如何在python中的for循环中更改变量？

Question

I am trying to make a program in python that calculates the remaining balance on a credit card. It is for MIT open courseware "Introduction to Computer Science and Programming" . I am doing problem set one .

The program has to ask the user for starting variables: starting balance, annual interest rate, and minimum monthly payment. Here is my code.

initialOutstandingBalance= float(raw_input('What is the outstanding balance on your  
card?'))
annualInterestRate=float(raw_input('What is the annual interest rate expressed as a   
decimal?'))
minimumMonthlyPaymentRate=float(raw_input('What is the minimum monthly payment rate on
your card expressed as a decimal?'))

for month in range(1,13):
    print("Month: "+ str(month))
    minimumMonthlyPayment=float(minimumMonthlyPaymentRate*initialOutstandingBalance)
    interestPaid=float((annualInterestRate)/(12*initialOutstandingBalance))
    principalPaid=float(minimumMonthlyPayment-interestPaid)
    newBalance=float(initialOutstandingBalance-principalPaid)
    print("Minimum monthly payment: $"+str(minimumMonthlyPayment))
    print("Principle paid: $"+str(principalPaid))
    print("Remaining Balance: $"+str(newBalance))

How do I get the remaining balance to properly update? I can't figure out how to update the remaining balance at the end of each month. So far, each month returns the same values for minimum monthly payment, principal paid, and remaining balance.

Answer 1

You're using the same initialOutstandingBalance variable throughout your loop, and never changing it. Instead, you should track the current balance. That will be equal to the initial outstanding balance at the start of the loop, but will change as it runs.

You also don't need to keep calling float .

current_balance = initialOutstandingBalance
for month in range(1,13):
    print("Month: "+ str(month))
    minimumMonthlyPayment = minimumMonthlyPaymentRate * current_balance
    # this calculation is almost certainly wrong, but is preserved from your starting code
    interestPaid = annualInterestRate / (12*current_balance)
    principalPaid = minimumMonthlyPayment - interestPaid
    current_balance = current_balance - principalPaid
    print("Minimum monthly payment: $"+str(minimumMonthlyPayment))
    print("Principle paid: $"+str(principalPaid))
    print("Remaining Balance: $"+str(current_balance))

Answer 2

You want to keep the variable newBalance outside the loop, or else it'll be reassigned each iteration. Also, you don't want to divide the interest rate by 12 times the balance, but instead divide it by 12 and then multiply the quotient by the balance. Finally, as noted above, you don't need all the float s.

This should work:

newBalance = initialOutstandingBalance

for month in range(1,13):
    print("Month: " + str(month))

    minimumMonthlyPayment = minimumMonthlyPaymentRate * newBalance
    interestPaid = annualInterestRate / 12 * newBalance
    principalPaid = minimumMonthlyPayment - interestPaid
    newBalance -= principalPaid

    print("Minimum monthly payment: $" + str(minimumMonthlyPayment))
    print("Principle paid: $" +  str(principalPaid))
    print("Remaining Balance: $" + str(newBalance))