Answer 7

ok so, think about when we want our program to stop running. What should be the state of our remaining balance when the program ends?

Answer 8

0 ..... thats why im questiong looping the month and not the balance

Answer 9

less then or equal to 0 actually

Answer 10

right, so we need to loop as long as the balance at the end of the year is greater than zero. In Pseudocode:
```
## while balance not paid off
## for each month
## update the balance
## if balance is paid off, end loop
## print results
```

Answer 11

ok so we are making a loop inside of a loop

Answer 12

as you mentioned though, we don't have a minimum balance as input. The pset states you are supposed to start with 10 payments, and use multiples thereof

Answer 13

that seems necessary to me, yes
this is because we need to run through the months of the year as in the previous problem, but if when we get to the end we still have a positive balance due, we need to be able to loop back and try again with a different monthly payment. this indicates we need a double loop

Answer 14

ok im going to attempt it and ill get back to you with my results!

Answer 15

so what you need to do is initialize a minimum monthly payment, and then, as long as the balance is not paid off at the end of the year, we need to increase it by $10

great, I'll be here to help if i'm still online

Answer 16

so the minimum monthly payment should start out as 10 and if it doesnt pay it off increment it by 10 until it does?

Answer 17

and what do I make the balance equal to in the while loop, im guess that it is already equal to what we put in and I dont need to set it = to zero or anything

Answer 18

exactly
one slightly tricky part is that if you initialize the value to 10 before the while loop, then if you increase the amount by 10 inside the while loop you will start off checking 20, not 10

I recommend initializing it to 0 outside the loop, and adding 10 each time before the for loop

Answer 19

the balance due is gotten from your raw_input statement. you need a different variable to represent the remaining balance inside the for loop, because if at the end of the 12 months the balance is not paid off, you need to be able to go back and access the original balance due

Answer 20

you have a line that says
```
monthly_Interest_Rate = annual_Interest_Rate / 12.0
updated_Balance_Each_Month = previous_Balance * (1 + monthly_Interest_Rate)
```
but you never defined updated_balance_each_month or previous_balance
you should make those two variables the same, and simply update the balance each time
```
currentBalance = currentBalance * (1 + monthlyInterestRate)
```
think about what currentBalance should be set to at the beginning of each year (every time we start the for loop)

Answer 21

awesome I was wondering about previous balance, ill be back!

Answer 22

good luck!

Answer 23

thanks again sir

Answer 24

oh save the formalities for your professor XD

Answer 25

lol gotcha

Answer 26

now im getting a syntax error with my while loop, I know Im not done im just trying to check out my work that ive done so far!

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance_Credit_Card = raw_input("Outstanding Balance on Credit Card:")
annual_Interest_Rate = raw_input("Annual Interest Rate:")

## while balance not paid off
outstanding_Balance_Credit_Card = outstanding_Balance_Credit_Card
while outstanding_Balance_Credit_Card (outstanding_Balance_Credit_Card > 1)
month = 1
for month range(1,12)
month + = 1

## update the balance
outstanding_Balance_Card_Card = current_Balance
monthly_Interest_Rate = annual_Interest_Rate / 12.0
current_Balance = current_Balance * (1 + monthly_Interest_Rate)

## if balance is paid off, end loop

## print results
print current_Balance

Answer 27

issues i see so far:
1)
```
outstanding_Balance_Credit_Card = outstanding_Balance_Credit_Card
```
what is the point of this line? it does nothing but rebind a variable to itself; it's pointless. You want to bind the original balance to a new variable that we will update as we go through the loop, but this binding should occur *after* the while loop starts, but before the for loop (this way we reset the balance at the end of each year if we see the debt is not paid off

2)
```
while outstanding_Balance_Credit_Card (outstanding_Balance_Credit_Card > 1)
```
I don't understand the syntax of this line at all. Why do you have the varible twice, once compared to 1 ? aren't we concerned about when it reaches 0 ? Also, you forgot the colon at the end of the while statement. should be something like
```
while outstanding_Balance_Credit_Card > 0:
```
3)
```
month = 1
for month range(1,12)
month + = 1
```
many things wrong here.everything you seem to be trying to accomplish here can be done with
```
for month range(1,13):
```
(again you forgot the colon) will bind the variable 'month' to 1, and on each iteration update it to the next month until (but not including) 'month' takes the value 12. You don't need to initiate the month variable to 1, and you don't need to do month += 1 each time; the 'for' statement does this on it's own. Also, this will only count months from 1 to 11, since range(a,b) counts from a to b-1, it does *not* include b. You should review how the range function works.

Answer 28

as for the part of the pseudocode that says
## if balance is paid off, end loop
this should be something like a check each month to see if the remaining balance is <=0, and if it is, we should then leave the loop. This probably means using a 'break' statement.

Answer 29

sorry I didnt realize you answered it ill be back

Answer 30

the overall design should be something like
```
outstanding_Balance_Credit_Card = raw_input("Outstanding Balance on Credit Card:")
annual_Interest_Rate = raw_input("Annual Interest Rate:")
## initiate monthly payment (to what? you figure it out :P)
## while balance not paid off
## increase monthly payment by $10
##store outstanding balance in a new variable like current_balance
## for each month of the year
monthly_Interest_Rate = annual_Interest_Rate / 12.0
current_Balance = current_Balance * (1 + monthly_Interest_Rate)
## IF current balance is <0
## store month we are in as a new variable like 'last_month'
## exit loop
print last_month
print monthly_payment
print current_balance
```

Answer 31

you can also do the line
```
monthly_Interest_Rate = annual_Interest_Rate / 12.0
```
before any of the loops, actually, since that value won't change across the year and amounts for minMonthlyPayment

Answer 32

yeah some of those dumb mistakes where me just checking and guessing

Answer 33

Oh it's quite all right. Gotta write about 100 messy programs before they start looking nice. Just be careful about things like how the
```
for x in range(a,b):
```
work, because that kinda stuff is very important later on

Answer 34

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = int(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
minimum_Monthly_Payment = int(raw_input("Minimum Monthly Payment:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
## while balance not paid off
while outstanding_Balance > 0:
outstanding_Balance = current_Balance
for month in range(1,12):

## update the balance

outstanding_Balance = outstanding_Balance * (1 + monthly_Interest_Rate)
## if balance is paid off, end loop
if outstanding_Balance <= 0:
month = last_Month
break

print "RESULT"
print "Monthly Payment to Pay Off Debit in 1 Year:",minimum_Monthly_Payment
print "Number of Months Needed:",last_Month
print "Balance:",outstanding_Balance


this is what I got so far... I think the lack of sleep is really starting to get to me lol

Answer 35

read what you wrote again after some rest and see if you notice any mistakes

Answer 36

ok so I read it again and changed a few things around, basically I know I need to increment it by 10 each time so I added that but its not letting me assign outstanding balance to current balance. Obviously im doing something wrong so take a look at it if you can!

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = int(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
## while balance not paid off
while outstanding_Balance > 0:
outstanding_Balance = outstanding_Balance + 10

outstanding_Balance = current_Balance
for month in range(1,12):

## update the balance

current_Balance = current_Balance * (1 + monthly_Interest_Rate)
## if balance is paid off, end loop
if current_balance <= 0:
month = last_Month
break

print "RESULT"
print "Monthly Payment to Pay Off Debit in 1 Year:",minimum_Monthly_Payment
print "Number of Months Needed:",last_Month
print "Balance:",outstanding_Balance

Answer 37

this line is backwards
```
outstanding_Balance = current_Balance
```
the value in the variable on the right is being put into the one on the left, hence you are trying to put something you have not yet defined: current_Balance, into the variable containing outsanding_Balance.

Answer 38

assignments are read from right to left; you bind the value in the variable on the right to that on the left.

your 'for' statement is still not quite right, because when you do
```
for x in range (a, b):
```
you iterate over the elements [a, a+1, a+2, ..., b-1], so what you have will cause months only to go up to 11, not 12

Answer 39

```
## update the balance

current_Balance = current_Balance * (1 + monthly_Interest_Rate)
```
You are skipping a step in the monthly update: you also need to deduct the minimum payment, right? Your code will just make current_Balance get bigger and bigger....

Answer 40

```
if current_balance <= 0:
month = last_Month
break

print "RESULT"
print "Monthly Payment to Pay Off Debit in 1
```
indentation is wrong. after the 'break' statement you *exit the loop* which, in python, means you now write the code to be executed upon leaving the loop the the indentation outside the loop

Answer 41

```
if current_balance <= 0:
month = last_Month
break

print "RESULT"
print "Monthly Payment to Pay Off Debit in 1 Year:",minimum_Monthly_Payment
print "Number of Months Needed:",last_Month
print "Balance:",outstanding_Balance
```
The indentation on the print statements is wrong. After the 'break' statement you *exit the loop* which, in python, means you now write the code to be executed upon leaving the loop the the indentation outside the loop

Answer 42

by 'loop' i mean the loop before the 'if' statement, which, in this case, is the 'for' statement.

Answer 43

omg thank you I was about to throw my laptop out the window because I couldnt assign the variable!

Answer 44

ok so ive kind of gotten stumped, maybe im confused a bit with the break statement. So whenever I run the program it just breaks. ALSO so im trying to figure out how the program was running so I entered a print states ment and the while loop is running a bunch of times for month of the year so basically it run run the while loop 12 times before the current balance is changed so the month stays constant each time I print it out!

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
monthly_Payment = float(raw_input("Monthly Payment:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
current_Balance = outstanding_Balance
## while balance not paid off
while outstanding_Balance > 0:
current_Balance = current_Balance * (1 + monthly_Interest_Rate)- monthly_Payment
for month in range(1,13):
## if balance is paid off, end loop
if current_Balance <= 0:break
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",month
print "Balance:",current_Balance

Answer 45

with what you have written, the balance will change twice. If you inserted a print statement to check the balance in the for loop this would make you think it only changed once, perhaps
```
monthly_Payment = float(raw_input("Monthly Payment:")) (1)
current_Balance = outstanding_Balance ## (2)
## while balance not paid off
while outstanding_Balance > 0:
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - monthly_Payment
for month in range(1,13):
if current_Balance <= 0:break ## (3)
## if balance is paid off, end loop
```
(1) The specs of the problem say to start with 10$ payments, not whatever you want, so you need to hard code a number to initialize monthly_Payment such that, on the first trial, you pay 10$/month
. No raw_input statement should be here

(2) you assign outstanding_Balance to current_Balance before we enter the while loop, hence this value will never be reset if it is not paid off at the end of the year (end of the for loop), which was the point of writing it. For this line to reset the balance to the original amount due it needs to be inside the loop, otherwise how can that assignment ever happen again? It should be executed every time we see that after 12 months, current_Balance is still > 0;

(3) This is your update statement, but look where it is. You put it before the loop that iterates over the months of the year, so that means it is not included in what happens each month. i.e. You only update the balance once, and you do it at the beginning of the year.The only thing inside the for loop (the only thing you do *each month*) is the 'if' statement, which just checks to see if the balance has been paid off yet. So in effect, all your code does, is once a year (at the beginning of the while loop) update the balance. It never tries a new monthly amount to pay, and hence never pays any more than the last time you tried, and never goes below 0 (unless your initial amount pays off the debt on the first trial)
You need to include a line that, before iterating over the months, but still inside a loop that continues until the balance is paid off, changes the amount paid each month by adding 10$, as per the specs of the problem.
Also, i think break has to be indented on a new line:
```
if something:
break
```

Answer 46

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
## while balance not paid off
while outstanding_Balance > 0:
current_Balance = outstanding_Balance
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - 200
for month in range(1,13):
## if balance is paid off, end loop
if current_Balance <= 0:
break
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",month
print "Balance:",current_Balance

still getting nothing back! changed the monthly payment a few times

Answer 47

##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
monthly_Payment = 200
## while balance not paid off
while outstanding_Balance > 0:
current_Balance = outstanding_Balance
for month in range(1,13):
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - monthly_Payment
## if balance is paid off, end loop
if current_Balance <= 0:
break
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",month
print "Balance:",current_Balance

this is actually what I have now....... so everything is running smoothly except where the break statement is. I took the break statement out and put it under the print statements and it just kepts running and then when I put it beffore the print statements like it is now it just doesnt print anything!! confussedd!!!!!!

Answer 48

The problem I see is in the while loop. I do not know why you even have the for in there, but it seems you are never changing the outstanding_Balance, but you are using it over and over.

Answer 49

```
##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
monthly_Payment = 200
## while balance not paid off
while outstanding_Balance > 0:
current_Balance = outstanding_Balance
for month in range(1,13):
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - monthly_Payment
## if balance is paid off, end loop
if current_Balance <= 0:
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",month
print "Balance:",current_Balance
```

Answer 50

It is this sort of error:

```
x = 10
y = 10
while x > 0
print x
print y
y = y - 1
```

Because x never changes, the loop never ends.

```
while outstanding_Balance > 0:
bla
bla
```

You never change outstanding_Balance so the loop never ends.

Answer 51

What your program does is tell how many months it will take to pay things off if a certain fixed payment is made, but only sort of. See, if the payment is not enough to pay it off, it dies. Try yours, as adjusted in chat, with a ballance of 3000. Should get some interesting results.

Answer 52

yeah that was left from my last program
http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-00sc-introduction-to-computer-science-and-programming-spring-2011/unit-1/lecture-4-machine-interpretation-of-a-program/MIT6_00SCS11_ps1.pdf

problem 2

Answer 53

Yes, that is what your program says i is trying to do, find the pay off in a year. But it does not really do that.

Answer 54

for some reason I was under the impression that they wanted us to put it as 10 and increment it manually.........

Answer 55

Well, they are saying to try different payments and see what happens. They wanted people to look at it three different ways to help show the power of the bisection search. There are other ways to make that sort of thing efficent enough for personal use, but they are trying to teach how to optimize for industrial use.

Answer 56

Here is some logic for you.

Case 1:
Pay off 10 a month, if paying 10 a month does not make the outstanding balance do down to 0 in a year, pay off 20 a month. Keep increasing projected payments by 10 a month until you could pay it off in a year. Not too hard a calculation, but it has to loop through several failures before a success.

Case 2:
If you pay 1/12th of the outstanding balance, it seems like you will pay it off in a year. But there is interest. So paying off 1/12th falls short because of the interest. Still, it is closer than starting with a random payment value or just 10. Then you can increase up to what is needed. How much to increase? Well, look at what is left and /12. So you go up in smaller increments. The flow here is that you can have a situation where the increase is too small, so it never exists the loop.

Answer 57

ok so what im going to do is initiate the first loop set the payment to 0 and then nest another loop that does the month and the payment increasing it by 10.
sound right?

Answer 58

For the 10 one, that is a good plan.

Answer 59

As I said before, I do not always have the chat open. This is especially true if I am dealing with issues or answering questions. So it has been 40 min since you said something to me in chat, and I just saw it.

Answer 60

In psudocode:

```
inputs from user for balance and APR
testBalance = balance
payment = 10
while testBalance > 0
for x = 1 to 12:
testBallance = testBallance - payment
testBallance = testBallance + interest formula goes here
if testBallance > 0:
payment = payment + 10
testBalance = balance
print results
```

Answer 61

If you look at that, it will stick in the while loop until the payment is high enough to pay off the debt in a year. It increases by 10 each time around. The for loop is to do the 12 monthly payments. You have two balances because you may need to reset.

For large balances with a high APR, this is slow because it only jumps up a small bit each time and always starts at 10. As I said before, if you start with say `payment = balance / 12` then it will be a lot closer to what is needed and take less time.

Answer 62

```
##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
monthly_Payment = 0
current_Balance = outstanding_Balance
## while balance not paid off
while current_Balance > 0:
months = 0
monthly_Payment += 10
while current_Balance > 0 and months < 12:
months += 1
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - monthly_Payment
## if balance is paid off, end loop
if current_Balance <= 0:
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",months
print "Balance:",round(current_Balance, 2)
```
ahhh ok this is what I had I guess I need to move the monthly_Payment+=10 lower so that it adds the 10 after the loop

Answer 63

Remember to reset each time through the outer loop.

Answer 64

```
## while balance not paid off
while current_Balance > 0:
current_Balance = outstanding_Balance
months = 0
```

See, that way it will do a fresh evaluation every loop.

Answer 65

```
##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
monthly_Payment = 0
## while balance not paid off
while outstanding_Balance > 0:
current_Balance = outstanding_Balance
months = 0
monthly_Payment += 10
while months < 12:
months += 1
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - monthly_Payment
## if balance is paid off, end loop
if current_Balance <= 0:
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",months
print "Balance:",round(current_Balance, 2)
```

so the program is running smoothly I just cant get the loop to stop when I need it to. I inserted a break statment but, it either doesnt stop or wont print out what I need lol. EXTREMLEY FRUSTRATING....... I even changed the nested while loop to while months < 12 and current_Balance >0 so I dont understand why the loop kepts going when the current balance reaches 0!!!

Answer 66

You are still making the mistake where you evaluate a temp variable but not doing the loop based on it.

Answer 67

Here, let me toss it out this way:

```
##calculates minimum fixed monthly payment needed in order to pay off credit card balance within 12 months
outstanding_Balance = float(raw_input("Outstanding Balance on Credit Card:"))
current_Balance = outstanding_Balance
annual_Interest_Rate = float(raw_input("Annual Interest Rate:"))
monthly_Interest_Rate = annual_Interest_Rate / 12.0
monthly_Payment = 0
## while balance not paid off
while current_Balance > 0:
current_Balance = outstanding_Balance
months = 0
monthly_Payment += 10
while months < 12:
months += 1
current_Balance = current_Balance * (1 + monthly_Interest_Rate) - monthly_Payment
## if balance is paid off, end loop
if current_Balance <= 0:
print "RESULT:"
print "Monthly payment to pay off debt in 1 year:",monthly_Payment
print "Number of months needed:",months
print "Balance:",round(current_Balance, 2)
```

I have not run it, but try it that way.