Calculate a running total during a for loop - Python

Question

Edit: Below is my working code based on the feedback/answers I recieved.

This question stems from my previous question that came up while learning Python/CS using open courseware from MIT. --See my previous question here--

I am using the following code to make a list of month payments and other things. However at the end of the loop I need to give a running total for the total amount that has been paid of the months.

Original Code

balance = float(raw_input("Outstanding Balance: "))
interestRate = float(raw_input("Interest Rate: "))
minPayRate = float(raw_input("Minimum Monthly Payment Rate: "))

for month in xrange(1, 12+1):
    interestPaid = round(interestRate / 12.0 * balance, 2)
    minPayment = round(minPayRate * balance, 2)
    principalPaid = round(minPayment - interestPaid, 2)
    remainingBalance = round(balance - principalPaid, 2)


    print 'Month: %d' % (month,)
    print 'Minimum monthly payment: %.2f' % (minPayment,)
    print 'Principle paid: %.2f' % (principalPaid,)
    print 'Remaining balance: %.2f' % (remainingBalance,)

    balance = remainingBalance

    if month in xrange(12, 12+1):
        print 'RESULTS'
        print 'Total amount paid: '
        print 'Remaining balance: %.2f' % (remainingBalance,)

The problem is that I have not been able to figure out how to keep a running total of the amounts paid. I tried adding totalPaid = round(interestPaid + principalPaid, 2) but that just led to a total for a single month, I cant seem to get it to keep that value for each month and then add them all up at the end to be printed out.

Also I know that the resulting amount should be 1131.12

I have found many examples of doing this when each value is know, via a list, but I cant seem to extrapolate that correctly.

Fixed Code

balance = float(raw_input("Outstanding Balance: "))
interestRate = float(raw_input("Interest Rate: "))
minPayRate = float(raw_input("Minimum Monthly Payment Rate: "))
totalPaid = 0

for month in xrange(1, 12+1):
    interestPaid = round(interestRate / 12.0 * balance, 2)
    minPayment = round(minPayRate * balance, 2)
    principalPaid = round(minPayment - interestPaid, 2)
    remainingBalance = round(balance - principalPaid, 2)
    totalPaid += round(minPayment, 2)

    print 'Month: %d' % (month,)
    print 'Minimum monthly payment: %.2f' % (minPayment,)
    print 'Principle paid: %.2f' % (principalPaid,)
    print 'Remaining balance: %.2f' % (remainingBalance,)

    balance = remainingBalance

    if month in xrange(12, 12+1):
        print 'RESULTS'
        print 'Total amount paid: %.2f' % (totalPaid,)
        print 'Remaining balance: %.2f' % (remainingBalance,)

Answer 1

Before your loop, initialize a variable to accumulate value:

total_paid = 0

And then, in the body of your loop, add the appropriate amount to it. You can use the += operator to add to an existing variable, e.g.

total_paid += 1

is a short form for total_paid = total_paid + 1. You don't want to give total_paid a new value each iteration, rather you want to add to its existing value.

I'm not sure about the specifics of your problem, but this is the general form for accumulating a value as you loop.

Answer 2

You always make the minimum payment? Just use minPayment instead of figuring out that math again. Keep a running total, then print it out after the loop.

balance = float(raw_input("Outstanding Balance: "))
interestRate = float(raw_input("Interest Rate: "))
minPayRate = float(raw_input("Minimum Monthly Payment Rate: "))
paid = 0

for month in xrange(1, 12+1):
    interestPaid = round(interestRate / 12.0 * balance, 2)
    minPayment = round(minPayRate * balance, 2)
    principalPaid = round(minPayment - interestPaid, 2)
    remainingBalance = round(balance - principalPaid, 2)
    paid += minPayment

    print  # Make the output easier to read.
    print 'Month: %d' % (month,)
    print 'Minimum monthly payment: %.2f' % (minPayment,)
    print 'Principle paid: %.2f' % (principalPaid,)
    print 'Remaining balance: %.2f' % (remainingBalance,)

    balance = remainingBalance

print
print 'RESULTS'
print 'Total amount paid:', paid
print 'Remaining balance: %.2f' % (remainingBalance,)

Also notice that range has exactly one value, so you'd just check month == 12, but it's simply not necessary here.

Answer 3

You actually have to initialize totalPaid to 0 and then

totalPaid = round(interestPaid + principalPaid, 2) + totalPaid

Inside the loop. Your problem is that you're not accumulating the total, you're just setting a new one on each iteration.

Answer 4

sounds like you were close. The problem is that you were overwriting the total each time. Try something like this:

totalPaid = totalPaid + round(interestPaid + principalPaid, 2)