如何在python中的for循环中更改变量？

Question

我试图用python编写一个程序，计算信用卡上的余额。 它是针对MIT开放式课件“计算机科学与程序设计导论” 。 我正在做第一个问题 。

该程序必须询问用户初始变量：初始余额，年利率和每月最低还款额。 这是我的代码。

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))

如何获取剩余余额以正确更新？ 我不知道如何在每个月底更新余额。 到目前为止，每个月对于最低每月还款额，已付本金和剩余余额返回相同的值。

Answer 1

您在整个循环中使用相同的initialOutstandingBalance变量，并且从不对其进行更改。 相反，您应该跟踪当前余额。 这将等于循环开始时的初始未偿余额，但是会随着循环的进行而变化。

您也不需要继续调用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

您希望将变量newBalance保留在循环之外，否则将在每次迭代时将其重新分配。 同样，您也不想将利率除以余额的12倍，而是将其除以12，然后再将商乘以余额。 最后，如上所述，您不需要所有的float 。

这应该工作：

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))